Programs & Examples On #Operating system

An operating System (OS) is a basic software whose role is to be an abstract layer between software requisitions for resources and the hardware available, manage input/output, memory allocation/deallocation, file systems, among other basic tasks a device (not necessarily a computer) should do.

What is the difference between Trap and Interrupt?

A trap is called by code like programs and used e. g. to call OS routines (i. e. normally synchronous). An interrupt is called by events (many times hardware, like the network card having received data, or the CPU timer), and - as the name suggests - interrupts the normal control flow, as the CPU has to switch to the driver routine to handle the event.

Best way to find os name and version in Unix/Linux platform

With and Linux::Distribution, the cleanest solution for an old problem :


perl -e '
    use Linux::Distribution qw(distribution_name distribution_version);

    my $linux = Linux::Distribution->new;
    if(my $distro = $linux->distribution_name()) {
          my $version = $linux->distribution_version();
          print "you are running $distro";
          print " version $version" if $version;
          print "\n";
    } else {
          print "distribution unknown\n";

DNS caching in linux

On Linux (and probably most Unix), there is no OS-level DNS caching unless nscd is installed and running. Even then, the DNS caching feature of nscd is disabled by default at least in Debian because it's broken. The practical upshot is that your linux system very very probably does not do any OS-level DNS caching.

You could implement your own cache in your application (like they did for Squid, according to diegows's comment), but I would recommend against it. It's a lot of work, it's easy to get it wrong (nscd got it wrong!!!), it likely won't be as easily tunable as a dedicated DNS cache, and it duplicates functionality that already exists outside your application.

If an end user using your software needs to have DNS caching because the DNS query load is large enough to be a problem or the RTT to the external DNS server is long enough to be a problem, they can install a caching DNS server such as Unbound on the same machine as your application, configured to cache responses and forward misses to the regular DNS resolvers.

What is the difference between user and kernel modes in operating systems?

  1. Kernel Mode

    In Kernel mode, the executing code has complete and unrestricted access to the underlying hardware. It can execute any CPU instruction and reference any memory address. Kernel mode is generally reserved for the lowest-level, most trusted functions of the operating system. Crashes in kernel mode are catastrophic; they will halt the entire PC.

  2. User Mode

    In User mode, the executing code has no ability to directly access hardware or reference memory. Code running in user mode must delegate to system APIs to access hardware or memory. Due to the protection afforded by this sort of isolation, crashes in user mode are always recoverable. Most of the code running on your computer will execute in user mode.

Read more

Understanding User and Kernel Mode

How to run regasm.exe from command line other than Visual Studio command prompt?

Like Cheeso said:

You don't need the directory on your path. You could put it on your path, but you don't NEED to do that. If you are calling regasm rarely, or calling it from a batch file, you may find it is simpler to just invoke regasm via the fully-qualified pathname on the exe, eg:

%SystemRoot%\Microsoft.NET\Framework\v2.0.50727\regasm.exe MyAssembly.dll

How do I set a Windows scheduled task to run in the background?

As noted by Mattias Nordqvist in the comments below, you can also select the radio button option "Run whether user is logged on or not". When saving the task, you will be prompted once for the user password. bambams noted that this wouldn't grant System permissions to the process, and also seems to hide the command window.

It's not an obvious solution, but to make a Scheduled Task run in the background, change the User running the task to "SYSTEM", and nothing will appear on your screen.

enter image description here

enter image description here

enter image description here

How to find the operating system version using JavaScript?

I started to write a Script to read OS and browser version that can be tested on Fiddle. Feel free to use and extend.
Breaking Change:
Since September 2020 the new Edge gets detected. So 'Microsoft Edge' is the new version based on Chromium and the old Edge is now detected as 'Microsoft Legacy Edge'!

 * JavaScript Client Detection
 * (C) viazenetti GmbH (Christian Ludwig)
(function (window) {
        var unknown = '-';

        // screen
        var screenSize = '';
        if (screen.width) {
            width = (screen.width) ? screen.width : '';
            height = (screen.height) ? screen.height : '';
            screenSize += '' + width + " x " + height;

        // browser
        var nVer = navigator.appVersion;
        var nAgt = navigator.userAgent;
        var browser = navigator.appName;
        var version = '' + parseFloat(navigator.appVersion);
        var majorVersion = parseInt(navigator.appVersion, 10);
        var nameOffset, verOffset, ix;

        // Opera
        if ((verOffset = nAgt.indexOf('Opera')) != -1) {
            browser = 'Opera';
            version = nAgt.substring(verOffset + 6);
            if ((verOffset = nAgt.indexOf('Version')) != -1) {
                version = nAgt.substring(verOffset + 8);
        // Opera Next
        if ((verOffset = nAgt.indexOf('OPR')) != -1) {
            browser = 'Opera';
            version = nAgt.substring(verOffset + 4);
        // Legacy Edge
        else if ((verOffset = nAgt.indexOf('Edge')) != -1) {
            browser = 'Microsoft Legacy Edge';
            version = nAgt.substring(verOffset + 5);
        // Edge (Chromium)
        else if ((verOffset = nAgt.indexOf('Edg')) != -1) {
            browser = 'Microsoft Edge';
            version = nAgt.substring(verOffset + 4);
        // MSIE
        else if ((verOffset = nAgt.indexOf('MSIE')) != -1) {
            browser = 'Microsoft Internet Explorer';
            version = nAgt.substring(verOffset + 5);
        // Chrome
        else if ((verOffset = nAgt.indexOf('Chrome')) != -1) {
            browser = 'Chrome';
            version = nAgt.substring(verOffset + 7);
        // Safari
        else if ((verOffset = nAgt.indexOf('Safari')) != -1) {
            browser = 'Safari';
            version = nAgt.substring(verOffset + 7);
            if ((verOffset = nAgt.indexOf('Version')) != -1) {
                version = nAgt.substring(verOffset + 8);
        // Firefox
        else if ((verOffset = nAgt.indexOf('Firefox')) != -1) {
            browser = 'Firefox';
            version = nAgt.substring(verOffset + 8);
        // MSIE 11+
        else if (nAgt.indexOf('Trident/') != -1) {
            browser = 'Microsoft Internet Explorer';
            version = nAgt.substring(nAgt.indexOf('rv:') + 3);
        // Other browsers
        else if ((nameOffset = nAgt.lastIndexOf(' ') + 1) < (verOffset = nAgt.lastIndexOf('/'))) {
            browser = nAgt.substring(nameOffset, verOffset);
            version = nAgt.substring(verOffset + 1);
            if (browser.toLowerCase() == browser.toUpperCase()) {
                browser = navigator.appName;
        // trim the version string
        if ((ix = version.indexOf(';')) != -1) version = version.substring(0, ix);
        if ((ix = version.indexOf(' ')) != -1) version = version.substring(0, ix);
        if ((ix = version.indexOf(')')) != -1) version = version.substring(0, ix);

        majorVersion = parseInt('' + version, 10);
        if (isNaN(majorVersion)) {
            version = '' + parseFloat(navigator.appVersion);
            majorVersion = parseInt(navigator.appVersion, 10);

        // mobile version
        var mobile = /Mobile|mini|Fennec|Android|iP(ad|od|hone)/.test(nVer);

        // cookie
        var cookieEnabled = (navigator.cookieEnabled) ? true : false;

        if (typeof navigator.cookieEnabled == 'undefined' && !cookieEnabled) {
            document.cookie = 'testcookie';
            cookieEnabled = (document.cookie.indexOf('testcookie') != -1) ? true : false;

        // system
        var os = unknown;
        var clientStrings = [
            {s:'Windows 10', r:/(Windows 10.0|Windows NT 10.0)/},
            {s:'Windows 8.1', r:/(Windows 8.1|Windows NT 6.3)/},
            {s:'Windows 8', r:/(Windows 8|Windows NT 6.2)/},
            {s:'Windows 7', r:/(Windows 7|Windows NT 6.1)/},
            {s:'Windows Vista', r:/Windows NT 6.0/},
            {s:'Windows Server 2003', r:/Windows NT 5.2/},
            {s:'Windows XP', r:/(Windows NT 5.1|Windows XP)/},
            {s:'Windows 2000', r:/(Windows NT 5.0|Windows 2000)/},
            {s:'Windows ME', r:/(Win 9x 4.90|Windows ME)/},
            {s:'Windows 98', r:/(Windows 98|Win98)/},
            {s:'Windows 95', r:/(Windows 95|Win95|Windows_95)/},
            {s:'Windows NT 4.0', r:/(Windows NT 4.0|WinNT4.0|WinNT|Windows NT)/},
            {s:'Windows CE', r:/Windows CE/},
            {s:'Windows 3.11', r:/Win16/},
            {s:'Android', r:/Android/},
            {s:'Open BSD', r:/OpenBSD/},
            {s:'Sun OS', r:/SunOS/},
            {s:'Chrome OS', r:/CrOS/},
            {s:'Linux', r:/(Linux|X11(?!.*CrOS))/},
            {s:'iOS', r:/(iPhone|iPad|iPod)/},
            {s:'Mac OS X', r:/Mac OS X/},
            {s:'Mac OS', r:/(Mac OS|MacPPC|MacIntel|Mac_PowerPC|Macintosh)/},
            {s:'QNX', r:/QNX/},
            {s:'UNIX', r:/UNIX/},
            {s:'BeOS', r:/BeOS/},
            {s:'OS/2', r:/OS\/2/},
            {s:'Search Bot', r:/(nuhk|Googlebot|Yammybot|Openbot|Slurp|MSNBot|Ask Jeeves\/Teoma|ia_archiver)/}
        for (var id in clientStrings) {
            var cs = clientStrings[id];
            if (cs.r.test(nAgt)) {
                os = cs.s;

        var osVersion = unknown;

        if (/Windows/.test(os)) {
            osVersion = /Windows (.*)/.exec(os)[1];
            os = 'Windows';

        switch (os) {
            case 'Mac OS':
            case 'Mac OS X':
            case 'Android':
                osVersion = /(?:Android|Mac OS|Mac OS X|MacPPC|MacIntel|Mac_PowerPC|Macintosh) ([\.\_\d]+)/.exec(nAgt)[1];

            case 'iOS':
                osVersion = /OS (\d+)_(\d+)_?(\d+)?/.exec(nVer);
                osVersion = osVersion[1] + '.' + osVersion[2] + '.' + (osVersion[3] | 0);
        // flash (you'll need to include swfobject)
        /* script src="//" */
        var flashVersion = 'no check';
        if (typeof swfobject != 'undefined') {
            var fv = swfobject.getFlashPlayerVersion();
            if (fv.major > 0) {
                flashVersion = fv.major + '.' + fv.minor + ' r' + fv.release;
            else  {
                flashVersion = unknown;

    window.jscd = {
        screen: screenSize,
        browser: browser,
        browserVersion: version,
        browserMajorVersion: majorVersion,
        mobile: mobile,
        os: os,
        osVersion: osVersion,
        cookies: cookieEnabled,
        flashVersion: flashVersion

    'OS: ' + jscd.os +' '+ jscd.osVersion + '\n' +
    'Browser: ' + jscd.browser +' '+ jscd.browserMajorVersion +
      ' (' + jscd.browserVersion + ')\n' + 
    'Mobile: ' + + '\n' +
    'Flash: ' + jscd.flashVersion + '\n' +
    'Cookies: ' + jscd.cookies + '\n' +
    'Screen Size: ' + jscd.screen + '\n\n' +
    'Full User Agent: ' + navigator.userAgent

How do I check the operating system in Python?

If you want to know on which platform you are on out of "Linux", "Windows", or "Darwin" (Mac), without more precision, you should use:

>>> import platform
>>> platform.system()
'Linux'  # or 'Windows'/'Darwin'

The platform.system function uses uname internally.

How do I check CPU and Memory Usage in Java?

JMX, The MXBeans (ThreadMXBean, etc) provided will give you Memory and CPU usages.

OperatingSystemMXBean operatingSystemMXBean = (OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();

What are the differences between virtual memory and physical memory?

Virtual memory is, among other things, an abstraction to give the programmer the illusion of having infinite memory available on their system.

Virtual memory mappings are made to correspond to actual physical addresses. The operating system creates and deals with these mappings - utilizing the page table, among other data structures to maintain the mappings. Virtual memory mappings are always found in the page table or some similar data structure (in case of other implementations of virtual memory, we maybe shouldn't call it the "page table"). The page table is in physical memory as well - often in kernel-reserved spaces that user programs cannot write over.

Virtual memory is typically larger than physical memory - there wouldn't be much reason for virtual memory mappings if virtual memory and physical memory were the same size.

Only the needed part of a program is resident in memory, typically - this is a topic called "paging". Virtual memory and paging are tightly related, but not the same topic. There are other implementations of virtual memory, such as segmentation.

I could be assuming wrong here, but I'd bet the things you are finding hard to wrap your head around have to do with specific implementations of virtual memory, most likely paging. There is no one way to do paging - there are many implementations and the one your textbook describes is likely not the same as the one that appears in real OSes like Linux/Windows - there are probably subtle differences.

I could blab a thousand paragraphs about paging... but I think that is better left to a different question targeting specifically that topic.

Find Process Name by its Process ID

The basic one, ask tasklist to filter its output and only show the indicated process id information

tasklist /fi "pid eq 4444" 

To only get the process name, the line must be splitted

for /f "delims=," %%a in ('
    tasklist /fi "pid eq 4444" /nh /fo:csv
') do echo %%~a

In this case, the list of processes is retrieved without headers (/nh) in csv format (/fo:csv). The commas are used as token delimiters and the first token in the line is the image name

note: In some windows versions (one of them, my case, is the spanish windows xp version), the pid filter in the tasklist does not work. In this case, the filter over the list of processes must be done out of the command

for /f "delims=," %%a in ('
    tasklist /fo:csv /nh ^| findstr /b /r /c:"[^,]*,\"4444\","
') do echo %%~a

This will generate the task list and filter it searching for the process id in the second column of the csv output.

edited: alternatively, you can suppose what has been made by the team that translated the OS to spanish. I don't know what can happen in other locales.

tasklist /fi "idp eq 4444" 

How to run a program without an operating system?

Runnable examples

Let's create and run some minuscule bare metal hello world programs that run without an OS on:

We will also try them out on the QEMU emulator as much as possible, as that is safer and more convenient for development. The QEMU tests have been on an Ubuntu 18.04 host with the pre-packaged QEMU 2.11.1.

The code of all x86 examples below and more is present on this GitHub repo.

How to run the examples on x86 real hardware

Remember that running examples on real hardware can be dangerous, e.g. you could wipe your disk or brick the hardware by mistake: only do this on old machines that don't contain critical data! Or even better, use cheap semi-disposable devboards such as the Raspberry Pi, see the ARM example below.

For a typical x86 laptop, you have to do something like:

  1. Burn the image to an USB stick (will destroy your data!):

    sudo dd if=main.img of=/dev/sdX
  2. plug the USB on a computer

  3. turn it on

  4. tell it to boot from the USB.

    This means making the firmware pick USB before hard disk.

    If that is not the default behavior of your machine, keep hitting Enter, F12, ESC or other such weird keys after power-on until you get a boot menu where you can select to boot from the USB.

    It is often possible to configure the search order in those menus.

For example, on my T430 I see the following.

After turning on, this is when I have to press Enter to enter the boot menu:

enter image description here

Then, here I have to press F12 to select the USB as the boot device:

enter image description here

From there, I can select the USB as the boot device like this:

enter image description here

Alternatively, to change the boot order and choose the USB to have higher precedence so I don't have to manually select it every time, I would hit F1 on the "Startup Interrupt Menu" screen, and then navigate to:

enter image description here

Boot sector

On x86, the simplest and lowest level thing you can do is to create a Master Boot Sector (MBR), which is a type of boot sector, and then install it to a disk.

Here we create one with a single printf call:

printf '\364%509s\125\252' > main.img
sudo apt-get install qemu-system-x86
qemu-system-x86_64 -hda main.img


enter image description here

Note that even without doing anything, a few characters are already printed on the screen. Those are printed by the firmware, and serve to identify the system.

And on the T430 we just get a blank screen with a blinking cursor:

enter image description here

main.img contains the following:

  • \364 in octal == 0xf4 in hex: the encoding for a hlt instruction, which tells the CPU to stop working.

    Therefore our program will not do anything: only start and stop.

    We use octal because \x hex numbers are not specified by POSIX.

    We could obtain this encoding easily with:

    echo hlt > a.S
    as -o a.o a.S
    objdump -S a.o

    which outputs:

    a.o:     file format elf64-x86-64
    Disassembly of section .text:
    0000000000000000 <.text>:
       0:   f4                      hlt

    but it is also documented in the Intel manual of course.

  • %509s produce 509 spaces. Needed to fill in the file until byte 510.

  • \125\252 in octal == 0x55 followed by 0xaa.

    These are 2 required magic bytes which must be bytes 511 and 512.

    The BIOS goes through all our disks looking for bootable ones, and it only considers bootable those that have those two magic bytes.

    If not present, the hardware will not treat this as a bootable disk.

If you are not a printf master, you can confirm the contents of main.img with:

hd main.img

which shows the expected:

00000000  f4 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |.               |
00000010  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
000001f0  20 20 20 20 20 20 20 20  20 20 20 20 20 20 55 aa  |              U.|

where 20 is a space in ASCII.

The BIOS firmware reads those 512 bytes from the disk, puts them into memory, and sets the PC to the first byte to start executing them.

Hello world boot sector

Now that we have made a minimal program, let's move to a hello world.

The obvious question is: how to do IO? A few options:

  • ask the firmware, e.g. BIOS or UEFI, to do it for us

  • VGA: special memory region that gets printed to the screen if written to. Can be used in Protected Mode.

  • write a driver and talk directly to the display hardware. This is the "proper" way to do it: more powerful, but more complex.

  • serial port. This is a very simple standardized protocol that sends and receives characters from a host terminal.

    On desktops, it looks like this:

    enter image description here


    It is unfortunately not exposed on most modern laptops, but is the common way to go for development boards, see the ARM examples below.

    This is really a shame, since such interfaces are really useful to debug the Linux kernel for example.

  • use debug features of chips. ARM calls theirs semihosting for example. On real hardware, it requires some extra hardware and software support, but on emulators it can be a free convenient option. Example.

Here we will do a BIOS example as it is simpler on x86. But note that it is not the most robust method.


    mov $msg, %si
    mov $0x0e, %ah
    or %al, %al
    jz halt
    int $0x10
    jmp loop
    .asciz "hello world"

GitHub upstream.


    /* The BIOS loads the code from the disk to this location.
     * We must tell that to the linker so that it can properly
     * calculate the addresses of symbols we might jump to.
    . = 0x7c00;
    .text :
        __start = .;
        /* Place the magic boot bytes at the end of the first 512 sector. */
        . = 0x1FE;

Assemble and link with:

as -g -o main.o main.S
ld --oformat binary -o main.img -T link.ld main.o
qemu-system-x86_64 -hda main.img


enter image description here

And on the T430:

enter image description here

Tested on: Lenovo Thinkpad T430, UEFI BIOS 1.16. Disk generated on an Ubuntu 18.04 host.

Besides the standard userland assembly instructions, we have:

  • .code16: tells GAS to output 16-bit code

  • cli: disable software interrupts. Those could make the processor start running again after the hlt

  • int $0x10: does a BIOS call. This is what prints the characters one by one.

The important link flags are:

  • --oformat binary: output raw binary assembly code, don't wrap it inside an ELF file as is the case for regular userland executables.

To better understand the linker script part, familiarize yourself with the relocation step of linking: What do linkers do?

Cooler x86 bare metal programs

Here are a few more complex bare metal setups that I've achieved:

Use C instead of assembly

Summary: use GRUB multiboot, which will solve a lot of annoying problems you never thought about. See the section below.

The main difficulty on x86 is that the BIOS only loads 512 bytes from the disk to memory, and you are likely to blow up those 512 bytes when using C!

To solve that, we can use a two-stage bootloader. This makes further BIOS calls, which load more bytes from the disk into memory. Here is a minimal stage 2 assembly example from scratch using the int 0x13 BIOS calls:


  • if you only need it to work in QEMU but not real hardware, use the -kernel option, which loads an entire ELF file into memory. Here is an ARM example I've created with that method.
  • for the Raspberry Pi, the default firmware takes care of the image loading for us from an ELF file named kernel7.img, much like QEMU -kernel does.

