What are file descriptors explained in simple terms

Question

What would be a more simplified description of file descriptors compared to Wikipedia s  Why are they required  Say  take shell processes as an example and how does it apply for it  Does a process table contain more than one file descriptor  If yes  why

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File descriptors are nothing but references for any open resource. As soon as you open a resource the kernel assumes you will be doing some operations on it. All the communication via your program and the resource happens over an interface and this interface is provided by the file-descriptor.

Since a process can open more than one resource, it is possible for a resource to have more than one file-descriptors.
You can view all file-descriptors linked to the process by simply running, ls -li /proc/<pid>/fd/ here pid is the process-id of your process

User · Answer

A file descriptor is an opaque handle that is used in the interface between user and kernel space to identify file socket resources  Therefore  when you use open   or socket    system calls to interface to the kernel   you are given a file descriptor  which is an integer  it is actually an index into the processes u structure - but that is not important   Therefore  if you want to interface directly with the kernel  using system calls to read    write    close   etc  the handle you use is a file descriptor   There is a layer of abstraction overlaid on the system calls  which is the stdio interface  This provides more functionality features than the basic system calls do  For this interface  the opaque handle you get is a FILE   which is returned by the fopen   call  There are many many functions that use the stdio interface fprintf    fscanf    fclose    which are there to make your life easier  In C  stdin  stdout  and stderr are FILE   which in UNIX respectively map to file descriptors 0  1 and 2

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Any operating system has processes  p s  running  say p1  p2  p3 and so forth  Each process usually makes an ongoing usage of files   Each process is consisted of a process tree  or a process table  in another phrasing    Usually  Operating systems represent each file in each process by a number  that is to say  in each process tree table    The first file used in the process is file0  second is file1  third is file2  and so forth   Any such number is a file descriptor   File descriptors are usually integers  0  1  2 and not 0 5  1 5  2 5    Given we often describe processes as  process-tables   and given that tables has rows  entries  we can say that the file descriptor cell in each entry  uses to represent the whole entry   In a similar way  when you open a network socket  it has a socket descriptor   In some operating systems  you can run out of file descriptors  but such case is extremely rare  and the average computer user shouldn t worry from that   File descriptors might be global  process A starts in say 0  and ends say in 1   Process B starts say in 2  and ends say in 3  and so forth  but as far as I know  usually in modern operating systems  file descriptors are not global  and are actually process-specific  process A starts in say 0 and ends say in 5  while process B starts in 0 and ends say in 10

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Other answers added great stuff  I will add just my 2 cents    According to Wikipedia we know for sure  a file descriptor is a non-negative integer  The most important thing I think is missing  would be to say   File descriptors are bound to a process ID   We know most famous file descriptors are 0  1 and 2   0 corresponds to STDIN  1 to STDOUT  and 2 to STDERR      Say  take shell processes as an example and how does it apply for it    Check out this code    gt sleep 1000  amp   12  14726   We created a process with the id 14726  PID   Using the lsof -p 14726 we can get the things like this   COMMAND   PID USER   FD   TYPE DEVICE SIZE OFF    NODE NAME sleep   14726 root  cwd    DIR    8 1     4096 1201140  home x sleep   14726 root  rtd    DIR    8 1     4096       2   sleep   14726 root  txt    REG    8 1    35000  786587  bin sleep sleep   14726 root  mem    REG    8 1 11864720 1186503  usr lib locale locale-archive sleep   14726 root  mem    REG    8 1  2030544  137184  lib x86 64-linux-gnu libc-2 27 so sleep   14726 root  mem    REG    8 1   170960  137156  lib x86 64-linux-gnu ld-2 27 so sleep   14726 root    0u   CHR  136 6      0t0       9  dev pts 6 sleep   14726 root    1u   CHR  136 6      0t0       9  dev pts 6 sleep   14726 root    2u   CHR  136 6      0t0       9  dev pts 6   The 4-th column FD and the very next column TYPE correspond to the File Descriptor and the File Descriptor type   Some of the values for the FD can be   cwd     Current Working Directory txt     Text file mem     Memory mapped file mmap     Memory mapped device   But the real file descriptor is under   NUMBER     Represent the actual file descriptor     The character after the number i e  1u   represents the mode in which the file is opened  r for read  w for write  u for read and write   TYPE specifies the type of the file  Some of the values of TYPEs are   REG     Regular File DIR     Directory FIFO     First In First Out   But all file descriptors are      CHR     Character special file  or character device file   Now  we can identify the File Descriptors for STDIN  STDOUT and STDERR easy with lsof -p PID  or we can see the same if we ls  proc PID fd   Note also that file descriptor table that kernel keeps track of is not the same as files table or inodes table  These are separate  as some other answers explained     You may ask yourself where are these file descriptors physically and what is stored in  dev pts 6 for instance  sleep   14726 root    0u   CHR  136 6      0t0       9  dev pts 6 sleep   14726 root    1u   CHR  136 6      0t0       9  dev pts 6 sleep   14726 root    2u   CHR  136 6      0t0       9  dev pts 6   Well   dev pts 6 lives purely in memory  These are not regular files  but so called character device files  You can check this with  ls -l  dev pts 6 and they will start with c  in my case crw--w----   Just to recall most Linux like OS define seven types of files     Regular files  Directories  Character device files  Block device files Local domain sockets  Named pipes  FIFOs  and Symbolic links

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File descriptors         To Kernel all open files are referred to by file descriptors       A file descriptor is a non - negative integer      When we open an existing or create a new file  the kernel returns a file descriptor to a process       When we want to read or write on a file  we identify the file with file descriptor that was retuned by open or create  as an argument to either read or write            Each UNIX process has 20 file descriptors and it disposal  numbered 0 through 19 but      it was extended to 63 by many systems           The first three are already opened when the process begins        0  The standard input        1  The standard output        2  The standard error output           When the parent process forks a process  the child process inherits the file descriptors of the parent

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File Descriptors  FD      In Linux Unix  everything is a file  Regular file  Directories  and even Devices are files  Every File has an associated number called File Descriptor  FD   Your screen also has a File Descriptor  When a program is executed the output is sent to File Descriptor of the screen  and you see program output on your monitor  If the output is sent to File Descriptor of the printer  the program output would have been printed  Error Redirection   Whenever you execute a program command at the terminal  3 files are always open standard input standard output standard error  These files are always present whenever a program is run  As explained before a file descriptor  is associated with each of these files   File nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  File Descriptor Standard Input STDIN  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp 0 Standard Output STDOUT  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp 1 Standard Error STDERR  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp 2  For instance  while searching for files  one typically gets permission denied errors or some other kind of errors  These errors can be saved to a  particular file  Example 1        ls mydir 2 errorsfile txt   The file descriptor for standard error is 2  If there is no any directory named as mydir then the output of command will be save to file errorfile txt Using  2   we re-direct the error output to a file named