For educational purposes only, here is a one stage minimal C example:


void main(void) {
    int i;
    char s[] = {'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'};
    for (i = 0; i < sizeof(s); ++i) {
        __asm__ (
            "int $0x10" : : "a" ((0x0e << 8) | s[i])
    while (1) {
        __asm__ ("hlt");


.global mystart
    ljmp $0, $.setcs
    xor %ax, %ax
    mov %ax, %ds
    mov %ax, %es
    mov %ax, %ss
    mov $__stack_top, %esp
    call main


  . = 0x7c00;
  .text : {
    __bss_start = .;
    /* COMMON vs BSS: */
    __bss_end = .;
  /* */
  .sig : AT(ADDR(.text) + 512 - 2)
  /DISCARD/ : {
  __stack_bottom = .;
  . = . + 0x1000;
  __stack_top = .;


set -eux
as -ggdb3 --32 -o entry.o entry.S
gcc -c -ggdb3 -m16 -ffreestanding -fno-PIE -nostartfiles -nostdlib -o main.o -std=c99 main.c
ld -m elf_i386 -o main.elf -T linker.ld entry.o main.o
objcopy -O binary main.elf main.img
qemu-system-x86_64 -drive file=main.img,format=raw

C standard library

Things get more fun if you also want to use the C standard library however, since we don't have the Linux kernel, which implements much of the C standard library functionality through POSIX.

A few possibilities, without going to a full-blown OS like Linux, include:

  • Write your own. It's just a bunch of headers and C files in the end, right? Right??

  • Newlib

    Detailed example at:

    Newlib implements all the boring non-OS specific things for you, e.g. memcmp, memcpy, etc.

    Then, it provides some stubs for you to implement the syscalls that you need yourself.

    For example, we can implement exit() on ARM through semihosting with:

    void _exit(int status) {
        __asm__ __volatile__ ("mov r0, #0x18; ldr r1, =#0x20026; svc 0x00123456");

    as shown at in this example.

    For example, you could redirect printf to the UART or ARM systems, or implement exit() with semihosting.

  • embedded operating systems like FreeRTOS and Zephyr.

    Such operating systems typically allow you to turn off pre-emptive scheduling, therefore giving you full control over the runtime of the program.

    They can be seen as a sort of pre-implemented Newlib.

GNU GRUB Multiboot

Boot sectors are simple, but they are not very convenient:

  • you can only have one OS per disk
  • the load code has to be really small and fit into 512 bytes
  • you have to do a lot of startup yourself, like moving into protected mode

It is for those reasons that GNU GRUB created a more convenient file format called multiboot.

Minimal working example:

I also use it on my GitHub examples repo to be able to easily run all examples on real hardware without burning the USB a million times.

QEMU outcome:

enter image description here


enter image description here

If you prepare your OS as a multiboot file, GRUB is then able to find it inside a regular filesystem.

This is what most distros do, putting OS images under /boot.

Multiboot files are basically an ELF file with a special header. They are specified by GRUB at:

You can turn a multiboot file into a bootable disk with grub-mkrescue.


In truth, your boot sector is not the first software that runs on the system's CPU.

What actually runs first is the so-called firmware, which is a software:

  • made by the hardware manufacturers
  • typically closed source but likely C-based
  • stored in read-only memory, and therefore harder / impossible to modify without the vendor's consent.

Well known firmwares include:

  • BIOS: old all-present x86 firmware. SeaBIOS is the default open source implementation used by QEMU.
  • UEFI: BIOS successor, better standardized, but more capable, and incredibly bloated.
  • Coreboot: the noble cross arch open source attempt

The firmware does things like:

  • loop over each hard disk, USB, network, etc. until you find something bootable.

    When we run QEMU, -hda says that main.img is a hard disk connected to the hardware, and hda is the first one to be tried, and it is used.

  • load the first 512 bytes to RAM memory address 0x7c00, put the CPU's RIP there, and let it run

  • show things like the boot menu or BIOS print calls on the display

Firmware offers OS-like functionality on which most OS-es depend. E.g. a Python subset has been ported to run on BIOS / UEFI:

It can be argued that firmwares are indistinguishable from OSes, and that firmware is the only "true" bare metal programming one can do.

As this CoreOS dev puts it:

The hard part

When you power up a PC, the chips that make up the chipset (northbridge, southbridge and SuperIO) are not yet initialized properly. Even though the BIOS ROM is as far removed from the CPU as it could be, this is accessible by the CPU, because it has to be, otherwise the CPU would have no instructions to execute. This does not mean that BIOS ROM is completely mapped, usually not. But just enough is mapped to get the boot process going. Any other devices, just forget it.

When you run Coreboot under QEMU, you can experiment with the higher layers of Coreboot and with payloads, but QEMU offers little opportunity to experiment with the low level startup code. For one thing, RAM just works right from the start.

Post BIOS initial state

Like many things in hardware, standardization is weak, and one of the things you should not rely on is the initial state of registers when your code starts running after BIOS.

So do yourself a favor and use some initialization code like the following:

Registers like %ds and %es have important side effects, so you should zero them out even if you are not using them explicitly.

Note that some emulators are nicer than real hardware and give you a nice initial state. Then when you go run on real hardware, everything breaks.

El Torito

Format that can be burnt to CDs:

It is also possible to produce a hybrid image that works on either ISO or USB. This is can be done with grub-mkrescue (example), and is also done by the Linux kernel on make isoimage using isohybrid.


In ARM, the general ideas are the same.

There is no widely available semi-standardized pre-installed firmware like BIOS for us to use for the IO, so the two simplest types of IO that we can do are:

  • serial, which is widely available on devboards
  • blink the LED

I have uploaded:

Some differences from x86 include:

  • IO is done by writing to magic addresses directly, there is no in and out instructions.

    This is called memory mapped IO.

  • for some real hardware, like the Raspberry Pi, you can add the firmware (BIOS) yourself to the disk image.

    That is a good thing, as it makes updating that firmware more transparent.


What are some resources for getting started in operating system development?

I would like to include this repo How-to-Make-a-Computer-Operating-System by Samy Pesse. Is a work-in-progress. Very interesting.

Context.startForegroundService() did not then call Service.startForeground()

I have a work around for this problem. I have verified this fix in my own app(300K+ DAU), which can reduce at least 95% of this kind of crash, but still cannot 100% avoid this problem.

This problem happens even when you ensure to call startForeground() just after service started as Google documented. It may be because the service creation and initialization process already cost more than 5 seconds in many scenarios, then no matter when and where you call startForeground() method, this crash is unavoidable.

My solution is to ensure that startForeground() will be executed within 5 seconds after startForegroundService() method, no matter how long your service need to be created and initialized. Here is the detailed solution.

  1. Do not use startForegroundService at the first place, use bindService() with auto_create flag. It will wait for the service initialization. Here is the code, my sample service is MusicService:

    final Context applicationContext = context.getApplicationContext();
    Intent intent = new Intent(context, MusicService.class);
    applicationContext.bindService(intent, new ServiceConnection() {
        public void onServiceConnected(ComponentName name, IBinder binder) {
            if (binder instanceof MusicBinder) {
                MusicBinder musicBinder = (MusicBinder) binder;
                MusicService service = musicBinder.getService();
                if (service != null) {
                    // start a command such as music play or pause.
                    // force the service to run in foreground here.
                    // the service is already initialized when bind and auto_create.
        public void onServiceDisconnected(ComponentName name) {
    }, Context.BIND_AUTO_CREATE);
  2. Then here is MusicBinder implementation:

     * Use weak reference to avoid binder service leak.
     public class MusicBinder extends Binder {
         private WeakReference<MusicService> weakService;
          * Inject service instance to weak reference.
         public void onBind(MusicService service) {
             this.weakService = new WeakReference<>(service);
         public MusicService getService() {
             return weakService == null ? null : weakService.get();
  3. The most important part, MusicService implementation, forceForeground() method will ensure that startForeground() method is called just after startForegroundService():

    public class MusicService extends MediaBrowserServiceCompat {
        private final MusicBinder musicBind = new MusicBinder();
        public IBinder onBind(Intent intent) {
            return musicBind;
        public void forceForeground() {
            // API lower than 26 do not need this work around.
            if (Build.VERSION.SDK_INT >= 26) {
                Intent intent = new Intent(this, MusicService.class);
                // service has already been initialized.
                // startForeground method should be called within 5 seconds.
                ContextCompat.startForegroundService(this, intent);
                Notification notification = mNotificationHandler.createNotification(this);
                // call startForeground just after startForegroundService.
                startForeground(Constants.NOTIFICATION_ID, notification);
  4. If you want to run the step 1 code snippet in a pending intent, such as if you want to start a foreground service in a widget (a click on widget button) without opening your app, you can wrap the code snippet in a broadcast receiver, and fire a broadcast event instead of start service command.

That is all. Hope it helps. Good luck.

Comprehensive methods of viewing memory usage on Solaris

# echo ::memstat | mdb -k
Page Summary                Pages                MB  %Tot
------------     ----------------  ----------------  ----
Kernel                       7308                57   23%
Anon                         9055                70   29%
Exec and libs                1968                15    6%
Page cache                   2224                17    7%
Free (cachelist)             6470                50   20%
Free (freelist)              4641                36   15%

Total                       31666               247
Physical                    31256               244

Difference between multitasking, multithreading and multiprocessing?

Both multiprogramming and multitasking solve different problems, though they use similar method of switching between the processes.

Multiprogramming : In the early days, it was seen that at times certain processes where using peripherals (e.g.: I/O). In such cases, the CPU remained idle. To use the CPU more efficiently, it was prudent to load multiple processes in the memory. This way, if a certain process were to use the peripheral, certain other process would use the CPU. This was multiprogramming in action.

Multitasking : To the end user, multiple processes had to appear running at the same time. This was mocked by switching between different processes and by making them run on the CPU simultaneously. This was the idea behind multitasking.

What is the return value of os.system() in Python?

Based on the answer of @AlokThakur (thanks!):

def run_system_command(command):
    return_value = os.system(command)
    # Calculate the return value code
    return_value = int(bin(return_value).replace("0b", "").rjust(16, '0')[:8], 2)
    if return_value != 0:
        raise RuntimeError(f'The system command\n{command}\nexited with return code {return_value}')

Difference between binary semaphore and mutex


Until recently, the only sleeping lock in the kernel was the semaphore. Most users of semaphores instantiated a semaphore with a count of one and treated them as a mutual exclusion lock—a sleeping version of the spin-lock. Unfortunately, semaphores are rather generic and do not impose any usage constraints. This makes them useful for managing exclusive access in obscure situations, such as complicated dances between the kernel and userspace. But it also means that simpler locking is harder to do, and the lack of enforced rules makes any sort of automated debugging or constraint enforcement impossible. Seeking a simpler sleeping lock, the kernel developers introduced the mutex.Yes, as you are now accustomed to, that is a confusing name. Let’s clarify.The term “mutex” is a generic name to refer to any sleeping lock that enforces mutual exclusion, such as a semaphore with a usage count of one. In recent Linux kernels, the proper noun “mutex” is now also a specific type of sleeping lock that implements mutual exclusion.That is, a mutex is a mutex.

The simplicity and efficiency of the mutex come from the additional constraints it imposes on its users over and above what the semaphore requires. Unlike a semaphore, which implements the most basic of behaviour in accordance with Dijkstra’s original design, the mutex has a stricter, narrower use case: n Only one task can hold the mutex at a time. That is, the usage count on a mutex is always one.

  1. Whoever locked a mutex must unlock it. That is, you cannot lock a mutex in one context and then unlock it in another. This means that the mutex isn’t suitable for more complicated synchronizations between kernel and user-space. Most use cases, however, cleanly lock and unlock from the same context.
  2. Recursive locks and unlocks are not allowed. That is, you cannot recursively acquire the same mutex, and you cannot unlock an unlocked mutex.
  3. A process cannot exit while holding a mutex.
  4. A mutex cannot be acquired by an interrupt handler or bottom half, even with mutex_trylock().
  5. A mutex can be managed only via the official API: It must be initialized via the methods described in this section and cannot be copied, hand initialized, or reinitialized.

[1] Linux Kernel Development, Third Edition Robert Love

Get operating system info

If you want very few info like a class in your html for common browsers for instance, you could use:

function get_browser()
    $browser = '';
    $ua = strtolower($_SERVER['HTTP_USER_AGENT']);
    if (preg_match('~(?:msie ?|trident.+?; ?rv: ?)(\d+)~', $ua, $matches)) $browser = 'ie ie'.$matches[1];
    elseif (preg_match('~(safari|chrome|firefox)~', $ua, $matches)) $browser = $matches[1];

    return $browser;

which will return 'safari' or 'firefox' or 'chrome', or 'ie ie8', 'ie ie9', 'ie ie10', 'ie ie11'.

How to make parent wait for all child processes to finish?

pid_t child_pid, wpid;
int status = 0;

//Father code (before child processes start)

for (int id=0; id<n; id++) {
    if ((child_pid = fork()) == 0) {
        //child code

while ((wpid = wait(&status)) > 0); // this way, the father waits for all the child processes 

//Father code (After all child processes end)

wait waits for a child process to terminate, and returns that child process's pid. On error (eg when there are no child processes), -1 is returned. So, basically, the code keeps waiting for child processes to finish, until the waiting errors out, and then you know they are all finished.

python: get directory two levels up

You can use pathlib. Unfortunately this is only available in the stdlib for Python 3.4. If you have an older version you'll have to install a copy from PyPI here. This should be easy to do using pip.

from pathlib import Path

p = Path(__file__).parents[1]

# /absolute/path/to/two/levels/up

This uses the parents sequence which provides access to the parent directories and chooses the 2nd one up.

Note that p in this case will be some form of Path object, with their own methods. If you need the paths as string then you can call str on them.

How do I programmatically determine operating system in Java?

You can use:


P.S. You may find this code useful:

class ShowProperties {
    public static void main(String[] args) {

All it does is print out all the properties provided by your Java implementations. It'll give you an idea of what you can find out about your Java environment via properties. :-)

Get Windows version in a batch file

This will return 5.1 for xp or 6/1 for windows 7

for /f "tokens=4-7 delims=[.] " %%i in ('ver') do (
if %%i == Version set OSVersion=%%j.%%k
if %%i neq Version set OSVersion=%%i.%%j

How to uninstall a windows service and delete its files without rebooting

If in .net ( I'm not sure if it works for all windows services)

  • Stop the service (THis may be why you're having a problem.)
  • InstallUtil -u [name of executable]
  • Installutil -i [name of executable]
  • Start the service again...

Unless I'm changing the service's public interface, I often deploy upgraded versions of my services without even unistalling/reinstalling... ALl I do is stop the service, replace the files and restart the service again...

Detect Windows version in .net

Like R. Bemrose suggested, if you are doing Windows 7 specific features, you should look at the Windows® API Code Pack for Microsoft® .NET Framework.

It contains a CoreHelpers class that let you determine the OS you are currently on (XP and above only, its a requirement for .NET nowaday)

It also provide multiple helper methods. For example, suppose that you want to use the jump list of Windows 7, there is a class TaskbarManager that provide a property called IsPlatformSupported and it will return true if you are on Windows 7 and above.

What are file descriptors, explained in simple terms?

Addition to above all simplified responses.

If you are working with files in bash script, it's better to use file descriptor.

For example: If you want to read and write from/to the file "test.txt", use the file descriptor as show below:

FILE=$1 # give the name of file in the command line
exec 5<>$FILE # '5' here act as the file descriptor

# Reading from the file line by line using file descriptor
while read LINE; do
    echo "$LINE"
done <&5

# Writing to the file using descriptor
echo "Adding the date: `date`" >&5 
exec 5<&- # Closing a file descriptor

How do I check OS with a preprocessor directive?

Based on nadeausoftware and Lambda Fairy's answer.

#include <stdio.h>

 * Determination a platform of an operation system
 * Fully supported supported only GNU GCC/G++, partially on Clang/LLVM

#if defined(_WIN32)
    #define PLATFORM_NAME "windows" // Windows
#elif defined(_WIN64)
    #define PLATFORM_NAME "windows" // Windows
#elif defined(__CYGWIN__) && !defined(_WIN32)
    #define PLATFORM_NAME "windows" // Windows (Cygwin POSIX under Microsoft Window)
#elif defined(__ANDROID__)
    #define PLATFORM_NAME "android" // Android (implies Linux, so it must come first)
#elif defined(__linux__)
    #define PLATFORM_NAME "linux" // Debian, Ubuntu, Gentoo, Fedora, openSUSE, RedHat, Centos and other
#elif defined(__unix__) || !defined(__APPLE__) && defined(__MACH__)
    #include <sys/param.h>
    #if defined(BSD)
        #define PLATFORM_NAME "bsd" // FreeBSD, NetBSD, OpenBSD, DragonFly BSD
#elif defined(__hpux)
    #define PLATFORM_NAME "hp-ux" // HP-UX
#elif defined(_AIX)
    #define PLATFORM_NAME "aix" // IBM AIX
#elif defined(__APPLE__) && defined(__MACH__) // Apple OSX and iOS (Darwin)
    #include <TargetConditionals.h>
        #define PLATFORM_NAME "ios" // Apple iOS
    #elif TARGET_OS_IPHONE == 1
        #define PLATFORM_NAME "ios" // Apple iOS
    #elif TARGET_OS_MAC == 1
        #define PLATFORM_NAME "osx" // Apple OSX
#elif defined(__sun) && defined(__SVR4)
    #define PLATFORM_NAME "solaris" // Oracle Solaris, Open Indiana

// Return a name of platform, if determined, otherwise - an empty string
const char *get_platform_name() {
    return (PLATFORM_NAME == NULL) ? "" : PLATFORM_NAME;

int main(int argc, char *argv[]) {
    return 0;

Tested with GCC and clang on:

  • Debian 8
  • Windows (MinGW)
  • Windows (Cygwin)

What is the difference between a process and a thread?

For those who are more comfortable with learning by visualizing, here is a handy diagram I created to explain Process and Threads.
I used the information from MSDN - About Processes and Threads

Processes and Threads

How can I safely create a nested directory?

If you consider the following:


means a directory (path) exists AND is a directory. So for me this way does what I need. So I can make sure it is folder (not a file) and exists.

What is the difference between user variables and system variables?

Environment variable (can access anywhere/ dynamic object) is a type of variable. They are of 2 types system environment variables and user environment variables.

System variables having a predefined type and structure. That are used for system function. Values that produced by the system are stored in the system variable. They generally indicated by using capital letters Example: HOME,PATH,USER

User environment variables are the variables that determined by the user,and are represented by using small letters.

How to detect my browser version and operating system using JavaScript?

Detecting browser's details:

var nVer = navigator.appVersion;
var nAgt = navigator.userAgent;
var browserName  = navigator.appName;
var fullVersion  = ''+parseFloat(navigator.appVersion); 
var majorVersion = parseInt(navigator.appVersion,10);
var nameOffset,verOffset,ix;

// In Opera, the true version is after "Opera" or after "Version"
if ((verOffset=nAgt.indexOf("Opera"))!=-1) {
 browserName = "Opera";
 fullVersion = nAgt.substring(verOffset+6);
 if ((verOffset=nAgt.indexOf("Version"))!=-1) 
   fullVersion = nAgt.substring(verOffset+8);
// In MSIE, the true version is after "MSIE" in userAgent
else if ((verOffset=nAgt.indexOf("MSIE"))!=-1) {
 browserName = "Microsoft Internet Explorer";
 fullVersion = nAgt.substring(verOffset+5);
// In Chrome, the true version is after "Chrome" 
else if ((verOffset=nAgt.indexOf("Chrome"))!=-1) {
 browserName = "Chrome";
 fullVersion = nAgt.substring(verOffset+7);
// In Safari, the true version is after "Safari" or after "Version" 
else if ((verOffset=nAgt.indexOf("Safari"))!=-1) {
 browserName = "Safari";
 fullVersion = nAgt.substring(verOffset+7);
 if ((verOffset=nAgt.indexOf("Version"))!=-1) 
   fullVersion = nAgt.substring(verOffset+8);
// In Firefox, the true version is after "Firefox" 
else if ((verOffset=nAgt.indexOf("Firefox"))!=-1) {
 browserName = "Firefox";
 fullVersion = nAgt.substring(verOffset+8);
// In most other browsers, "name/version" is at the end of userAgent 
else if ( (nameOffset=nAgt.lastIndexOf(' ')+1) < 
          (verOffset=nAgt.lastIndexOf('/')) ) 
 browserName = nAgt.substring(nameOffset,verOffset);
 fullVersion = nAgt.substring(verOffset+1);
 if (browserName.toLowerCase()==browserName.toUpperCase()) {
  browserName = navigator.appName;
// trim the fullVersion string at semicolon/space if present
if ((ix=fullVersion.indexOf(";"))!=-1)
if ((ix=fullVersion.indexOf(" "))!=-1)

majorVersion = parseInt(''+fullVersion,10);
if (isNaN(majorVersion)) {
 fullVersion  = ''+parseFloat(navigator.appVersion); 
 majorVersion = parseInt(navigator.appVersion,10);

 +'Browser name  = '+browserName+'<br>'
 +'Full version  = '+fullVersion+'<br>'
 +'Major version = '+majorVersion+'<br>'
 +'navigator.appName = '+navigator.appName+'<br>'
 +'navigator.userAgent = '+navigator.userAgent+'<br>'

Source JavaScript: browser name.
See JSFiddle to detect Browser Details.

Detecting OS:

// This script sets OSName variable as follows:
// "Windows"    for all versions of Windows
// "MacOS"      for all versions of Macintosh OS
// "Linux"      for all versions of Linux
// "UNIX"       for all other UNIX flavors 
// "Unknown OS" indicates failure to detect the OS

var OSName="Unknown OS";
if (navigator.appVersion.indexOf("Win")!=-1) OSName="Windows";
if (navigator.appVersion.indexOf("Mac")!=-1) OSName="MacOS";
if (navigator.appVersion.indexOf("X11")!=-1) OSName="UNIX";
if (navigator.appVersion.indexOf("Linux")!=-1) OSName="Linux";

document.write('Your OS: '+OSName);

source JavaScript: OS detection.
See JSFiddle to detect OS Details.

    var nVer = navigator.appVersion;_x000D_
    var nAgt = navigator.userAgent;_x000D_
    var browserName  = navigator.appName;_x000D_
    var fullVersion  = ''+parseFloat(navigator.appVersion); _x000D_
    var majorVersion = parseInt(navigator.appVersion,10);_x000D_
    var nameOffset,verOffset,ix;_x000D_
    // In Opera, the true version is after "Opera" or after "Version"_x000D_
    if ((verOffset=nAgt.indexOf("Opera"))!=-1) {_x000D_
     browserName = "Opera";_x000D_
     fullVersion = nAgt.substring(verOffset+6);_x000D_
     if ((verOffset=nAgt.indexOf("Version"))!=-1) _x000D_
       fullVersion = nAgt.substring(verOffset+8);_x000D_
    // In MSIE, the true version is after "MSIE" in userAgent_x000D_
    else if ((verOffset=nAgt.indexOf("MSIE"))!=-1) {_x000D_
     browserName = "Microsoft Internet Explorer";_x000D_
     fullVersion = nAgt.substring(verOffset+5);_x000D_
    // In Chrome, the true version is after "Chrome" _x000D_
    else if ((verOffset=nAgt.indexOf("Chrome"))!=-1) {_x000D_
     browserName = "Chrome";_x000D_
     fullVersion = nAgt.substring(verOffset+7);_x000D_
    // In Safari, the true version is after "Safari" or after "Version" _x000D_
    else if ((verOffset=nAgt.indexOf("Safari"))!=-1) {_x000D_
     browserName = "Safari";_x000D_
     fullVersion = nAgt.substring(verOffset+7);_x000D_
     if ((verOffset=nAgt.indexOf("Version"))!=-1) _x000D_
       fullVersion = nAgt.substring(verOffset+8);_x000D_
    // In Firefox, the true version is after "Firefox" _x000D_
    else if ((verOffset=nAgt.indexOf("Firefox"))!=-1) {_x000D_
     browserName = "Firefox";_x000D_
     fullVersion = nAgt.substring(verOffset+8);_x000D_
    // In most other browsers, "name/version" is at the end of userAgent _x000D_
    else if ( (nameOffset=nAgt.lastIndexOf(' ')+1) < _x000D_
              (verOffset=nAgt.lastIndexOf('/')) ) _x000D_
     browserName = nAgt.substring(nameOffset,verOffset);_x000D_
     fullVersion = nAgt.substring(verOffset+1);_x000D_
     if (browserName.toLowerCase()==browserName.toUpperCase()) {_x000D_
      browserName = navigator.appName;_x000D_
    // trim the fullVersion string at semicolon/space if present_x000D_
    if ((ix=fullVersion.indexOf(";"))!=-1)_x000D_
    if ((ix=fullVersion.indexOf(" "))!=-1)_x000D_
    majorVersion = parseInt(''+fullVersion,10);_x000D_
    if (isNaN(majorVersion)) {_x000D_
     fullVersion  = ''+parseFloat(navigator.appVersion); _x000D_
     majorVersion = parseInt(navigator.appVersion,10);_x000D_
     +'Browser name  = '+browserName+'<br>'_x000D_
     +'Full version  = '+fullVersion+'<br>'_x000D_
     +'Major version = '+majorVersion+'<br>'_x000D_
     +'navigator.appName = '+navigator.appName+'<br>'_x000D_
     +'navigator.userAgent = '+navigator.userAgent+'<br>'_x000D_
    // This script sets OSName variable as follows:_x000D_
    // "Windows"    for all versions of Windows_x000D_
    // "MacOS"      for all versions of Macintosh OS_x000D_
    // "Linux"      for all versions of Linux_x000D_
    // "UNIX"       for all other UNIX flavors _x000D_
    // "Unknown OS" indicates failure to detect the OS_x000D_
    var OSName="Unknown OS";_x000D_
    if (navigator.appVersion.indexOf("Win")!=-1) OSName="Windows";_x000D_
    if (navigator.appVersion.indexOf("Mac")!=-1) OSName="MacOS";_x000D_
    if (navigator.appVersion.indexOf("X11")!=-1) OSName="UNIX";_x000D_
    if (navigator.appVersion.indexOf("Linux")!=-1) OSName="Linux";_x000D_
    document.write('Your OS: '+OSName);

Fork() function in C

int a = fork(); 

Creates a duplicate process "clone?", which shares the execution stack. The difference between the parent and the child is the return value of the function.

The child getting 0 returned, and the parent getting the new pid.

Each time the addresses and the values of the stack variables are copied. The execution continues at the point it already got to in the code.

At each fork, only one value is modified - the return value from fork.

What is the difference between the kernel space and the user space?

In Linux there are two space 1st is user space and another one is kernal space. user space consist of only user application which u want to run. as the kernal service there is process management, file management, signal handling, memory management, thread management, and so many services are present there. if u run the application from the user space that appliction interact with only kernal service. and that service is interact with device driver which is present between hardware and kernal. the main benefit of kernal space and user space seperation is we can acchive a security by the virus.bcaz of all user application present in user space, and service is present in kernal space. thats why linux doesn,t affect from the virus.

What resources are shared between threads?

Tell the interviewer that it depends entirely on the implementation of the OS.

Take Windows x86 for example. There are only 2 segments [1], Code and Data. And they're both mapped to the whole 2GB (linear, user) address space. Base=0, Limit=2GB. They would've made one but x86 doesn't allow a segment to be both Read/Write and Execute. So they made two, and set CS to point to the code descriptor, and the rest (DS, ES, SS, etc) to point to the other [2]. But both point to the same stuff!

The person interviewing you had made a hidden assumption that he/she did not state, and that is a stupid trick to pull.

So regarding

Q. So tell me which segment thread share?

The segments are irrelevant to the question, at least on Windows. Threads share the whole address space. There is only 1 stack segment, SS, and it points to the exact same stuff that DS, ES, and CS do [2]. I.e. the whole bloody user space. 0-2GB. Of course, that doesn't mean threads only have 1 stack. Naturally each has its own stack, but x86 segments are not used for this purpose.

Maybe *nix does something different. Who knows. The premise the question was based on was broken.

  1. At least for user space.
  2. From ntsd notepad: cs=001b ss=0023 ds=0023 es=0023

What languages are Windows, Mac OS X and Linux written in?

  • windows: C++
  • linux: C
  • mac: Objective C
  • android: JAVA, C, C++
  • Solaris: C, C++
  • iOS 7: Objective-C,Swift,C, C++

What is the difference between the operating system and the kernel?

The difference between an operating system and a kernel:

The kernel is a part of an operating system. The operating system is the software package that communicates directly to the hardware and our application. The kernel is the lowest level of the operating system. The kernel is the main part of the operating system and is responsible for translating the command into something that can be understood by the computer. The main functions of the kernel are:

  1. memory management
  2. network management
  3. device driver
  4. file management
  5. process management

Calculating Waiting Time and Turnaround Time in (non-preemptive) FCFS queue

wt = tt - cpu tm.
Tt = cpu tm + wt.

Where wt is a waiting time and tt is turnaround time. Cpu time is also called burst time.

What is private bytes, virtual bytes, working set?

You should not try to use perfmon, task manager or any tool like that to determine memory leaks. They are good for identifying trends, but not much else. The numbers they report in absolute terms are too vague and aggregated to be useful for a specific task such as memory leak detection.

A previous reply to this question has given a great explanation of what the various types are.

You ask about a tool recommendation: I recommend Memory Validator. Capable of monitoring applications that make billions of memory allocations.

Disclaimer: I designed Memory Validator.

Running windows shell commands with python

You can use the subprocess package with the code as below:

import subprocess
cmdCommand = "python"   #specify your cmd command
process = subprocess.Popen(cmdCommand.split(), stdout=subprocess.PIPE)
output, error = process.communicate()
print output

What is an OS kernel ? How does it differ from an operating system?

A kernel is the part of the operating system that mediates access to system resources. It's responsible for enabling multiple applications to effectively share the hardware by controlling access to CPU, memory, disk I/O, and networking.

An operating system is the kernel plus applications that enable users to get something done (i.e compiler, text editor, window manager, etc).

How is TeamViewer so fast?

would take time to route through TeamViewer's servers (TeamViewer bypasses corporate Symmetric NATs by simply proxying traffic through their servers)

You'll find that TeamViewer rarely needs to relay traffic through their own servers. TeamViewer penetrates NAT and networks complicated by NAT using NAT traversal (I think it is UDP hole-punching, like Google's libjingle).

They do use their own servers to middle-man in order to do the handshake and connection set-up, but most of the time the relationship between client and server will be P2P (best case, when the hand-shake is successful). If NAT traversal fails, then TeamViewer will indeed relay traffic through its own servers.

I've only ever seen it do this when a client has been behind double-NAT, though.

IE 8: background-size fix

I created jquery.backgroundSize.js: a 1.5K jquery plugin that can be used as a IE8 fallback for "cover" and "contain" values. Have a look at the demo.

Solving your problem could be as simple as:

$("h2#news").css({backgroundSize: "cover"});

How to create a notification with NotificationCompat.Builder?

You can try this code this works fine for me:

    NotificationCompat.Builder mBuilder= new NotificationCompat.Builder(this);

    Intent i = new Intent(noti.this, Xyz_activtiy.class);
    PendingIntent pendingIntent= PendingIntent.getActivity(this,0,i,0);

    mBuilder.setContentTitle("New notification title");
    mBuilder.setContentText("Notification text");

    NotificationManager notificationManager= (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);

Reading an Excel file in python using pandas

This is much simple and easy way.

import pandas
df = pandas.read_excel(open('your_xls_xlsx_filename','rb'), sheetname='Sheet 1')
# or using sheet index starting 0
df = pandas.read_excel(open('your_xls_xlsx_filename','rb'), sheetname=2)

check out documentation full details

FutureWarning: The sheetname keyword is deprecated for newer Pandas versions, use sheet_name instead.

Spring MVC: How to return image in @ResponseBody?

By using Spring 3.1.x and 3.2.x, this is how you should do it:

The controller method:

public @ResponseBody byte[] testphoto() throws IOException {
    InputStream in = servletContext.getResourceAsStream("/images/no_image.jpg");
    return IOUtils.toByteArray(in);

And the mvc annotation in servlet-context.xml file:

        <bean class="org.springframework.http.converter.ByteArrayHttpMessageConverter">
            <property name="supportedMediaTypes">

Get multiple elements by Id

You can't have duplicate ids. Ids are supposed to be unique. You might want to use a specialized class instead.

Spring @Value is not resolving to value from property file

Have a read of pedjaradenkovic's comment.

Further to the link he provides, the reason this isn't working is that @Value processing requires a PropertySourcesPlaceholderConfigurer instead of a PropertyPlaceholderConfigurer.

How can I write an anonymous function in Java?

With the introduction of lambda expression in Java 8 you can now have anonymous methods.

Say I have a class Alpha and I want to filter Alphas on a specific condition. To do this you can use a Predicate<Alpha>. This is a functional interface which has a method test that accepts an Alpha and returns a boolean.

Assuming that the filter method has this signature:

List<Alpha> filter(Predicate<Alpha> filterPredicate)

With the old anonymous class solution you would need to something like:

filter(new Predicate<Alpha>() {
   boolean test(Alpha alpha) {
      return alpha.centauri > 1;

With the Java 8 lambdas you can do:

filter(alpha -> alpha.centauri > 1);

For more detailed information see the Lambda Expressions tutorial

How do I get values from a SQL database into textboxes using C#?

The line reader.Read() is missing in your code. You should add it. It is the function which actually reads data from the database:

string conString = "Data Source=localhost;Initial Catalog=LoginScreen;Integrated Security=True";
SqlConnection con = new SqlConnection(conString);

string selectSql = "select * from Pending_Tasks";
SqlCommand com = new SqlCommand(selectSql, con);


    using (SqlDataReader read = cmd.ExecuteReader())
            CustID.Text = (read["Customer_ID"].ToString());
            CustName.Text = (read["Customer_Name"].ToString());
            Add1.Text = (read["Address_1"].ToString());
            Add2.Text = (read["Address_2"].ToString());
            PostBox.Text = (read["Postcode"].ToString());
            PassBox.Text = (read["Password"].ToString());
            DatBox.Text = (read["Data_Important"].ToString());
            LanNumb.Text = (read["Landline"].ToString());
            MobNumber.Text = (read["Mobile"].ToString());
            FaultRep.Text = (read["Fault_Report"].ToString());

EDIT : This code works supposing you want to write the last record to your textboxes. If you want to apply a different scenario, like for example to read all the records from database and to change data in the texboxes when you click the Next button, you should create and use your own Model, or you can store data in the DataTable and refer to them later if you wish.

How to truncate text in Angular2?

I've been using this module ng2 truncate, its pretty easy, import module and u are ready to go... in {{ data.title | truncate : 20 }}

Show data on mouseover of circle

There is an awesome library for doing that that I recently discovered. It's simple to use and the result is quite neat: d3-tip.

You can see an example here:

enter image description here

Basically, all you have to do is to download(index.js), include the script:

<script src="index.js"></script>

and then follow the instructions from here (same link as example)

But for your code, it would be something like:

define the method:

var tip = d3.tip()
  .attr('class', 'd3-tip')
  .offset([-10, 0])
  .html(function(d) {
    return "<strong>Frequency:</strong> <span style='color:red'>" + d.frequency + "</span>";

create your svg (as you already do)

var svg = ...

call the method:;

add tip to your object:

   .on('mouseout', tip.hide)

Don't forget to add the CSS:

.d3-tip {
  line-height: 1;
  font-weight: bold;
  padding: 12px;
  background: rgba(0, 0, 0, 0.8);
  color: #fff;
  border-radius: 2px;

/* Creates a small triangle extender for the tooltip */
.d3-tip:after {
  box-sizing: border-box;
  display: inline;
  font-size: 10px;
  width: 100%;
  line-height: 1;
  color: rgba(0, 0, 0, 0.8);
  content: "\25BC";
  position: absolute;
  text-align: center;

/* Style northward tooltips differently */
.d3-tip.n:after {
  margin: -1px 0 0 0;
  top: 100%;
  left: 0;

SQL like search string starts with

You need to use the wildcard % :

SELECT * from games WHERE (lower(title) LIKE 'age of empires III%');

Get month name from date in Oracle

If you are trying to pull the value from a field, you could use:

select extract(month from [field_name])
from [table_name]

You can also insert day or year for the "month" extraction value above.

row-level trigger vs statement-level trigger

The main difference is not what can be modified by the trigger, that depends on the DBMS. A trigger (row or statement level) may modify one or many rows*, of the same or other tables as well and may have cascading effects (trigger other actions/triggers) but all these depend on the DBMS of course.

The main difference is how many times the trigger is activated. Imagine you have a 1M rows table and you run:

SET columnX = columnX + 1

A statement-level trigger will be activated once (and even if no rows are updated). A row-level trigger will be activated a million times, once for every updated row.

Another difference is the order or activation. For example in Oracle the 4 different types of triggers will be activated in the following order:

Before the triggering statement executes
Before each row that the triggering statement affects
After each row that the triggering statement affects
After the triggering statement executes

In the previous example, we'd have something like:

Before statement-level trigger executes

  Before row-level trigger executes
  One row is updated
  After row-level trigger executes

  Before row-level trigger executes
  Second row is updated
  After row-level trigger executes


  Before row-level trigger executes
  Millionth row is updated
  After row-level trigger executes

After statement-level trigger executes


* Regarding what rows can be modified by a trigger: Different DBMS have different limitations on this, depending on the specific implementation or triggers in the DBMS. For example, Oracle may show a "mutating table" errors for some cases, e.g. when a row-level trigger selects from the whole table (SELECT MAX(col) FROM tablename) or if it modifies other rows or the whole table and not only the row that is related to / triggered from.

It is perfectly valid of course for a row-level trigger (in Oracle or other) to modify the row that its change has triggered it and that is a very common use. Example in

Other DBMS may have different limitations on what any type of trigger can do and even what type of triggers are offered (some do not have BEFORE triggers for example, some do not have statement level triggers at all, etc).

Using GitLab token to clone without authentication

If you already has a repository and just changed the way you do authentication to MFA, u can change your remote origin HTTP URI to use your new api token as follows:

git remote set-url origin https://oauth2:TOKEN@ANY_GIT_PROVIDER_DOMAIN/YOUR_PROJECT/YOUR_REPO.git

And you wont need to re-clone the repository at all.

How to generate an entity-relationship (ER) diagram using Oracle SQL Developer

Oracle used to have a component in SQL Developer called Data Modeler. It no longer exists in the product since at least

It's now a separate download that you can find here:

Android appcompat v7:23

Ran into a similar issue using React Native

> Could not find

the Support Libraries are Local Maven repository for Support Libraries

enter image description here

Android: Quit application when press back button

nobody seems to have recommended noHistory="true" in manifest.xml to prevent certain activity to appear after you press back button which by default calling method finish()

In Spring MVC, how can I set the mime type header when using @ResponseBody

Register org.springframework.http.converter.json.MappingJacksonHttpMessageConverter as the message converter and return the object directly from the method.

<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
  <property name="webBindingInitializer">
    <bean class=""/>
  <property name="messageConverters">
      <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter"/>

and the controller:

@RequestMapping(method=RequestMethod.GET, value="foo/bar")
public @ResponseBody Object fooBar(){
    return myService.getActualObject();

This requires the dependency org.springframework:spring-webmvc.

Empty an array in Java / processing


Arrays.fill(myArray, null);

Not that it does anything different than you'd do on your own (it just loops through every element and sets it to null). It's not native in that it's pure Java code that performs this, but it is a library function if maybe that's what you meant.

This of course doesn't allow you to resize the array (to zero), if that's what you meant by "empty". Array sizes are fixed, so if you want the "new" array to have different dimensions you're best to just reassign the reference to a new array as the other answers demonstrate. Better yet, use a List type like an ArrayList which can have variable size.

How to set column header text for specific column in Datagridview C#

For info, if you are binding to a class, you can do this in your type via DisplayNameAttribute:

[DisplayName("Access key")]
public string AccessKey { get {...} set {...} }

Now the header-text on auto-generated columns will be "Access key".

Keystore type: which one to use?

Here is a post which introduces different types of keystore in Java and the differences among different types of keystore.

Below are the descriptions of different keystores from the post:

JKS, Java Key Store. You can find this file at This keystore is Java specific, it usually has an extension of jks. This type of keystore can contain private keys and certificates, but it cannot be used to store secret keys. Since it's a Java specific keystore, so it cannot be used in other programming languages.

JCEKS, JCE key store. You can find this file at com.sun.crypto.provider.JceKeyStore. This keystore has an extension of jceks. The entries which can be put in the JCEKS keystore are private keys, secret keys and certificates.

PKCS12, this is a standard keystore type which can be used in Java and other languages. You can find this keystore implementation at It usually has an extension of p12 or pfx. You can store private keys, secret keys and certificates on this type.

PKCS11, this is a hardware keystore type. It servers an interface for the Java library to connect with hardware keystore devices such as Luna, nCipher. You can find this implementation at When you load the keystore, you no need to create a specific provider with specific configuration. This keystore can store private keys, secret keys and cetrificates. When loading the keystore, the entries will be retrieved from the keystore and then converted into software entries.

How do I prevent a parent's onclick event from firing when a child anchor is clicked?

Events bubble to the highest point in the DOM at which a click event has been attached. So in your example, even if you didn't have any other explicitly clickable elements in the div, every child element of the div would bubble their click event up the DOM to until the DIV's click event handler catches it.

There are two solutions to this is to check to see who actually originated the event. jQuery passes an eventargs object along with the event:

$("#clickable").click(function(e) {
    var senderElement =;
    // Check if sender is the <div> element e.g.
    // if($("div")) {
    window.location = url;
    return true;

You can also attach a click event handler to your links which tell them to stop event bubbling after their own handler executes:

$("#clickable a").click(function(e) {
   // Do something

PHP Warning: Module already loaded in Unknown on line 0

Just for the record as it might help others who are on shared hosting (cPanel).

I had error on shared hosting php7.2: Module 'imagick' already loaded in Unknown on line 0

In the beginning hosting provider said it was my bad configuration (running Yii2.16). But after I showed them, that all Internet related this issue to server configuration -they started to listen to me. After I proved, that there was no error on php7.1 they started to search for the error.

As they told to me - the error was due to configuration in PERL modules or PEAR package, but they did not told me the real issue.

So, if you are on shared hosting - talk to you provider and experiment with PHP versions (if you can change them).

How to uninstall with msiexec using product id guid without .msi file present

Thanks all for the help - turns out it was a WiX issue.

When the Product ID GUID was left explicit & hardcoded as in the question, the resulting .msi had no ProductCode property but a Product ID property instead when inspected with orca.

Once I changed the GUID to '*' to auto-generate, the ProductCode showed up and all works fine with syntax confirmed by the other answers.

How to add ASP.NET 4.0 as Application Pool on IIS 7, Windows 7

I just encountered this and whilst we already had .NET 4.0 installed on the server it turns out we only had the "Client Profile" version and not the "Full" version. Installing the latter fixed the problem.

Change bullets color of an HTML list without using span

I tried this today and typed this:
I needed to display color markers in my lists (both bullets and numbers). I came upon this tip and wrote in in my stylesheet whith mutualization of the properties:

ol {
  list-style: none;
  padding: 0;
  margin: 0 0 0 15px;
ul {}
ol {
  counter-reset: li;
li {
  padding-left: 1em; 
ul li {}
ul li::before,
ol li::before {
  color: #91be3c;
  display: inline-block;
  width: 1em;
ul li::before {
  content: "\25CF";
  margin: 0 0.1em 0 -1.1em;
ol li {
  counter-increment: li;
ol li::before {
  content: counter(li);
  margin: 0 0 0 -1em;

I chose a different character to display a bullet, watching it here. I needed to adjust the margin accoardingly, maybe the values won't apply with the font you chose (the numbers use your webfont).

How to split a string into an array of characters in Python?

To split a string s, the easiest way is to pass it to list(). So,

s = 'abc'
s_l = list(s) #  s_l is now ['a', 'b', 'c']

You can also use a list comprehension, which works but is not as concise as the above:

s_l = [c for c in s]

There are other ways, as well, but these should suffice. Later, if you want to recombine them, a simple call to "".join(s_l) will return your list to all its former glory as a string...

Changing MongoDB data store directory

Copy the contents of /var/lib/mongodb to /data/db. The files you should be looking for should have names like your_db_name.ns and your_dbname.n where n is a number starting with 0. If you do not see such files under /var/lib/mongodb, search for them on your filesystem.

Once copied over, use --dbpath=/data/db when starting MongoDB via the mongod command.

How to call a stored procedure from Java and JPA

For me, only the following worked with Oracle 11g and Glassfish 2.1 (Toplink):

Query query = entityManager.createNativeQuery("BEGIN PROCEDURE_NAME(); END;");

The variant with curly braces resulted in ORA-00900.

How can I export a GridView.DataSource to a datatable or dataset?

I have used below line of code and it works, Try this

DataTable dt =  dataSource.Tables[0];

Algorithm to find Largest prime factor of a number

Actually there are several more efficient ways to find factors of big numbers (for smaller ones trial division works reasonably well).

One method which is very fast if the input number has two factors very close to its square root is known as Fermat factorisation. It makes use of the identity N = (a + b)(a - b) = a^2 - b^2 and is easy to understand and implement. Unfortunately it's not very fast in general.

The best known method for factoring numbers up to 100 digits long is the Quadratic sieve. As a bonus, part of the algorithm is easily done with parallel processing.

Yet another algorithm I've heard of is Pollard's Rho algorithm. It's not as efficient as the Quadratic Sieve in general but seems to be easier to implement.

Once you've decided on how to split a number into two factors, here is the fastest algorithm I can think of to find the largest prime factor of a number:

Create a priority queue which initially stores the number itself. Each iteration, you remove the highest number from the queue, and attempt to split it into two factors (not allowing 1 to be one of those factors, of course). If this step fails, the number is prime and you have your answer! Otherwise you add the two factors into the queue and repeat.

How to clear all input fields in a specific div with jQuery?

inspired from but I use the selector instead of a class and prefer if over switch:

function clearAllInputs(selector) {
  $(selector).find(':input').each(function() {
    if(this.type == 'submit'){
          //do nothing
      else if(this.type == 'checkbox' || this.type == 'radio') {
        this.checked = false;
      else if(this.type == 'file'){
        var control = $(this);
        control.replaceWith( control = control.clone( true ) );

this should take care of almost all input inside any selector.

How to check if array element exists or not in javascript?

This way is easiest one in my opinion.

var nameList = new Array('item1','item2','item3','item4');

// Using for loop to loop through each item to check if item exist.

for (var i = 0; i < nameList.length; i++) {
if (nameList[i] === 'item1') 
   alert('Value exist');
   alert('Value doesn\'t exist');

And Maybe Another way to do it is.

   if( == 'item1')
          alert('Item Exist');

How to install Java SDK on CentOS?

@Sventeck, perfecto.

redhat docs are always a great source - good tutorial that explains how to install JDK via yum and then setting the path can be found here (have fun!) - Install OpenJDK and set $JAVA_HOME path

OpenJDK 6:

yum install java-1.6.0-openjdk-devel

OpenJDK 7:

yum install java-1.7.0-openjdk-devel

To list all available java openjdk-devel packages try:

yum list "java-*-openjdk-devel"

Datatype for storing ip address in SQL Server

The following answer is based on answers by M. Turnhout and Jerry Birchler to this question but with the following improvements:

  • Replaced the use of undocumented functions (sys.fn_varbintohexsubstring, fn_varbintohexstr) with CONVERT() for binary styles
  • Replaced XML "hacks" ( CAST('' as xml).value('xs:hexBinary()) ) with CONVERT() for binary styles
  • Fixed bug in Jerry Birchler's implementation of fn_ConvertIpAddressToBinary (as pointed out by C.Plock)
  • Add minor syntax sugar

The code has been tested in SQL Server 2014 and SQL Server 2016 (see test cases at the end)


Converts 4 bytes values to IPV4 and 16 byte values to IPV6 string representations. Note that this function does not shorten addresses.

ALTER FUNCTION dbo.IPAddressVarbinaryToString
    @varbinaryValue VARBINARY( 16 )
    IF @varbinaryValue IS NULL
        RETURN NULL;
    ELSE IF DATALENGTH( @varbinaryValue ) = 4
            CONVERT( VARCHAR(3), CONVERT(TINYINT, SUBSTRING( @varbinaryValue, 1, 1 ))) + '.' +
            CONVERT( VARCHAR(3), CONVERT(TINYINT, SUBSTRING( @varbinaryValue, 2, 1 ))) + '.' +
            CONVERT( VARCHAR(3), CONVERT(TINYINT, SUBSTRING( @varbinaryValue, 3, 1 ))) + '.' +
            CONVERT( VARCHAR(3), CONVERT(TINYINT, SUBSTRING( @varbinaryValue, 4, 1 )));
    ELSE IF DATALENGTH( @varbinaryValue ) = 16
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue,  1, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue,  3, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue,  5, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue,  7, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue,  9, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue, 11, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue, 13, 2 ), 2 ) + ':' +
            CONVERT( VARCHAR(4), SUBSTRING( @varbinaryValue, 15, 2 ), 2 );

    RETURN 'Invalid';

Test Cases:

SELECT dbo.IPAddressVarbinaryToString(0x00000000000000000000000000000000) -- 0000:0000:0000:0000:0000:0000:0000:0000 (no address shortening)
SELECT dbo.IPAddressVarbinaryToString(0x00010002000300400500060070000089) -- 0001:0002:0003:0040:0500:0600:7000:0089
SELECT dbo.IPAddressVarbinaryToString(0xC0A80148) --
SELECT dbo.IPAddressVarbinaryToString(0x7F000001) -- (no address shortening)
SELECT dbo.IPAddressVarbinaryToString(NULL) -- NULL


Converts IPV4 and IPV6 string representations to 4 byte and 16 bytes binary values respectively. Note that this function is able to parse most (all of the commonly used) of shorthand address representations (e.g. 127...1 and 2001:db8::1319:370:7348). To force this function to always return 16 byte binary values uncomment leading 0s concatenation at the end of the function.

ALTER FUNCTION [dbo].[IPAddressStringToVarbinary]
    @IPAddress VARCHAR( 39 )

IF @ipAddress IS NULL

DECLARE @bytes VARBINARY(16), @token VARCHAR(4),
    @vbytes VARBINARY(16) = 0x, @vbzone VARBINARY(2),
    @tIPAddress VARCHAR( 40 ),
    @colIndex TINYINT,
    @delim CHAR(1) = '.',
    @prevColIndex TINYINT = 0,
    @parts TINYINT = 0, @limit TINYINT = 4;

-- Get position if IPV4 delimiter
SET @colIndex = CHARINDEX( @delim, @ipAddress );

-- If not IPV4, then assume IPV6
IF @colIndex = 0
    SELECT @delim = ':', @limit = 8, @colIndex = CHARINDEX( @delim, @ipAddress );

    -- Get number of parts (delimiters)
    WHILE @colIndex > 0
        SELECT @parts += 1, @colIndex = CHARINDEX( @delim, @ipAddress, @colIndex + 1 );

    SET @colIndex = CHARINDEX( @delim, @ipAddress );

    IF @colIndex = 0
        RETURN NULL;

-- Add trailing delimiter (need new variable of larger size)
SET @tIPAddress = @IPAddress + @delim;

WHILE @colIndex > 0
    SET @token = SUBSTRING( @tIPAddress, @prevColIndex + 1, @Colindex - @prevColIndex - 1 );

    IF @delim = ':'
        SELECT @vbzone = CONVERT( VARBINARY(2), RIGHT( '0000' + @token, 4 ), 2 ), @vbytes += @vbzone;

        -- Handles consecutive sections of zeros representation rule (i.e. ::)(
        IF @token = ''
            WHILE @parts + 1 < @limit
                SELECT @vbytes += @vbzone, @parts += 1;
        SELECT @vbzone = CONVERT( VARBINARY(1), CONVERT( TINYINT, @token )), @vbytes += @vbzone

    SELECT @prevColIndex = @colIndex, @colIndex = CHARINDEX( @delim, @tIPAddress, @colIndex + 1 ) 

SET @bytes =
    CASE @delim
        WHEN ':' THEN @vbytes
        ELSE /*0x000000000000000000000000 +*/ @vbytes -- Return IPV4 addresses as 4 byte binary (uncomment leading 0s section to force 16 byte binary)

RETURN @bytes


Test Cases

Valid cases

SELECT dbo.IPAddressStringToVarbinary( '0000:0000:0000:0000:0000:0000:0000:0001' ) -- 0x0000000000000000000000000001 (check bug fix)
SELECT dbo.IPAddressStringToVarbinary( '0001:0002:0003:0040:0500:0600:7000:0089' ) -- 0x00010002000300400500060070000089
SELECT dbo.IPAddressStringToVarbinary( '2001:db8:85a3:8d3:1319::370:7348' )     -- 0x20010DB885A308D31319000003707348 (check short hand)
SELECT dbo.IPAddressStringToVarbinary( '2001:db8:85a3:8d3:1319:0000:370:7348' ) -- 0x20010DB885A308D31319000003707348
SELECT dbo.IPAddressStringToVarbinary( '' ) -- 0xC0A80148
SELECT dbo.IPAddressStringToVarbinary( '127...1' ) -- 0x7F000001 (check short hand)
SELECT dbo.IPAddressStringToVarbinary( NULL ) -- NULL
SELECT dbo.IPAddressStringToVarbinary( '' ) -- NULL
-- Check that conversions return original address
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '0001:0002:0003:0040:0500:0600:7000:0089' )) -- '0001:0002:0003:0040:0500:0600:7000:0089' 
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '127...1' )) --
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '' )) --
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '2001:db8:85a3:8d3:1319::370:7348' ))     -- 2001:0db8:85a3:08d3:1319:0000:0370:7348
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '2001:db8:85a3:8d3:1314:0000:370:7348' )) -- 2001:0db8:85a3:08d3:1319:0000:0370:7348
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '2001:db8:85a3:8d3::370:7348' )) -- 2001:0DB8:85A3:08D3:0000:0000:0370:7348
-- This is technically an invalid IPV6 (according to Wikipedia) but it parses correctly
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '2001:db8::1319::370:7348' )) -- 2001:0DB8:0000:0000:1319:0000:0370:7348

Invalid cases

SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '2001:db8::1319::7348' )) -- 2001:0DB8:0000:0000:0000:1319:0000:7348 (ambiguous address)
SELECT dbo.IPAddressStringToVarbinary( '127.1' ) -- (not supported short-hand)
SELECT dbo.IPAddressVarbinaryToString( dbo.IPAddressStringToVarbinary( '127.1' )) -- (not supported short-hand)
SELECT dbo.IPAddressStringToVarbinary( '0300.0000.0002.0353' ) -- octal byte values
SELECT dbo.IPAddressStringToVarbinary( '0xC0.0x00.0x02.0xEB' ) -- hex values
SELECT dbo.IPAddressStringToVarbinary( 'C0.00.02.EB' ) -- hex values

How to determine whether a year is a leap year?

The whole formula can be contained in a single expression:

def is_leap_year(year):
    return (year % 4 == 0 and year % 100 != 0) or year % 400 == 0

print n, " is a leap year" if is_leap_year(n) else " is not a leap year"

Is there any way to do HTTP PUT in python

I've used a variety of python HTTP libs in the past, and I've settled on 'Requests' as my favourite. Existing libs had pretty useable interfaces, but code can end up being a few lines too long for simple operations. A basic PUT in requests looks like:

payload = {'username': 'bob', 'email': '[email protected]'}
>>> r = requests.put("", data=payload)

You can then check the response status code with:


or the response with:


Requests has a lot synactic sugar and shortcuts that'll make your life easier.

MySQL create stored procedure syntax with delimiter

Here is my code to create procedure in MySQL :

CREATE DEFINER=`root`@`localhost` PROCEDURE `procedureName`(IN comId int)
select * from tableName 
         (add joins OR sub query as per your requirement)
         Where (where condition here)
END $$

To call this procedure use this query :

call procedureName(); // without parameter
call procedureName(id,pid); // with parameter

Detail :

1) DEFINER : root is the user name and change it as per your username of mysql localhost is the host you can change it with ip address of the server if you are execute this query on hosting server.

Read here for more detail

Protect .NET code from reverse engineering?

.NET Reflector can only open "managed code" which basically means ".NET code". So you can't use it to disassemble COM DLL files, native C++, classic Visual Basic 6.0 code, etc. The structure of compiled .NET code makes it very convenient, portable, discoverable, verifiable, etc. .NET Reflector takes advantage of this to let you peer into compiled assemblies but decompilers and disassemblers are by no means specific to .NET and have been around as long as compilers have been around.

You can use obfuscators to make the code more difficult to read, but you can't exactly prevent it from being decompiled without also making it unreadable to .NET. There are a handful of products out there (usually expensive) that claim to "link" your managed code application into a native code application, but even if these actually work, a determined person will always find a way.

When it comes to obfuscation however, you get what you pay for. So if your code is so proprietary that you must go to such great lengths to protect it, you should be willing to invest money in a good obfuscator.

However, in my 15 or so years of experience writing code I've realized that being over-protective of your source code is a waste of time and has little benefit. Just trying to read original source code without supporting documentation, comments, etc. can be very difficult to understand. Add to that the senseless variable names that decompilers come up with and the spaghetti code that modern obfuscators create - you probably don't have to worry too much about people stealing your intellectual property.

How to set default vim colorscheme

OS: Redhat enterprise edition

colo schema_name works fine if you are facing problems with colorscheme.

Removing empty lines in Notepad++

1) Ctrl + H ( Or Search Replace..) to open Replace window.

2) Select 'Search Mode' 'Regular expression'

3) In 'Find What' type ^(\s*)(.*)(\s*)$ & in 'Replace With' type \2

  • ^ - Matches start of line character
  • (\s*) - Matches empty space characters
  • (.*) - Matches any characters
  • (\s*) - Matches empty spaces characters
  • $ - Matches end of line character
  • \2 - Denotes the matching contend of the 2nd bracket

enter image description here Refer for more on regex.

Vector of Vectors to create matrix

As it is, both dimensions of your vector are 0.

Instead, initialize the vector as this:

vector<vector<int> > matrix(RR);
for ( int i = 0 ; i < RR ; i++ )

This will give you a matrix of dimensions RR * CC with all elements set to 0.

psql: FATAL: role "postgres" does not exist

This is the only one that fixed it for me :

createuser -s -U $USER

How to find the minimum value of a column in R?

If you prefer using column names, you could do something like this as an alternative:


Command-line Git on Windows

I had the same issue and resolved it by adding the /bin directory location to the PATH Environment Variable.

  1. Search for the file location where Git was installed, mine is C:\Users\(My UserName)\AppData\Local\GitHub. It may also be C:\Program Files (x86)\Git

  2. Once you have the location of Git you should see a /bin sub-folder. It may be in a PortableGit folder (mine is PortableGit_015aa71ef18c047ce8509ffb2f9e4bb0e3e73f13). Copy this path.

  3. Go to Control Panel > System > System Protection > Advanced > Environment Variables

  4. Choose PATH, click edit and paste the bin path there. If there are already any values in your PATH paste your Git path at the end separated with a semi-colon.

Now you can access Git command from CMD.

Converting Array to List

If you are dealing with String[] instead of int[], We can use

ArrayList<String> list = new ArrayList<>();

How to get just the date part of getdate()?

Try this:


Bug? #1146 - Table 'xxx.xxxxx' doesn't exist

As pprakash mentions above, copying the table.frm files AND the ibdata1 file was what worked for me.

In short:

  1. Shut your DB explorer client (e.g. Workbench).
  2. Stop the MySQL service (Windows host).
  3. Make a safe copy of virtually everything!
  4. Save a copy of the table file(s) (eg mytable.frm) to the schema data folder (e.g. MySQL Server/data/{yourschema}).
  5. Save a copy of the ibdata1 file to the data folder (i.e., MySQL Server/data).
  6. Restart the MySQL service.
  7. Check that the tables are now accessible, queryable, etc. in your DB explorer client.

After that, all was well. (Don't forget to backup if you have success!)

static constructors in C++? I need to initialize private static objects

How about creating a template to mimic the behavior of C#.

template<class T> class StaticConstructor
    bool m_StaticsInitialised = false;

    typedef void (*StaticCallback)(void);

    StaticConstructor(StaticCallback callback)
        if (m_StaticsInitialised)


        m_StaticsInitialised = true;

template<class T> bool StaticConstructor<T>::m_StaticsInitialised;

class Test : public StaticConstructor<Test>
    static std::vector<char> letters_;

    static void _Test()
        for (char c = 'a'; c <= 'z'; c++)

    Test() : StaticConstructor<Test>(&_Test)
        // non static stuff

Cannot obtain value of local or argument as it is not available at this instruction pointer, possibly because it has been optimized away

Go to Project Properties and under Build Make sure that the "Optimize Code" checkbox is unchecked.

Also, set the "Debug Info" dropdown to "Full" in the Advanced Options (Under Build tab).

Reverse engineering from an APK file to a project

If you are looking for a professional alternative, have a look at JEB Decompiler from PNF Software.

There is a demo version that will let you decompile most code.

Error: Uncaught SyntaxError: Unexpected token <

I too got this error, when developing a Backbone application using HTML5 push state in conjunction with an .htaccess which redirects any unknown files to index.html.

It turns out that when visiting a URL such as /something/5, my /index.html was effectively being served at /something/index.html (thanks to my .htaccess). This had the effect of breaking all the relative URLs to my JavaScript files (from inside the index.html ), which meant that they should have 404'd on me.

However, again due to my htaccess, instead of 404'ing when attempting to retrieve the JS files, they instead returned my index.html. Thus the browser was given an index.html for every JavaScript file it tried to pull in, and when it evaluated the HTML as if it were JavaScript, it returned a JS error due to the leading < (from my tag in index.html).

The fix in my case (as I was serving the site from inside a subdirectory) was to add a base tag in my html head.

<base href="/my-app/">

Change the "From:" address in Unix "mail"

The answers provided before didn't work for me on CentOS5. I installed mutt. It has a lot of options. With mutt you do this this way:

export [email protected]
export [email protected]
mutt -s Testing [email protected]

Jinja2 template not rendering if-elif-else statement properly

You are testing if the values of the variables error and Already are present in RepoOutput[RepoName.index(repo)]. If these variables don't exist then an undefined object is used.

Both of your if and elif tests therefore are false; there is no undefined object in the value of RepoOutput[RepoName.index(repo)].

I think you wanted to test if certain strings are in the value instead:

{% if "error" in RepoOutput[RepoName.index(repo)] %}
    <td id="error"> {{ RepoOutput[RepoName.index(repo)] }} </td>
{% elif "Already" in RepoOutput[RepoName.index(repo) %}
    <td id="good"> {{ RepoOutput[RepoName.index(repo)] }} </td>
{% else %}
    <td id="error"> {{ RepoOutput[RepoName.index(repo)] }} </td>
{% endif %}

Other corrections I made:

  • Used {% elif ... %} instead of {$ elif ... %}.
  • moved the </tr> tag out of the if conditional structure, it needs to be there always.
  • put quotes around the id attribute

Note that most likely you want to use a class attribute instead here, not an id, the latter must have a value that must be unique across your HTML document.

Personally, I'd set the class value here and reduce the duplication a little:

{% if "Already" in RepoOutput[RepoName.index(repo)] %}
    {% set row_class = "good" %}
{% else %}
    {% set row_class = "error" %}
{% endif %}
<td class="{{ row_class }}"> {{ RepoOutput[RepoName.index(repo)] }} </td>

Absolute position of an element on the screen using jQuery

For the absolute coordinates of any jquery element I wrote this function, it probably doesnt work for all css position types but maybe its a good start for someone ..

function AbsoluteCoordinates($element) {
    var sTop = $(window).scrollTop();
    var sLeft = $(window).scrollLeft();
    var w = $element.width();
    var h = $element.height();
    var offset = $element.offset(); 
    var $p = $element;
    while(typeof $p == 'object') {
        var pOffset = $p.parent().offset();
        if(typeof pOffset == 'undefined') break;
        offset.left = offset.left + (pOffset.left); = + (;
        $p = $p.parent();

    var pos = {
          left: offset.left + sLeft,
          right: offset.left + w + sLeft,
          top: + sTop,
          bottom: + h + sTop,
    } = { x: pos.left, y: }; = { x: pos.right, y: }; = { x: pos.left, y: pos.bottom }; = { x: pos.right, y: pos.bottom };
    //console.log( 'left: ' + pos.left + ' - right: ' + pos.right +' - top: ' + +' - bottom: ' + pos.bottom  );
    return pos;

Date Conversion from String to sql Date in Java giving different output?

That is the simple way of converting string into util date and sql date

String startDate="12-31-2014";
SimpleDateFormat sdf1 = new SimpleDateFormat("MM-dd-yyyy");
java.util.Date date = sdf1.parse(startDate);
java.sql.Date sqlStartDate = new java.sql.Date(date.getTime()); 

RSA Public Key format

Starting from the decoded base64 data of an OpenSSL rsa-ssh Key, i've been able to guess a format:

  • 00 00 00 07: four byte length prefix (7 bytes)
  • 73 73 68 2d 72 73 61: "ssh-rsa"
  • 00 00 00 01: four byte length prefix (1 byte)
  • 25: RSA Exponent (e): 25
  • 00 00 01 00: four byte length prefix (256 bytes)
  • RSA Modulus (n):

    7f 9c 09 8e 8d 39 9e cc d5 03 29 8b c4 78 84 5f
    d9 89 f0 33 df ee 50 6d 5d d0 16 2c 73 cf ed 46 
    dc 7e 44 68 bb 37 69 54 6e 9e f6 f0 c5 c6 c1 d9 
    cb f6 87 78 70 8b 73 93 2f f3 55 d2 d9 13 67 32 
    70 e6 b5 f3 10 4a f5 c3 96 99 c2 92 d0 0f 05 60 
    1c 44 41 62 7f ab d6 15 52 06 5b 14 a7 d8 19 a1 
    90 c6 c1 11 f8 0d 30 fd f5 fc 00 bb a4 ef c9 2d 
    3f 7d 4a eb d2 dc 42 0c 48 b2 5e eb 37 3c 6c a0 
    e4 0a 27 f0 88 c4 e1 8c 33 17 33 61 38 84 a0 bb 
    d0 85 aa 45 40 cb 37 14 bf 7a 76 27 4a af f4 1b 
    ad f0 75 59 3e ac df cd fc 48 46 97 7e 06 6f 2d 
    e7 f5 60 1d b1 99 f8 5b 4f d3 97 14 4d c5 5e f8 
    76 50 f0 5f 37 e7 df 13 b8 a2 6b 24 1f ff 65 d1 
    fb c8 f8 37 86 d6 df 40 e2 3e d3 90 2c 65 2b 1f 
    5c b9 5f fa e9 35 93 65 59 6d be 8c 62 31 a9 9b 
    60 5a 0e e5 4f 2d e6 5f 2e 71 f3 7e 92 8f fe 8b

The closest validation of my theory i can find it from RFC 4253:

The "ssh-rsa" key format has the following specific encoding:

  string    "ssh-rsa"
  mpint     e
  mpint     n

Here the 'e' and 'n' parameters form the signature key blob.

But it doesn't explain the length prefixes.

Taking the random RSA PUBLIC KEY i found (in the question), and decoding the base64 into hex:

30 82 01 0a 02 82 01 01 00 fb 11 99 ff 07 33 f6 e8 05 a4 fd 3b 36 ca 68 
e9 4d 7b 97 46 21 16 21 69 c7 15 38 a5 39 37 2e 27 f3 f5 1d f3 b0 8b 2e 
11 1c 2d 6b bf 9f 58 87 f1 3a 8d b4 f1 eb 6d fe 38 6c 92 25 68 75 21 2d 
dd 00 46 87 85 c1 8a 9c 96 a2 92 b0 67 dd c7 1d a0 d5 64 00 0b 8b fd 80 
fb 14 c1 b5 67 44 a3 b5 c6 52 e8 ca 0e f0 b6 fd a6 4a ba 47 e3 a4 e8 94 
23 c0 21 2c 07 e3 9a 57 03 fd 46 75 40 f8 74 98 7b 20 95 13 42 9a 90 b0 
9b 04 97 03 d5 4d 9a 1c fe 3e 20 7e 0e 69 78 59 69 ca 5b f5 47 a3 6b a3 
4d 7c 6a ef e7 9f 31 4e 07 d9 f9 f2 dd 27 b7 29 83 ac 14 f1 46 67 54 cd 
41 26 25 16 e4 a1 5a b1 cf b6 22 e6 51 d3 e8 3f a0 95 da 63 0b d6 d9 3e 
97 b0 c8 22 a5 eb 42 12 d4 28 30 02 78 ce 6b a0 cc 74 90 b8 54 58 1f 0f 
fb 4b a3 d4 23 65 34 de 09 45 99 42 ef 11 5f aa 23 1b 15 15 3d 67 83 7a 
63 02 03 01 00 01

From RFC3447 - Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1:

A.1.1 RSA public key syntax

An RSA public key should be represented with the ASN.1 type RSAPublicKey:

  RSAPublicKey ::= SEQUENCE {
     modulus           INTEGER,  -- n
     publicExponent    INTEGER   -- e

The fields of type RSAPublicKey have the following meanings:

  • modulus is the RSA modulus n.
  • publicExponent is the RSA public exponent e.

Using Microsoft's excellent (and the only real) ASN.1 documentation:

30 82 01 0a       ;SEQUENCE (0x010A bytes: 266 bytes)
|  02 82 01 01    ;INTEGER  (0x0101 bytes: 257 bytes)
|  |  00          ;leading zero because high-bit, but number is positive
|  |  fb 11 99 ff 07 33 f6 e8 05 a4 fd 3b 36 ca 68 
|  |  e9 4d 7b 97 46 21 16 21 69 c7 15 38 a5 39 37 2e 27 f3 f5 1d f3 b0 8b 2e 
|  |  11 1c 2d 6b bf 9f 58 87 f1 3a 8d b4 f1 eb 6d fe 38 6c 92 25 68 75 21 2d 
|  |  dd 00 46 87 85 c1 8a 9c 96 a2 92 b0 67 dd c7 1d a0 d5 64 00 0b 8b fd 80 
|  |  fb 14 c1 b5 67 44 a3 b5 c6 52 e8 ca 0e f0 b6 fd a6 4a ba 47 e3 a4 e8 94 
|  |  23 c0 21 2c 07 e3 9a 57 03 fd 46 75 40 f8 74 98 7b 20 95 13 42 9a 90 b0 
|  |  9b 04 97 03 d5 4d 9a 1c fe 3e 20 7e 0e 69 78 59 69 ca 5b f5 47 a3 6b a3 
|  |  4d 7c 6a ef e7 9f 31 4e 07 d9 f9 f2 dd 27 b7 29 83 ac 14 f1 46 67 54 cd 
|  |  41 26 25 16 e4 a1 5a b1 cf b6 22 e6 51 d3 e8 3f a0 95 da 63 0b d6 d9 3e 
|  |  97 b0 c8 22 a5 eb 42 12 d4 28 30 02 78 ce 6b a0 cc 74 90 b8 54 58 1f 0f 
|  |  fb 4b a3 d4 23 65 34 de 09 45 99 42 ef 11 5f aa 23 1b 15 15 3d 67 83 7a 
|  |  63 
|  02 03          ;INTEGER (3 bytes)
|     01 00 01

giving the public key modulus and exponent:

  • modulus = 0xfb1199ff0733f6e805a4fd3b36ca68...837a63
  • exponent = 65,537

Update: My expanded form of this answer in another question

Is Tomcat running?

Create a Shell script that checks if tomcat is up or down and set a cron for sh to make it check every few minutes, and auto start tomcat if down. Sample Snippet of code below

TOMCAT_PID=$(ps -ef | awk '/[t]omcat/{print $2}')

if [ -z "$TOMCAT_PID" ]
    sudo /opt/tomcat/bin/

How to calculate a mod b in Python?

you can also try divmod(x, y) which returns a tuple (x // y, x % y)

Messagebox with input field

You can do it by making form and displaying it using ShowDialogBox....

Form.ShowDialog Method - Shows the form as a modal dialog box.


public void ShowMyDialogBox()
   Form2 testDialog = new Form2();

   // Show testDialog as a modal dialog and determine if DialogResult = OK.
   if (testDialog.ShowDialog(this) == DialogResult.OK)
      // Read the contents of testDialog's TextBox.
      this.txtResult.Text = testDialog.TextBox1.Text;
      this.txtResult.Text = "Cancelled";

Entity Framework - Code First - Can't Store List<String>

This answer is based on the ones provided by @Sasan and @CAD bloke.

If you wish to use this in .NET Standard 2 or don't want Newtonsoft, see Xaniff's answer below

Works only with EF Core 2.1+ (not .NET Standard compatible)(Newtonsoft JsonConvert)

builder.Entity<YourEntity>().Property(p => p.Strings)
        v => JsonConvert.SerializeObject(v),
        v => JsonConvert.DeserializeObject<List<string>>(v));

Using the EF Core fluent configuration we serialize/deserialize the List to/from JSON.

Why this code is the perfect mix of everything you could strive for:

  • The problem with Sasn's original answer is that it will turn into a big mess if the strings in the list contains commas (or any character chosen as the delimiter) because it will turn a single entry into multiple entries but it is the easiest to read and most concise.
  • The problem with CAD bloke's answer is that it is ugly and requires the model to be altered which is a bad design practice (see Marcell Toth's comment on Sasan's answer). But it is the only answer that is data-safe.

Convert a RGB Color Value to a Hexadecimal String

This is an adapted version of the answer given by Vivien Barousse with the update from Vulcan applied. In this example I use sliders to dynamically retreive the RGB values from three sliders and display that color in a rectangle. Then in method toHex() I use the values to create a color and display the respective Hex color code.

This example does not include the proper constraints for the GridBagLayout. Though the code will work, the display will look strange.

public class HexColor

  public static void main (String[] args)
   JSlider sRed = new JSlider(0,255,1);
   JSlider sGreen = new JSlider(0,255,1);
   JSlider sBlue = new JSlider(0,255,1);
   JLabel hexCode = new JLabel();
   JPanel myPanel = new JPanel();
   GridBagLayout layout = new GridBagLayout();
   JFrame frame = new JFrame();

   //set frame to organize components using GridBagLayout 

   //create gray filled rectangle 

   //In practice this code is replicated and applied to sGreen and sBlue. 
   //For the sake of brevity I only show sRed in this post.
         new ChangeListener()
             public void stateChanged(ChangeEvent e){
   //add each component to JFrame
} //end of main

  //creates JPanel filled rectangle
  protected void paintComponent(Graphics g)
      g.drawRect(360, 300, 10, 10);
      g.fillRect(360, 300, 10, 10);

  //changes the display color in JPanel
  private Color changeColor()
    int r = sRed.getValue();
    int b = sBlue.getValue();
    int g = sGreen.getValue();
    Color c;
    return  c = new Color(r,g,b);

  //Displays hex representation of displayed color
  private String toHex()
      Integer r = sRed.getValue();
      Integer g = sGreen.getValue();
      Integer b = sBlue.getValue();
      Color hC;
      hC = new Color(r,g,b);
      String hex = Integer.toHexString(hC.getRGB() & 0xffffff);
      while(hex.length() < 6){
          hex = "0" + hex;
      hex = "Hex Code: #" + hex;
      return hex;

A huge thank you to both Vivien and Vulcan. This solution works perfectly and was super simple to implement.

package R does not exist

When I experienced this, I created a new project, and copied the files from my old project to my new one.

  1. Start a new project
  2. Add the old files into the new project (copy and right click on Layout, drawables,com.packagename,...) to paste my old projects files

However, I still had the same problem: R does not exist. Then I realised that I had copied and pasted Manifest from my notepad into Android Studio manifests and I hadn't changed the package name in the manifest file.

Therefore, once I changed the package name in the manifest to the new project name, the new project I'd created worked fine.

Appending a vector to a vector

std::copy (b.begin(), b.end(), std::back_inserter(a));

This can be used in case the items in vector a have no assignment operator (e.g. const member).

In all other cases this solution is ineffiecent compared to the above insert solution.

Calendar date to yyyy-MM-dd format in java

A Java Date is a container for the number of milliseconds since January 1, 1970, 00:00:00 GMT.

When you use something like System.out.println(date), Java uses Date.toString() to print the contents.

The only way to change it is to override Date and provide your own implementation of Date.toString(). Now before you fire up your IDE and try this, I wouldn't; it will only complicate matters. You are better off formatting the date to the format you want to use (or display).

Java 8+

LocalDateTime ldt =;
DateTimeFormatter formmat1 = DateTimeFormatter.ofPattern("yyyy-MM-dd", Locale.ENGLISH);
// Output "2018-05-12T17:21:53.658"

String formatter = formmat1.format(ldt);
// 2018-05-12

Prior to Java 8

You should be making use of the ThreeTen Backport

The following is maintained for historical purposes (as the original answer)

What you can do, is format the date.

Calendar cal = Calendar.getInstance();
cal.add(Calendar.DATE, 1);
SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd");
// Output "Wed Sep 26 14:23:28 EST 2012"

String formatted = format1.format(cal.getTime());
// Output "2012-09-26"

// Output "Wed Sep 26 00:00:00 EST 2012"

These are actually the same date, represented differently.

find vs find_by vs where

There is a difference between find and find_by in that find will return an error if not found, whereas find_by will return null.

Sometimes it is easier to read if you have a method like find_by email: "haha", as opposed to .where(email: some_params).first.

What is difference between CrudRepository and JpaRepository interfaces in Spring Data JPA?

All the answers provide sufficient details to the question. However, let me add something more.

Why are we using these Interfaces:

  • They allow Spring to find your repository interfaces and create proxy objects for them.
  • It provides you with methods that allow you to perform some common operations (you can also define your custom method as well). I love this feature because creating a method (and defining query and prepared statements and then execute the query with connection object) to do a simple operation really sucks !

Which interface does what:

  • CrudRepository: provides CRUD functions
  • PagingAndSortingRepository: provides methods to do pagination and sort records
  • JpaRepository: provides JPA related methods such as flushing the persistence context and delete records in a batch

When to use which interface:

According to

Generally the best idea is to use CrudRepository or PagingAndSortingRepository depending on whether you need sorting and paging or not.

The JpaRepository should be avoided if possible, because it ties you repositories to the JPA persistence technology, and in most cases you probably wouldn’t even use the extra methods provided by it.

Strange PostgreSQL "value too long for type character varying(500)"

By specifying the column as VARCHAR(500) you've set an explicit 500 character limit. You might not have done this yourself explicitly, but Django has done it for you somewhere. Telling you where is hard when you haven't shown your model, the full error text, or the query that produced the error.

If you don't want one, use an unqualified VARCHAR, or use the TEXT type.

varchar and text are limited in length only by the system limits on column size - about 1GB - and by your memory. However, adding a length-qualifier to varchar sets a smaller limit manually. All of the following are largely equivalent:

column_name VARCHAR(500)

column_name VARCHAR CHECK (length(column_name) <= 500) 

column_name TEXT CHECK (length(column_name) <= 500) 

The only differences are in how database metadata is reported and which SQLSTATE is raised when the constraint is violated.

The length constraint is not generally obeyed in prepared statement parameters, function calls, etc, as shown:

regress=> \x
Expanded display is on.
regress=> PREPARE t2(varchar(500)) AS SELECT $1;
regress=> EXECUTE t2( repeat('x',601) );
-[ RECORD 1 ]-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
?column? | xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

and in explicit casts it result in truncation:

regress=> SELECT repeat('x',501)::varchar(1);
-[ RECORD 1 ]
repeat | x

so I think you are using a VARCHAR(500) column, and you're looking at the wrong table or wrong instance of the database.

explicit casting from super class to subclass

By using a cast you're essentially telling the compiler "trust me. I'm a professional, I know what I'm doing and I know that although you can't guarantee it, I'm telling you that this animal variable is definitely going to be a dog."

Since the animal isn't actually a dog (it's an animal, you could do Animal animal = new Dog(); and it'd be a dog) the VM throws an exception at runtime because you've violated that trust (you told the compiler everything would be ok and it's not!)

The compiler is a bit smarter than just blindly accepting everything, if you try and cast objects in different inheritence hierarchies (cast a Dog to a String for example) then the compiler will throw it back at you because it knows that could never possibly work.

Because you're essentially just stopping the compiler from complaining, every time you cast it's important to check that you won't cause a ClassCastException by using instanceof in an if statement (or something to that effect.)

CSS3 Transparency + Gradient

Yes. You can use rgba in both webkit and moz gradient declarations:

/* webkit example */
background-image: -webkit-gradient(
  linear, left top, left bottom, from(rgba(50,50,50,0.8)),
  to(rgba(80,80,80,0.2)), color-stop(.5,#333333)


/* mozilla example - FF3.6+ */
background-image: -moz-linear-gradient(
  rgba(255, 255, 255, 0.7) 0%, rgba(255, 255, 255, 0) 95%


Apparently you can even do this in IE, using an odd "extended hex" syntax. The first pair (in the example 55) refers to the level of opacity:

/* approximately a 33% opacity on blue */
filter: progid:DXImageTransform.Microsoft.gradient(
  startColorstr=#550000FF, endColorstr=#550000FF

/* IE8 uses -ms-filter for whatever reason... */
-ms-filter: progid:DXImageTransform.Microsoft.gradient(
  startColorstr=#550000FF, endColorstr=#550000FF


How to create global variables accessible in all views using Express / Node.JS?

One way to do this by updating the app.locals variable for that app in app.js

Set via following

var app = express();
app.locals.appName = "DRC on FHIR";

Get / Access

app.listen(3000, function () {
    console.log('[' + app.locals.appName + '] => app listening on port 3001!');

Elaborating with a screenshot from @RamRovi example with slight enhancement.

enter image description here

How do I search for an object by its ObjectId in the mongo console?

I just had this issue and was doing exactly as was documented and it still was not working.

Look at your error message and make sure you do not have any special characters copied in. I was getting the error

SyntaxError: illegal character @(shell):1:43

When I went to character 43 it was just the start of my object ID, after the open quotes, exactly as I pasted it in. I put my cursor there and hit backspace nothing appeared to happen when it should have removed the open quote. I hit backspace again and it removed the open quote, then I put the quote back in and executed the query and it worked, despite looking exactly the same.

I was doing development in WebMatrix and copied the object id from the console. Whenever you copy from the console in WebMatrix you're likely to pick up some invisible characters that will cause errors.

Limit characters displayed in span

max-length is used for input elements. Use text-overflow property of CSS.

.claimedRight {
    display:block; width: 250px;
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;

How to iterate over the file in python

This is probably because an empty line at the end of your input file.

Try this:

for x in f:
        print int(x.strip(),16)
    except ValueError:
        print "Invalid input:", x

bodyParser is deprecated express 4

What is your opinion to use express-generator it will generate skeleton project to start with, without deprecated messages appeared in your log

run this command

npm install express-generator -g

Now, create new Express.js starter application by type this command in your Node projects folder.

express node-express-app

That command tell express to generate new Node.js application with the name node-express-app.

then Go to the newly created project directory, install npm packages and start the app using the command

cd node-express-app && npm install && npm start

Permanently add a directory to PYTHONPATH?

Shortest path between A <-> B is a straight line;

import sys
if not 'NEW_PATH' in sys.path:
  sys.path += ['NEW_PATH']

How to detect reliably Mac OS X, iOS, Linux, Windows in C preprocessor?

There are predefined macros that are used by most compilers, you can find the list here. GCC compiler predefined macros can be found here. Here is an example for gcc:

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
   //define something for Windows (32-bit and 64-bit, this part is common)
   #ifdef _WIN64
      //define something for Windows (64-bit only)
      //define something for Windows (32-bit only)
#elif __APPLE__
    #include <TargetConditionals.h>
         // iOS Simulator
        // iOS device
    #elif TARGET_OS_MAC
        // Other kinds of Mac OS
    #   error "Unknown Apple platform"
#elif __linux__
    // linux
#elif __unix__ // all unices not caught above
    // Unix
#elif defined(_POSIX_VERSION)
    // POSIX
#   error "Unknown compiler"

The defined macros depend on the compiler that you are going to use.

The _WIN64 #ifdef can be nested into the _WIN32 #ifdef because _WIN32 is even defined when targeting the Windows x64 version. This prevents code duplication if some header includes are common to both (also WIN32 without underscore allows IDE to highlight the right partition of code).

Create database from command line

Change the user to postgres :

su - postgres

Create User for Postgres

$ createuser testuser

Create Database

$ createdb testdb

Acces the postgres Shell

psql ( enter the password for postgressql)

Provide the privileges to the postgres user

$ alter user testuser with encrypted password 'qwerty';
$ grant all privileges on database testdb to testuser;

Passing Objects By Reference or Value in C#

One more code sample to showcase this:

void Main()

    int k = 0;
    Console.WriteLine("TestPlain:" + k);

    TestRef(ref k);
    Console.WriteLine("TestRef:" + k);

    string t = "test";

    Console.WriteLine("TestObjPlain:" +t);

    TestObjRef(ref t);
    Console.WriteLine("TestObjRef:" + t);

public static void TestPlain(int i)
    i = 5;

public static void TestRef(ref int i)
    i = 5;

public static void TestObjPlain(string s)
    s = "TestObjPlain";

public static void TestObjRef(ref string s)
    s = "TestObjRef";

And the output:





Saving a high resolution image in R

You can do the following. Add your ggplot code after the first line of code and end with

tiff("test.tiff", units="in", width=5, height=5, res=300)
# insert ggplot code

res=300 specifies that you need a figure with a resolution of 300 dpi. The figure file named 'test.tiff' is saved in your working directory.

Change width and height in the code above depending on the desired output.

Note that this also works for other R plots including plot, image, and pheatmap.

Other file formats

In addition to TIFF, you can easily use other image file formats including JPEG, BMP, and PNG. Some of these formats require less memory for saving.

Split string into list in jinja?

If there are up to 10 strings then you should use a list in order to iterate through all values.

{% set list1 = variable1.split(';') %}
{% for list in list1 %}
<p>{{ list }}</p>
{% endfor %}

Get Android .apk file VersionName or VersionCode WITHOUT installing apk

    EditText ET1 = (EditText) findViewById(;

    PackageInfo pinfo;
    try {
        pinfo = getPackageManager().getPackageInfo(getPackageName(), 0);
        String versionName = pinfo.versionName;
    } catch (NameNotFoundException e) {
        // TODO Auto-generated catch block

Why compile Python code?

There is a performance increase in running compiled python. However when you run a .py file as an imported module, python will compile and store it, and as long as the .py file does not change it will always use the compiled version.

With any interpeted language when the file is used the process looks something like this:
1. File is processed by the interpeter.
2. File is compiled
3. Compiled code is executed.

obviously by using pre-compiled code you can eliminate step 2, this applies python, PHP and others.

Heres an interesting blog post explaining the differences
And here's an entry that explains the Python compile process

Change WPF window background image in C# code

img.UriSource = new Uri("pack://application:,,,/images/" + fileName, UriKind.Absolute);

Javascript getElementsByName.value not working

Here is the example for having one or more checkboxes value. If you have two or more checkboxes and need values then this would really help.

function myFunction() {_x000D_
  var selchbox = [];_x000D_
  var inputfields = document.getElementsByName("myCheck");_x000D_
  var ar_inputflds = inputfields.length;_x000D_
  for (var i = 0; i < ar_inputflds; i++) {_x000D_
    if (inputfields[i].type == 'checkbox' && inputfields[i].checked == true)_x000D_
  return selchbox;_x000D_
document.getElementById('btntest').onclick = function() {_x000D_
  var selchb = myFunction();_x000D_
<input type="checkbox" name="myCheck" value="UK">United Kingdom_x000D_
<input type="checkbox" name="myCheck" value="USA">United States_x000D_
<input type="checkbox" name="myCheck" value="IL">Illinois_x000D_
<input type="checkbox" name="myCheck" value="MA">Massachusetts_x000D_
<input type="checkbox" name="myCheck" value="UT">Utah_x000D_
<input type="button" value="Click" id="btntest" />

logout and redirecting session in php

header("Location: home.php");

Constructor of an abstract class in C#

Key Points About Abstract Class

  1. An abstract class cannot be instantiated.
  2. An abstract class can have constructor and destructor.
  3. An abstract class cannot be a sealed class because the sealed modifier prevents a class from being inherited.
  4. An abstract class contains abstract as well as non-abstract members.
  5. An abstract class members can be private, protected and internal.
  6. Abstract members cannot have a private access modifier.
  7. Abstract members are implicitly virtual and must be implemented by a non-abstract derived class.

How to get the first line of a file in a bash script?

to read first line using bash, use read statement. eg

read -r firstline<file

firstline will be your variable (No need to assign to another)

Date format Mapping to JSON Jackson

Building on @miklov-kriven's very helpful answer, I hope these two additional points of consideration prove helpful to someone:

(1) I find it a nice idea to include serializer and de-serializer as static inner classes in the same class. NB, using ThreadLocal for thread safety of SimpleDateFormat.

public class DateConverter {

    private static final ThreadLocal<SimpleDateFormat> sdf = 
                () -> {return new SimpleDateFormat("yyyy-MM-dd HH:mm a z");});

    public static class Serialize extends JsonSerializer<Date> {
        public void serialize(Date value, JsonGenerator jgen SerializerProvider provider) throws Exception {
            if (value == null) {
            else {

    public static class Deserialize extends JsonDeserializer<Date> {
        public Date deserialize(JsonParser jp, DeserializationContext ctxt) throws Exception {
            String dateAsString = jp.getText();
            try {
                if (Strings.isNullOrEmpty(dateAsString)) {
                    return null;
                else {
                    return new Date(sdf.get().parse(dateAsString).getTime());
            catch (ParseException pe) {
                throw new RuntimeException(pe);

(2) As an alternative to using @JsonSerialize and @JsonDeserialize annotations on each individual class member you could also consider overriding Jackson's default serialization by applying the custom serialization at an application level, that is all class members of type Date will be serialized by Jackson using this custom serialization without explicit annotation on each field. If you are using Spring Boot for example one way to do this would as follows:

public class Application {
    public static void main(String[] args) {, args);

    public Module customModule() {
        SimpleModule module = new SimpleModule();
        module.addSerializer(Date.class, new DateConverter.Serialize());
        module.addDeserializer(Date.class, new Dateconverter.Deserialize());
        return module;

Is it possible to install Xcode 10.2 on High Sierra (10.13.6)?

Based on responses and comments below, the following was the simple solution for my issue and THIS WORKED. Now my app, Match4app, is fully compatible with latest iOS versions!

  1. Download Xcode 10.2 from a direct link (not from App Store). (Estimated Size: ~6Gb)
  2. From the downloaded version just copy/paste the DeviceSupport/12.2 directory into "Applications/"
  3. You can discard the downloaded version now (we just need the small 12.2 directory!)

Delete an element from a dictionary

The del statement is what you're looking for. If you have a dictionary named foo with a key called 'bar', you can delete 'bar' from foo like this:

del foo['bar']

Note that this permanently modifies the dictionary being operated on. If you want to keep the original dictionary, you'll have to create a copy beforehand:

>>> foo = {'bar': 'baz'}
>>> fu = dict(foo)
>>> del foo['bar']
>>> print foo
>>> print fu
{'bar': 'baz'}

The dict call makes a shallow copy. If you want a deep copy, use copy.deepcopy.

Here's a method you can copy & paste, for your convenience:

def minus_key(key, dictionary):
    shallow_copy = dict(dictionary)
    del shallow_copy[key]
    return shallow_copy

User Authentication in ASP.NET Web API

I am working on a MVC5/Web API project and needed to be able to get authorization for the Web Api methods. When my index view is first loaded I make a call to the 'token' Web API method which I believe is created automatically.

The client side code (CoffeeScript) to get the token is:

getAuthenticationToken = (username, password) ->
    dataToSend = "username=" + username + "&password=" + password
    dataToSend += "&grant_type=password"
    $.post("/token", dataToSend).success saveAccessToken

If successful the following is called, which saves the authentication token locally:

saveAccessToken = (response) ->
    window.authenticationToken = response.access_token

Then if I need to make an Ajax call to a Web API method that has the [Authorize] tag I simply add the following header to my Ajax call:

{ "Authorization": "Bearer " + window.authenticationToken }

Force DOM redraw/refresh on Chrome/Mac

call window.getComputedStyle() should force a reflow

Specifying java version in maven - differences between properties and compiler plugin

Consider the alternative:


It should be the same thing of maven.compiler.source/ but the above solution works for me, otherwise the second one gets the parent specification (I have a matrioska of .pom)

A non well formed numeric value encountered

If the error is at the time of any calculation, double check that the values does not contains any comma(,). Values must be only in integer/ float format.

Increase bootstrap dropdown menu width

Usually we have the need to control the width of the dropdown menu; specially that's essential when the dropdown menu holds a form, e.g. login form --- then the dropdown menu and its items should be wide enough for ease of inputing username/email and password.

Besides, when the screen is smaller than 768px or when the window (containing the dropdown menu) is zoomed down to smaller than 768px, Bootstrap 3 responsively scales the dropdown menu to the whole width of the screen/window. We need to keep this reponsive action.

Hence, the following css class could do that:

@media (min-width: 768px) {
    .dropdown-menu {
        width: 300px !important;  /* change the number to whatever that you need */

(I had used it in my web app.)

AngularJS - difference between pristine/dirty and touched/untouched

$pristine/$dirty tells you whether the user actually changed anything, while $touched/$untouched tells you whether the user has merely been there/visited.

This is really useful for validation. The reason for $dirty was always to avoid showing validation responses until the user has actually visited a certain control. But, by using only the $dirty property, the user wouldn't get validation feedback unless they actually altered the value. So, an $invalid field still wouldn't show the user a prompt if the user didn't change/interact with the value. If the user entirely ignored a required field, everything looked OK.

With Angular 1.3 and ng-touched, you can now set a particular style on a control as soon as the user has blurred, regardless of whether they actually edited the value or not.

Here's a CodePen that shows the difference in behavior.

How to detect shake event with android?

Do the following:

private float xAccel, yAccel, zAccel;
private float xPreviousAccel, yPreviousAccel, zPreviousAccel;
private boolean firstUpdate = true;
private final float shakeThreshold = 1.5f;
private boolean shakeInitiated = false;
SensorEventListener mySensorEventListener;
SensorManager mySensorManager;

Put this in onCreate method.

mySensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);

And now the main part.

private boolean isAccelerationChanged() {
    float deltaX = Math.abs(xPreviousAccel - xAccel);
    float deltaY = Math.abs(yPreviousAccel - yAccel);
    float deltaZ = Math.abs(zPreviousAccel - zAccel);
    return (deltaX > shakeThreshold && deltaY > shakeThreshold)
            || (deltaX > shakeThreshold && deltaZ > shakeThreshold)
            || (deltaY > shakeThreshold && deltaZ > shakeThreshold);

private void updateAccelParameters(float xNewAccel, float yNewAccel, float zNewAccel) {
    if (firstUpdate) {
        xPreviousAccel = xNewAccel;
        yPreviousAccel = yNewAccel;
        zPreviousAccel = zNewAccel;
        firstUpdate = false;
        xPreviousAccel = xAccel;
        yPreviousAccel = yAccel;
        zPreviousAccel = zAccel;
    xAccel = xNewAccel;
    yAccel = yNewAccel;
    zAccel = zNewAccel;

private void executeShakeAction() {
    //this method is called when devices shakes

public void onSensorChanged(SensorEvent se) {
    updateAccelParameters(se.values[0], se.values[1], se.values[2]);
    if ((!shakeInitiated) && isAccelerationChanged()) {
        shakeInitiated = true;
    }else if ((shakeInitiated) && isAccelerationChanged()){
    }else if((shakeInitiated) && (!isAccelerationChanged())){
        shakeInitiated = false;

public void onAccuracyChanged(Sensor sensor, int accuracy) {
    //setting the accuracy

ImportError: No module named PyQt4.QtCore

As mentioned in the comments, you need to install the python-qt4 package - no need to recompile it yourself.

sudo apt-get install python-qt4

Calculating difference between two timestamps in Oracle in milliseconds

Better to use procedure like that:

start_time_in TIMESTAMP
, end_time_in TIMESTAMP
l_days NUMBER;
l_hours NUMBER;
l_minutes NUMBER;
l_seconds NUMBER;
l_milliseconds NUMBER;
SELECT extract(DAY FROM end_time_in-start_time_in)
, extract(HOUR FROM end_time_in-start_time_in)
, extract(MINUTE FROM end_time_in-start_time_in)
, extract(SECOND FROM end_time_in-start_time_in)
INTO l_days, l_hours, l_minutes, l_seconds
FROM dual;

l_milliseconds := l_seconds*1000 + l_minutes*60*1000 + l_hours*60*60*1000 + l_days*24*60*60*1000;
RETURN l_milliseconds;


You can check it by calling:

SELECT timestamp_diff (TO_TIMESTAMP('12.04.2017 12:00:00.00', 'DD.MM.YYYY HH24:MI:SS.FF'), 
                      TO_TIMESTAMP('12.04.2017 12:00:01.111', 'DD.MM.YYYY HH24:MI:SS.FF')) 
            as milliseconds

How do I simulate placeholder functionality on input date field?

use this input attribute events

onfocus="(this.type='date')" onblur="(this.type='text')"

<link rel="stylesheet" href="" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">_x000D_
<div class="container mt-3">_x000D_
  <label for="date">Date : </label>_x000D_
  <input placeholder="Your Date" class="form-control" type="text" onfocus="(this.type='date')" onblur="(this.type='text')" id="date">_x000D_

How To Inject AuthenticationManager using Java Configuration in a Custom Filter

In addition to what Angular University said above you may want to use @Import to aggregate @Configuration classes to the other class (AuthenticationController in my case) :

public class AuthenticationController {
private AuthenticationManager authenticationManager;
//some logic

Spring doc about Aggregating @Configuration classes with @Import: link

Is there a way to create key-value pairs in Bash script?

For persistent key/value storage, you can use kv-bash, a pure bash implementation of key/value database available at


git clone
source kv-bash/kv-bash

Try create some permanent variables

kvset myName  xyz
kvset myEmail [email protected]

#read the varible
kvget myEmail

#you can also use in another script with $(kvget keyname)
echo $(kvget myEmail)

How to get bean using application context in spring boot

You can Autowire the ApplicationContext, either as a field

private ApplicationContext context;

or a method

public void context(ApplicationContext context) { this.context = context; }

Finally use


How to pass variable number of arguments to a PHP function

This is now possible with PHP 5.6.x, using the ... operator (also known as splat operator in some languages):


function addDateIntervalsToDateTime( DateTime $dt, DateInterval ...$intervals )
    foreach ( $intervals as $interval ) {
        $dt->add( $interval );
    return $dt;

addDateIntervaslToDateTime( new DateTime, new DateInterval( 'P1D' ), 
        new DateInterval( 'P4D' ), new DateInterval( 'P10D' ) );

Read and write into a file using VBScript

This is for create a text file

For i = 1 to 10
    createFile( i )

Public Sub createFile(a)

    Dim fso,MyFile
    filePath = "C:\file_name" & a & ".txt"
    Set fso = CreateObject("Scripting.FileSystemObject")
    Set MyFile = fso.CreateTextFile(filePath)
    MyFile.WriteLine("This is a separate file")

End Sub

And this for read a text file

Dim fso
Set fso = CreateObject("Scripting.FileSystemObject")
Set dict = CreateObject("Scripting.Dictionary")
Set file = fso.OpenTextFile ("test.txt", 1)
row = 0
Do Until file.AtEndOfStream
  line = file.Readline
  dict.Add row, line
  row = row + 1


For Each line in dict.Items
  WScript.Echo line
  WScript.Sleep 1000

XPath to select element based on childs child value

Almost there. In your predicate, you want a relative path, so change

./book[/author/name = 'John'] 

to either

./book[author/name = 'John'] 


./book[./author/name = 'John'] 

and you will match your element. Your current predicate goes back to the root of the document to look for an author.

How do I initialize a TypeScript Object with a JSON-Object?

For simple objects, I like this method:

class Person {
    public id: String, 
    public name: String, 
    public title: String) {};

  static deserialize(input:any): Person {
    return new Person(,, input.title);

var person = Person.deserialize({id: 'P123', name: 'Bob', title: 'Mr'});

Leveraging the ability to define properties in the constructor lets it be concise.

This gets you a typed object (vs all the answers that use Object.assign or some variant, which give you an Object) and doesn't require external libraries or decorators.

Custom format for time command

The accepted answer gives me this output

# bash
Time in seconds: 51 line 12: unexpected EOF while looking for matching `"' line 21: syntax error: unexpected end of file

This is how I solved the issue


date1=$(date --date 'now' +%s) #date since epoch in seconds at the start of script
date2=$(date --date 'now' +%s) #date since epoch in seconds at the end of script
difference=$(echo "$((date2-$date1))") # difference between two values
date3=$(echo "scale=2 ; $difference/3600" | bc) # difference/3600 = seconds in hours
echo SCRIPT TOOK $date3 HRS TO COMPLETE # 3rd variable for a pretty output.

how to display employee names starting with a and then b in sql

Regular expressions work well if needing to find a range of starting characters. The following finds all employee names starting with A, B, C or D and adds the “UPPER” call in case a name is in the database with a starting lowercase letter. My query works in Oracle (I did not test other DB's). The following would return for example:

This query also ignores case in the ORDER BY via the "lower" call:

SELECT employee_name 
FROM employees
WHERE REGEXP_LIKE(UPPER(TRIM(employee_name)), '^[A-D]')
ORDER BY lower(employee_name)

Convert a positive number to negative in C#

As previously mentioned, just multiplying by -1 is not cool, as int.MinValue * -1 == int.MinValue

It really depends on your application, but for mine (Inverting Joystick axes) I want to ensure that all extremes are still reachable, and maintain as much fidelity as possible.
To this end, I map Max <--> Min and everything in-between gets a * -1
Using this technique, -32767 is unreachable when inverting.

    private static int Invert(int value)
        if (value == int.MaxValue) return int.MinValue;
        if (value == int.MinValue) return int.MaxValue;
        return value * -1;

What is a 'multi-part identifier' and why can't it be bound?

You probably have a typo. For instance, if you have a table named Customer in a database named Sales, you could refer to it as Sales..Customer (although it is better to refer to it including the owner name (dbo is the default owner) like Sales.dbo.Customer.

If you typed Sales...Customer, you might have gotten the message you got.

trigger body click with jQuery

You should have something like this:

$('body').click(function() {
   // do something here

The callback function will be called when the user clicks somewhere on the web page. You can trigger the callback programmatically with:


Negative matching using grep (match lines that do not contain foo)

You can also use awk for these purposes, since it allows you to perform more complex checks in a clearer way:

Lines not containing foo:

awk '!/foo/'

Lines containing neither foo nor bar:

awk '!/foo/ && !/bar/'

Lines containing neither foo nor bar which contain either foo2 or bar2:

awk '!/foo/ && !/bar/ && (/foo2/ || /bar2/)'

And so on.

How do I update the password for Git?

To fix this on macOS, you can use

git config --global credential.helper osxkeychain

A username and password prompt will appear with your next Git action (pull, clone, push, etc.).

For Windows, it's the same command with a different argument:

git config --global credential.helper wincred

Error 6 (net::ERR_FILE_NOT_FOUND): The files c or directory could not be found

If this is an HTML file where you are using file://FileName then using a CDN like src="//" will not work.

You will have to include the .js file in your code.

Setting Column width in Apache POI

I answered my problem with a default width for all columns and cells, like below:

int width = 15; // Where width is number of caracters 

SQLite Query in Android to count rows

Use an SQLiteStatement.


 SQLiteStatement s = mDb.compileStatement( "select count(*) from users where uname='" + loginname + "' and pwd='" + loginpass + "'; " );

  long count = s.simpleQueryForLong();

How to disable copy/paste from/to EditText

You can do this by disabling the long press of the EditText

To implement it, just add the following line in the xml -


Using bind variables with dynamic SELECT INTO clause in PL/SQL

Put the select statement in a dynamic PL/SQL block.

CREATE OR REPLACE FUNCTION get_num_of_employees (p_loc VARCHAR2, p_job VARCHAR2) 
  v_query_str VARCHAR2(1000);
  v_num_of_employees NUMBER;
  v_query_str := 'begin SELECT COUNT(*) INTO :into_bind FROM emp_' 
                 || p_loc
                 || ' WHERE job = :bind_job; end;';
    USING out v_num_of_employees, p_job;
  RETURN v_num_of_employees;

PHP: get the value of TEXTBOX then pass it to a VARIABLE

Inside testing2.php you should print the $_POST array which contains all the data from the post. Also, $_POST['name'] should be available. For more info check $_POST on

httpd: Could not reliably determine the server's fully qualified domain name, using for ServerName

Make sure you're editing the right httpd.conf file, then the error about unreliable server's domain name should be gone (this is the most common mistake).

To locate your httpd.conf Apache configuration file, run:

apachectl -t -D DUMP_INCLUDES

Then edit the file and uncomment or change ServerName line into:

ServerName localhost

Then restart your apache by: sudo apachectl restart

What 'additional configuration' is necessary to reference a .NET 2.0 mixed mode assembly in a .NET 4.0 project?

Add following at this location C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\x64 FileName: sgen.exe.config(If you dont find this file, create and add one)

 <?xml version ="1.0"?>_x000D_
 <runtime>        _x000D_
        <generatePublisherEvidence enabled="false"/>    _x000D_
    <startup useLegacyV2RuntimeActivationPolicy="true">_x000D_
                <supportedRuntime version="v4.0" />_x000D_
    </startup>    _x000D_

Doing this resolved the issue

Android Relative Layout Align Center

This will definately work for you.

    android:background="@drawable/top_bg" >

        android:background="@drawable/btn_edit" />

        android:textStyle="bold" />

        android:background="@drawable/btn_refresh" />

XCOPY: Overwrite all without prompt in BATCH

The solution is the /Y switch:

xcopy "C:\Users\ADMIN\Desktop\*.*" "D:\Backup\" /K /D /H /Y

Get first element in PHP stdObject

Update PHP 7.4

Curly brace access syntax is deprecated since PHP 7.4

Update 2019

Moving on to the best practices of OOPS, @MrTrick's answer must be marked as correct, although my answer provides a hacked solution its not the best method.

Simply iterate its using {}



This way your object is not destroyed and you can easily iterate through object.

For PHP 5.6 and below use this


Both array() and the stdClass objects can be accessed using the current() key() next() prev() reset() end() functions.

So, if your object looks like

object(stdClass)#19 (3) {
  object(stdClass)#20 (22) {
    string(1) "123"

Then you can just do;

$id = reset($obj)->id; //Gets the 'id' attr of the first entry in the object

If you need the key for some reason, you can do;

reset($obj); //Ensure that we're at the first element
$key = key($obj);

Hope that works for you. :-) No errors, even in super-strict mode, on PHP 5.4

2022 Update:
After PHP 7.4, using current(), end(), etc functions on objects is deprecated.

In newer versions of PHP, use the ArrayIterator class:

$objIterator = new ArrayIterator($obj);

$id = $objIterator->current()->id; // Gets the 'id' attr of the first entry in the object

$key = $objIterator->key(); // and gets the key

How do I use grep to search the current directory for all files having the a string "hello" yet display only .h and .cc files?

If I read your question carefully, you ask to "grep to search the current directory for any and all files containing the string "hello" and display only .h and .cc files". So to meet your precise requirements here is my submission:

This displays the file names:

grep -lR hello * | egrep '(cc|h)$'

...and this display the file names and contents:

grep hello `grep -lR hello * | egrep '(cc|h)$'`

What's the best way to add a full screen background image in React Native

import React, { Component } from 'react';
import { Image, StyleSheet } from 'react-native';

export default class App extends Component {
  render() {
    return (
      <Image source={{uri: ''}} style={s.backgroundImage} />

const s = StyleSheet.create({
  backgroundImage: {
      flex: 1,
      width: null,
      height: null,

You can try it at: (from:


What is the difference between "long", "long long", "long int", and "long long int" in C++?

Historically, in early C times, when processors had 8 or 16 bit wordlength,intwas identical to todays short(16 bit). In a certain sense, int is a more abstract data type thanchar,short,longorlong long, as you cannot be sure about the bitwidth.

When definingint n;you could translate this with "give me the best compromise of bitwidth and speed on this machine for n". Maybe in the future you should expect compilers to translateintto be 64 bit. So when you want your variable to have 32 bits and not more, better use an explicitlongas data type.

[Edit: #include <stdint.h> seems to be the proper way to ensure bitwidths using the int##_t types, though it's not yet part of the standard.]

Get the selected option id with jQuery

$('#my_select option:selected').attr('id');

How to check programmatically if an application is installed or not in Android?

A simpler implementation using Kotlin

fun PackageManager.isAppInstalled(packageName: String): Boolean =
                .firstOrNull { it.packageName == packageName } != null

And call it like this (seeking for Spotify app):


How to backup MySQL database in PHP?

Take a look here! It is a native solution written in php. You won't need to exec mysqldump, or cope with incomplete scripts. This is a full mysqldump clone, without dependencies, output compression and sane defaults.

Out of the box, mysqldump-php supports backing up table structures, the data itself, views, triggers and events.

MySQLDump-PHP is the only library that supports:

  • output binary blobs as hex.
  • resolves view dependencies (using Stand-In tables).
  • output compared against original mysqldump. Linked to travis-ci testing system (testing from php 5.3 to 7.1 & hhvm)
  • dumps stored procedures.
  • dumps events.
  • does extended-insert and/or complete-insert.
  • supports virtual columns from MySQL 5.7.

You can install it using composer, or just download the php file, and it is as easy as doing:

use Ifsnop\Mysqldump as IMysqldump;

try {
    $dump = new IMysqldump\Mysqldump('database', 'username', 'password');
} catch (\Exception $e) {
    echo 'mysqldump-php error: ' . $e->getMessage();

All the options are explained at the github page, but more or less are auto-explicative:

$dumpSettingsDefault = array(
    'include-tables' => array(),
    'exclude-tables' => array(),
    'compress' => Mysqldump::NONE,
    'init_commands' => array(),
    'no-data' => array(),
    'reset-auto-increment' => false,
    'add-drop-database' => false,
    'add-drop-table' => false,
    'add-drop-trigger' => true,
    'add-locks' => true,
    'complete-insert' => false,
    'databases' => false,
    'default-character-set' => Mysqldump::UTF8,
    'disable-keys' => true,
    'extended-insert' => true,
    'events' => false,
    'hex-blob' => true, /* faster than escaped content */
    'net_buffer_length' => self::MAXLINESIZE,
    'no-autocommit' => true,
    'no-create-info' => false,
    'lock-tables' => true,
    'routines' => false,
    'single-transaction' => true,
    'skip-triggers' => false,
    'skip-tz-utc' => false,
    'skip-comments' => false,
    'skip-dump-date' => false,
    'skip-definer' => false,
    'where' => '',
    /* deprecated */
    'disable-foreign-keys-check' => true

In a javascript array, how do I get the last 5 elements, excluding the first element?

var y = [1,2,3,4,5,6,7,8,9,10];_x000D_
console.log(y.slice((y.length - 5), y.length))

you can do this!

Including external jar-files in a new jar-file build with Ant

Two options, either reference the new jars in your classpath or unpack all classes in the enclosing jars and re-jar the whole lot! As far as I know packaging jars within jars is not recommeneded and you'll forever have the class not found exception!

Remove all whitespaces from NSString

I strongly suggest placing this somewhere in your project:

extension String {
    func trim() -> String {
        return self.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet())

    func trim(withSet: NSCharacterSet) -> String {
        return self.stringByTrimmingCharactersInSet(withSet)

ProgressDialog in AsyncTask

 * this class performs all the work, shows dialog before the work and dismiss it after
public class ProgressTask extends AsyncTask<String, Void, Boolean> {

    public ProgressTask(ListActivity activity) {
        this.activity = activity;
        dialog = new ProgressDialog(activity);

    /** progress dialog to show user that the backup is processing. */
    private ProgressDialog dialog;
    /** application context. */
    private ListActivity activity;

    protected void onPreExecute() {
        this.dialog.setMessage("Progress start");;

    protected void onPostExecute(final Boolean success) {
        if (dialog.isShowing()) {

        MessageListAdapter adapter = new MessageListAdapter(activity, titles);

        if (success) {
            Toast.makeText(context, "OK", Toast.LENGTH_LONG).show();
        } else {
            Toast.makeText(context, "Error", Toast.LENGTH_LONG).show();

    protected Boolean doInBackground(final String... args) {
          BaseFeedParser parser = new BaseFeedParser();
          messages = parser.parse();
          List<Message> titles = new ArrayList<Message>(messages.size());
          for (Message msg : messages){
          return true;
       } catch (Exception e)
          Log.e("tag", "error", e);
          return false;

public class Soirees extends ListActivity {
    private List<Message> messages;
    private TextView tvSorties;
    private MyProgressDialog dialog;
    public void onCreate(Bundle icicle) {



        tvSorties.setText("Programme des soirées");

        // just call here the task
        AsyncTask task = new ProgressTask(this).execute();

   public void setMessages(List<Message> msgs) {
      messages = msgs;


How do you post to an iframe?

If you want to change inputs in an iframe then submit the form from that iframe, do this

var el = document.getElementById('targetFrame');

var doc, frame_win = getIframeWindow(el); // getIframeWindow is defined below

if (frame_win) {
  doc = (window.contentDocument || window.document);

if (doc) {
  doc.forms[0].someInputName.value = someValue;

Normally, you can only do this if the page in the iframe is from the same origin, but you can start Chrome in a debug mode to disregard the same origin policy and test this on any page.

function getIframeWindow(iframe_object) {
  var doc;

  if (iframe_object.contentWindow) {
    return iframe_object.contentWindow;

  if (iframe_object.window) {
    return iframe_object.window;

  if (!doc && iframe_object.contentDocument) {
    doc = iframe_object.contentDocument;

  if (!doc && iframe_object.document) {
    doc = iframe_object.document;

  if (doc && doc.defaultView) {
   return doc.defaultView;

  if (doc && doc.parentWindow) {
    return doc.parentWindow;

  return undefined;

How to set proxy for wget?

export http_proxy=http://proxy_host:proxy_port/
export https_proxy=https://proxy_host:proxy_port/


export http_proxy=http://username:password@proxy_host:proxy_port/
export https_proxy=https://username:password@proxy_host:proxy_port/

As all others explained here, these environment variable helps to pass on proxies.

Note: But please not that if the password contains any special character then that needs to be configured as %<hex_value_of_special_char>.

Example: If the password is pass#123, need to be used as pass%23123 in above export commands.

YouTube: How to present embed video with sound muted

For me works using &autoplay=1&mute=1

Shrink to fit content in flexbox, or flex-basis: content workaround?

It turns out that it was shrinking and growing correctly, providing the desired behaviour all along; except that in all current browsers flexbox wasn't accounting for the vertical scrollbar! Which is why the content appears to be getting cut off.

You can see here, which is the original code I was using before I added the fixed widths, that it looks like the column isn't growing to accomodate the text:

However if you make the content in that column wider, you'll see that it always cuts it off by the same amount, which is the width of the scrollbar.

So the fix is very, very simple - add enough right padding to account for the scrollbar:

  main > section {_x000D_
    overflow-y: auto;_x000D_
    padding-right: 2em;_x000D_

It was when I was trying some things suggested by Michael_B (specifically adding a padding buffer) that I discovered this, thanks so much!

Edit: I see that he also posted a fiddle which does the same thing - again, thanks so much for all your help

SQL Server : SUM() of multiple rows including where clauses

Try this:

   SUM(Amount) as TOTAL_COSTS
   EndDate IS NULL

How to run .jar file by double click on Windows 7 64-bit?

change the default application for JAR files from java.exe to javaw.exe from your JAVA_HOME/bin folder.

This is because, java.exe is console application only, but JAR file needs a window rendered execution. Since javaw.exe is a window application, it is preferred for the execution of JAR files.

An alternative to this is that to some extent you can use command prompt to run your JAR files by simply using java keyword with -jar attrib.

How do I clear my local working directory in Git?

All the answers so far retain local commits. If you're really serious, you can discard all local commits and all local edits by doing:

git reset --hard origin/branchname

For example:

git reset --hard origin/master

This makes your local repository exactly match the state of the origin (other than untracked files).

If you accidentally did this after just reading the command, and not what it does :), use git reflog to find your old commits.

How to Convert the value in DataTable into a string array in c#

            string[] result = new string[table.Columns.Count];
            DataRow dr = table.Rows[0];
            for (int i = 0; i < dr.ItemArray.Length; i++)
                result[i] = dr[i].ToString();
            foreach (string str in result)

Sending images using Http Post

I struggled a lot trying to implement posting a image from Android client to servlet using httpclient-4.3.5.jar, httpcore-4.3.2.jar, httpmime-4.3.5.jar. I always got a runtime error. I found out that basically you cannot use these jars with Android as Google is using older version of HttpClient in Android. The explanation is here You need to get the httpclientandroidlib-1.2.1 jar from android http-client library. Then change your imports from or.apache.http.client to ch.boye.httpclientandroidlib. Hope this helps.

How do I convert seconds to hours, minutes and seconds?

If you need to get datetime.time value, you can use this trick:

my_time = (datetime(1970,1,1) + timedelta(seconds=my_seconds)).time()

You cannot add timedelta to time, but can add it to datetime.

UPD: Yet another variation of the same technique:

my_time = (datetime.fromordinal(1) + timedelta(seconds=my_seconds)).time()

Instead of 1 you can use any number greater than 0. Here we use the fact that datetime.fromordinal will always return datetime object with time component being zero.

SHA1 vs md5 vs SHA256: which to use for a PHP login?

Let's assume the next point : the hackers steal our database including the users and password (encrypted). And the hackers created a fake account with a password that they know.

MD5 is weak because its short and popular and practically every hash generation without password is weak of a dictionary attack. But..

So, let's say that we are still using MD5 with a SALT. The hackers don't know the SALT but they know the password of a specific user. So they can test : ?????12345 where 12345 is the know password and ????? is the salt. The hackers sooner or later can guess the SALT.

However, if we used a MD5+SALT and we applied MD5, then there is not way to recover the information. However, i repeat, MD5 is still short.

For example, let's say that my password is : 12345. The SALT is BILLCLINTON

md5 : 827ccb0eea8a706c4c34a16891f84e7b

md5 with the hash : 56adb0f19ac0fb50194c312d49b15378

mD5 with the hash over md5 : 28a03c0bc950decdd9ee362907d1798a I tried to use those online service and i found none that was able to crack it. And its only MD5! (may be as today it will be crackeable because i generated the md5 online)

If you want to overkill then SHA256 is more than enough if its applied with a salt and twice.

tldr MD5(HASH+MD5(password)) = ok but short, SHA256 is more than enough.

Which port(s) does XMPP use?

The ports required will be different for your XMPP Server and any XMPP Clients. Most "modern" XMPP Servers follow the defined IANA Ports for Server-to-Server 5269 and for Client-to-Server 5222. Any additional ports depends on what features you enable on the Server, i.e. if you offer BOSH then you may need to open port 80.

File Transfer is highly dependent on both the Clients you use and the Server as to what port it will use, but most of them also negotiate the connect via your existing XMPP Client-to-Server link so the required port opening will be client side (or proxied via port 80.)

Hive: Filtering Data between Specified Dates when Date is a String

You have to convert string formate to required date format as following and then you can get your required result.

hive> select * from salesdata01 where from_unixtime(unix_timestamp(Order_date, 'dd-MM-yyyy'),'yyyy-MM-dd') >= from_unixtime(unix_timestamp('2010-09-01', 'yyyy-MM-dd'),'yyyy-MM-dd') and from_unixtime(unix_timestamp(Order_date, 'dd-MM-yyyy'),'yyyy-MM-dd') <= from_unixtime(unix_timestamp('2011-09-01', 'yyyy-MM-dd'),'yyyy-MM-dd') limit 10;
1   3   13-10-2010  Low 6.0 261.54  0.04    Regular Air -213.25 38.94
80  483 10-07-2011  High    30.0    4965.7593   0.08    Regular Air 1198.97 195.99
97  613 17-06-2011  High    12.0    93.54   0.03    Regular Air -54.04  7.3
98  613 17-06-2011  High    22.0    905.08  0.09    Regular Air 127.7   42.76
103 643 24-03-2011  High    21.0    2781.82 0.07    Express Air -695.26 138.14
127 807 23-11-2010  Medium  45.0    196.85  0.01    Regular Air -166.85 4.28
128 807 23-11-2010  Medium  32.0    124.56  0.04    Regular Air -14.33  3.95
160 995 30-05-2011  Medium  46.0    1815.49 0.03    Regular Air 782.91  39.89
229 1539    09-03-2011  Low 33.0    511.83  0.1 Regular Air -172.88 15.99
230 1539    09-03-2011  Low 38.0    184.99  0.05    Regular Air -144.55 4.89
Time taken: 0.166 seconds, Fetched: 10 row(s)
hive> select * from salesdata01 where from_unixtime(unix_timestamp(Order_date, 'dd-MM-yyyy'),'yyyy-MM-dd') >= from_unixtime(unix_timestamp('2010-09-01', 'yyyy-MM-dd'),'yyyy-MM-dd') and from_unixtime(unix_timestamp(Order_date, 'dd-MM-yyyy'),'yyyy-MM-dd') <= from_unixtime(unix_timestamp('2010-12-01', 'yyyy-MM-dd'),'yyyy-MM-dd') limit 10;
1   3   13-10-2010  Low 6.0 261.54  0.04    Regular Air -213.25 38.94
127 807 23-11-2010  Medium  45.0    196.85  0.01    Regular Air -166.85 4.28
128 807 23-11-2010  Medium  32.0    124.56  0.04    Regular Air -14.33  3.95
256 1792    08-11-2010  Low 28.0    370.48  0.04    Regular Air -5.45   13.48
381 2631    23-09-2010  Low 27.0    1078.49 0.08    Regular Air 252.66  40.96
656 4612    19-09-2010  Medium  9.0 89.55   0.06    Regular Air -375.64 4.48
769 5506    07-11-2010  Critical    22.0    129.62  0.05    Regular Air 4.41    5.88
1457    10499   16-11-2010  Not Specified   29.0    6250.936    0.01    Delivery Truck  31.21   262.11
1654    11911   10-11-2010  Critical    25.0    397.84  0.0 Regular Air -14.75  15.22
2323    16741   30-09-2010  Medium  6.0 157.97  0.01    Regular Air -42.38  22.84
Time taken: 0.17 seconds, Fetched: 10 row(s)

Link to reload current page

For me, the answers previously provided here, were opening a new tab/window (probably because of my browser settings). But i wanted to reload/refresh on the same page. So, the above solutions did not work for me.

However, the good news is, the following (either of two) worked for me.

  1. <a onclick="javascript:window.location.reload();"> refresh</a>
  2. <a onclick="window.location.href=this">refresh</a>

How to print table using Javascript?

My fellows,

In January 2019 I used a code made before:

 <script type="text/javascript">   
    function imprimir() {
        var divToPrint=document.getElementById("ConsutaBPM");

To undestand: ConsutaBPM is a DIV which contains inside phrases and tables. I wanted to print ALL, titles, table, and others. The problem was when TRIED to print the TABLE...

The table mas be defined with BORDER and CELLPADDING:

<table border='1' cellpadding='1' id='Tablbpm1' >

It worked fine!!!

How can I get Git to follow symlinks?

NOTE: This advice is now out-dated as per comment since Git 1.6.1. Git used to behave this way, and no longer does.

Git by default attempts to store symlinks instead of following them (for compactness, and it's generally what people want).

However, I accidentally managed to get it to add files beyond the symlink when the symlink is a directory.


  /bar/foo --> /foo

by doing

 git add /bar/foo/baz

it appeared to work when I tried it. That behavior was however unwanted by me at the time, so I can't give you information beyond that.

Concatenate text files with Windows command line, dropping leading lines

I know you said that you couldn't install any software, but I'm not sure how tight that restriction is. Anyway, I had the same issue (trying to concatenate two files with presumably the same headers) and I thought I'd provide an alternative answer for others who arrive at this page, since it worked just great for me.

After trying a whole bunch of commands in windows and being severely frustrated, and also trying all sorts of graphical editors that promised to be able to open large files, but then couldn't, I finally got back to my Linux roots and opened my Cygwin prompt. Two commands:

cp file1.csv out.csv
tail -n+2 file2.csv >> out.csv

For file1.csv 800MB and file2.csv 400MB, those two commands took under 5 seconds on my machine. In a Cygwin prompt, no less. I thought Linux commands were supposed to be slow in Cygwin but that approach took far less effort and was way easier than any windows approach I could find.

Difference between Hive internal tables and external tables?

Consider this scenario which best suits for External Table:

A MapReduce (MR) job filters a huge log file to spit out n sub log files (e.g. each sub log file contains a specific message type log) and the output i.e n sub log files are stored in hdfs.

These log files are to be loaded into Hive tables for performing further analytic, in this scenario I would recommend an External Table(s), because the actual log files are generated and owned by an external process i.e. a MR job besides you can avoid an additional step of loading each generated log file into respective Hive table as well.

Check cell for a specific letter or set of letters

Just use = IF(A1="Bla*","YES","NO"). When you insert the asterisk, it acts as a wild card for any amount of characters after the specified text.

Nested classes' scope?

You might be better off if you just don't use nested classes. If you must nest, try this:

x = 1
class OuterClass:
    outer_var = x
    class InnerClass:
        inner_var = x

Or declare both classes before nesting them:

class OuterClass:
    outer_var = 1

class InnerClass:
    inner_var = OuterClass.outer_var

OuterClass.InnerClass = InnerClass

(After this you can del InnerClass if you need to.)

Escape quote in web.config connection string

Use &quot; instead of " to escape it.

web.config is an XML file so you should use XML escaping.

connectionString="Server=dbsrv;User ID=myDbUser;Password=somepass&quot;word"

See this forum thread.


&quot; should work, but as it doesn't, have you tried some of the other string escape sequences for .NET? \" and ""?

Update 2:

Try single quotes for the connectionString:

connectionString='Server=dbsrv;User ID=myDbUser;Password=somepass"word'


connectionString='Server=dbsrv;User ID=myDbUser;Password=somepass&quot;word'

Update 3:

From MSDN (SqlConnection.ConnectionString Property):

To include values that contain a semicolon, single-quote character, or double-quote character, the value must be enclosed in double quotation marks. If the value contains both a semicolon and a double-quote character, the value can be enclosed in single quotation marks.


connectionString="Server=dbsrv;User ID=myDbUser;Password='somepass&quot;word'"

The issue is not with web.config, but the format of the connection string. In a connection string, if you have a " in a value (of the key-value pair), you need to enclose the value in '. So, while Password=somepass"word does not work, Password='somepass"word' does.

How to run Maven from another directory (without cd to project dir)?

You can use the parameter -f (or --file) and specify the path to your pom file, e.g. mvn -f /path/to/pom.xml

This runs maven "as if" it were in /path/to for the working directory.

Group dataframe and get sum AND count?

Just in case you were wondering how to rename columns during aggregation, here's how for

pandas >= 0.25: Named Aggregation

df.groupby('Company Name')['Amount'].agg(MySum='sum', MyCount='count')


df.groupby('Company Name').agg(MySum=('Amount', 'sum'), MyCount=('Amount', 'count'))

                       MySum  MyCount
Company Name                       
Vifor Pharma UK Ltd  4207.93        5

How can I set the opacity or transparency of a Panel in WinForms?

This does work for me. In below example, Alpha range can be a value between 0 to 255. Previously, I made a mistake by thinking that it must be a value of percentage.

Dim x as integer = 230 Panel1.BackColor = Color.FromArgb(x, Color.Blue)

tkinter: how to use after method

I believe, the 500ms run in the background, while the rest of the code continues to execute and empties the list.

Then after 500ms nothing happens, as no function-call is implemented in the after-callup (same as frame.after(500, function=None))

How to add a TextView to LinearLayout in Android

LinearLayout.LayoutParams layoutParams ;
layoutParams= new LinearLayout.LayoutParams(LinearLayout.LayoutParams.FILL_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);

How to export all collections in MongoDB?

you can create zip file by using following command .It will create zip file of database {dbname} provided.You can later import the following zip file in you mongo DB.

Window filepath=C:\Users\Username\mongo 

mongodump --archive={filepath}\+{filename}.gz --gzip --db {dbname}

Error: Selection does not contain a main type

I hope you are trying to run the main class in this way, see screenshot:
screenshot of Eclipse file context menu

If not, then try this way. If yes, then please make sure that your class you are trying to run has a main method, that is, the same method definition as below:

public static void main(String[] args) {
    // some code here

I hope this will help you.

Installing Apache Maven Plugin for Eclipse

Eclipse > Help > Eclipse Marketplace...

Search for m2e

Install Maven Integration for Eclipse (Juno and newer). [It works for Indigo also]

PHPExcel Make first row bold


That way you get the complete first row

How do I determine k when using k-means clustering?

First build a minimum spanning tree of your data. Removing the K-1 most expensive edges splits the tree into K clusters,
so you can build the MST once, look at cluster spacings / metrics for various K, and take the knee of the curve.

This works only for Single-linkage_clustering, but for that it's fast and easy. Plus, MSTs make good visuals.
See for example the MST plot under stats.stackexchange visualization software for clustering.

How to enable php7 module in apache?

I found the solution on the following thread :

Im my case not only the php wasn't working but phpmyadmin aswell i did step by step like that

sudo apt install php libapache2-mod-php
sudo apt install php7.0-mbstring
sudo a2dismod mpm_event
sudo a2enmod mpm_prefork
service apache2 restart

And then to:

gksu gedit /etc/apache2/apache2.conf

In the last line I do add Include /etc/phpmyadmin/apache.conf

That make a deal with all problems


If it solves your problem, up vote this solution in the original post.

Does an HTTP Status code of 0 have any meaning?

from documentation means a request was cancelled before going anywhere

How can I find a specific element in a List<T>?

You can solve your problem most concisely with a predicate written using anonymous method syntax:

MyClass found = list.Find(item => item.GetID() == ID);

On linux SUSE or RedHat, how do I load Python 2.7

If you want to install Python 2.7 on Oracle Linux, you can proceed as follows:

Enable the Software Collection in /etc/yum.repos.d/public-yum-ol6.repo.

vim /etc/yum.repos.d/public-yum-ol6.repo

name=Software Collection Library release 1.2 packages for Oracle Linux 6 
    enabled=1 <==============change from 0 to 1

After making this change to the yum repository you can simply run the yum command to install the Python:

yum install gcc libffi libffi-devel python27 python27-python-devel openssl-devel python27-MySQL-python

edit bash_profile with follow variables:

vim ~/.bash_profile
PATH=$PATH:$HOME/bin:/opt/rh/python27/root/usr/bin export PATH
LD_LIBRARY_PATH=/opt/rh/python27/root/usr/lib64 export LD_LIBRARY_PATH
PKG_CONFIG_PATH=/opt/rh/python27/root/usr/lib64/pkgconfig export PKG_CONFIG_PATH

Now you can use python2.7 and pip for install Python modules:

/opt/rh/python27/root/usr/bin/pip install pynacl
/opt/rh/python27/root/usr/bin/python2.7 --version

How to configure "Shorten command line" method for whole project in IntelliJ

Thanks to Rajesh Goel in Android Studio:

Run > Edit Configurations...

enter image description here

Select a test (better to select a parent test class) and set a Shorten command line: option to classpath file. Then OK (or Apply, OK).

enter image description here

Oracle PL/SQL string compare issue

To the first question:

Probably the message wasn't print out because you have the output turned off. Use these commands to turn it back on:

set serveroutput on
exec dbms_output.enable(1000000);

On the second question:

My PLSQL is quite rusty so I can't give you a full snippet, but you'll need to loop over the result set of the SQL query and CONCAT all the strings together.

How to add new column to MYSQL table?

Something like:

$db = mysqli_connect("localhost", "user", "password", "database");
$name = $db->mysqli_real_escape_string($name);
$query = 'ALTER TABLE assesment ADD ' . $name . ' TINYINT NOT NULL DEFAULT \'0\'';
if($db->query($query)) {
    echo "It worked";

Haven't tested it but should work.

Get safe area inset top and bottom heights

In iOS 11 there is a method that tells when the safeArea has changed.

override func viewSafeAreaInsetsDidChange() {
    let top =
    let bottom = view.safeAreaInsets.bottom

View markdown files offline

An easy solution for most situations: copy/paste the markdown into a viewer in the "cloud." Here are two choices:

  2. Dingus

Nothing to install! Cross platform! Cross browser! Always available!

Disadvantages: could be hassle for large files, standard cloud application security issues.

Force flushing of output to a file while bash script is still running

script -c <PROGRAM> -f OUTPUT.txt

Key is -f. Quote from man script:

-f, --flush
     Flush output after each write.  This is nice for telecooperation: one person
     does 'mkfifo foo; script -f foo', and another can supervise real-time what is
     being done using 'cat foo'.

Run in background:

nohup script -c <PROGRAM> -f OUTPUT.txt

How do I access Configuration in any class in ASP.NET Core?

Using the Options pattern in ASP.NET Core is the way to go. I just want to add, if you need to access the options within your startup.cs, I recommend to do it this way:


public class CosmosDbOptions
    public string ConnectionString { get; set; }


public void ConfigureServices(IServiceCollection services)
    // This is how you can access the Connection String:
    var connectionString = Configuration.GetSection(nameof(CosmosDbOptions))[nameof(CosmosDbOptions.ConnectionString)];

HTML how to clear input using javascript?

you can use attribute placeholder

<input type="text" name="email" placeholder="[email protected]" size="30" />

or try this for older browsers

<input type="text" name="email" value="[email protected]" size="30" onblur="if(this.value==''){this.value='[email protected]';}" onfocus="if(this.value=='[email protected]'){this.value='';}">

Exception is never thrown in body of corresponding try statement

Always remember that in case of checked exception you can catch only after throwing the exception(either you throw or any inbuilt method used in your code can throw) ,but in case of unchecked exception You an catch even when you have not thrown that exception.

How can I create a war file of my project in NetBeans?

Simplest way is to Check the Output - Build tab: It would display the location of war file.
It will have something like:

Installing D:\Project\target\Tool.war to C:\Users\myname.m2\repository\com\tool\1.0\Tool-1.0.war

Confirmation before closing of tab/browser

I read comments on answer set as Okay. Most of the user are asking that the button and some links click should be allowed. Here one more line is added to the existing code that will work.

<script type="text/javascript">
  var hook = true;
  window.onbeforeunload = function() {
    if (hook) {

      return "Did you save your stuff?"
  function unhook() {

Call unhook() onClick for button and links which you want to allow. E.g.

<a href="#" onClick="unhook()">This link will allow navigation</a>

Testing socket connection in Python

12 years later for anyone having similar problems.

    s.connect((address, '80'))
    alert('failed' + address, 'down')

doesn't work because the port '80' is a string. Your port needs to be int.

    s.connect((address, 80))

This should work. Not sure why even the best answer didnt see this.

Open page in new window without popup blocking

This is the only one that actually worked for me in all the browsers

let newTab =; newTab.location.href = url;

Using the "animated circle" in an ImageView while loading stuff

You can do this by using the following xml


With this style

<style name="GenericProgressBackground" parent="android:Theme">
    <item name="android:layout_width">fill_parent</item>    
    <item name="android:layout_height">fill_parent</item>
    <item name="android:background">#DD111111</item>    
    <item name="android:gravity">center</item>  
<style name="GenericProgressIndicator" parent="@android:style/Widget.ProgressBar.Small">
    <item name="android:layout_width">wrap_content</item>
    <item name="android:layout_height">wrap_content</item>
    <item name="android:indeterminate">true</item> 

To use this, you must hide your UI elements by setting the visibility value to GONE and whenever the data is loaded, call setVisibility(View.VISIBLE) on all your views to restore them. Don't forget to call findViewById( to hide the loading animation.

If you dont have a loading event/function but just want the loading panel to disappear after x seconds use a Handle to trigger the hiding/showing.

TypeError: 'function' object is not subscriptable - Python

You can use this:

bankHoliday= [1, 0, 1, 1, 2, 0, 0, 1, 0, 0, 0, 2] #gives the list of bank holidays in each month

def bank_holiday(month):
   month -= 1#Takes away the numbers from the months, as months start at 1 (January) not at 0. There is no 0 month.

bank_holiday(int(input("Which month would you like to check out: ")))

Creating a simple XML file using python

For such a simple XML structure, you may not want to involve a full blown XML module. Consider a string template for the simplest structures, or Jinja for something a little more complex. Jinja can handle looping over a list of data to produce the inner xml of your document list. That is a bit trickier with raw python string templates

For a Jinja example, see my answer to a similar question.

Here is an example of generating your xml with string templates.

import string
from xml.sax.saxutils import escape

inner_template = string.Template('    <field${id} name="${name}">${value}</field${id}>')

outer_template = string.Template("""<root>

data = [
    (1, 'foo', 'The value for the foo document'),
    (2, 'bar', 'The <value> for the <bar> document'),

inner_contents = [inner_template.substitute(id=id, name=name, value=escape(value)) for (id, name, value) in data]
result = outer_template.substitute(document_list='\n'.join(inner_contents))
print result


    <field1 name="foo">The value for the foo document</field1>
    <field2 name="bar">The &lt;value&gt; for the &lt;bar&gt; document</field2>

The downer of the template approach is that you won't get escaping of < and > for free. I danced around that problem by pulling in a util from xml.sax

How to convert date to timestamp?

Split the string into its parts and provide them directly to the Date constructor:


var myDate = "26-02-2012";
myDate = myDate.split("-");
var newDate = new Date( myDate[2], myDate[1] - 1, myDate[0]);

How to have comments in IntelliSense for function in Visual Studio?

All these others answers make sense, but are incomplete. Visual Studio will process XML comments but you have to turn them on. Here's how to do that:

Intellisense will use XML comments you enter in your source code, but you must have them enabled through Visual Studio Options. Go to Tools > Options > Text Editor. For Visual Basic, enable the Advanced > Generate XML documentation comments for ''' setting. For C#, enable the Advanced > Generate XML documentation comments for /// setting. Intellisense will use the summary comments when entered. They will be available from other projects after the referenced project is compiled.

To create external documentation, you need to generate an XML file through the Project Settings > Build > XML documentation file: path that controls the compiler's /doc option. You will need an external tool that will take the XML file as input and generate the documentation in your choice of output formats.

Be aware that generating the XML file can noticeably increase your compile time.

How to do URL decoding in Java?

This does not have anything to do with character encodings such as UTF-8 or ASCII. The string you have there is URL encoded. This kind of encoding is something entirely different than character encoding.

Try something like this:

try {
    String result =,;
} catch (UnsupportedEncodingException e) {
    // not going to happen - value came from JDK's own StandardCharsets

Java 10 added direct support for Charset to the API, meaning there's no need to catch UnsupportedEncodingException:

String result =, StandardCharsets.UTF_8);

Note that a character encoding (such as UTF-8 or ASCII) is what determines the mapping of characters to raw bytes. For a good intro to character encodings, see this article.

Error Importing SSL certificate : Not an X.509 Certificate

Does your cacerts.pem file hold a single certificate? Since it is a PEM, have a look at it (with a text editor), it should start with


and end with


Finally, to check it is not corrupted, get hold of openssl and print its details using

openssl x509 -in cacerts.pem -text

Determine if $.ajax error is a timeout

If your error event handler takes the three arguments (xmlhttprequest, textstatus, and message) when a timeout happens, the status arg will be 'timeout'.

Per the jQuery documentation:

Possible values for the second argument (besides null) are "timeout", "error", "notmodified" and "parsererror".

You can handle your error accordingly then.

I created this fiddle that demonstrates this.

    url: "/ajax_json_echo/",
    type: "GET",
    dataType: "json",
    timeout: 1000,
    success: function(response) { alert(response); },
    error: function(xmlhttprequest, textstatus, message) {
        if(textstatus==="timeout") {
            alert("got timeout");
        } else {

With jsFiddle, you can test ajax calls -- it will wait 2 seconds before responding. I put the timeout setting at 1 second, so it should error out and pass back a textstatus of 'timeout' to the error handler.

Hope this helps!

Correct path for img on React.js

  1. Make an images folder inside src(/src/images) And keep your image in it. Then import this image in your component(use your relative path). Like below-

    import imageSrc from './images/image-name.jpg';

    And then in your component.

    <img title="my-img" src={imageSrc} alt="my-img" />

  2. Another way is to keep images in public folder and import them using relative path. For this make an image folder in public folder and keep your image in it. And then in your component use it like below.

    <img title="my-img" src='/images/my-image.jpg' alt="my-img" />

    Both method work but first one is recommended because its cleaner way and images are handled by webpack during build time.

JAX-WS and BASIC authentication, when user names and passwords are in a database

BindingProvider.USERNAME_PROPERTY and BindingProvider.PASSWORD_PROPERTY are matching HTTP Basic Authentication mechanism that enable authentication process at the HTTP level and not at the application nor servlet level.

Basically, only the HTTP server will know the username and the password (and eventually application according to HTTP/application server specification, such with Apache/PHP). With Tomcat/Java, add a login config BASIC in your web.xml and appropriate security-constraint/security-roles (roles that will be later associated to users/groups of real users).


Then, connect the realm at the HTTP server (or application server) level with the appropriate user repository. For tomcat you may look at JAASRealm, JDBCRealm or DataSourceRealm that may suit your needs.

Rails: Address already in use - bind(2) (Errno::EADDRINUSE)

If the above solutions don't work on ubuntu/linux then you can try this

sudo fuser -k -n tcp port

Run it several times to kill processes on your port of choosing. port could be 3000 for example. You would have killed all the processes if you see no output after running the command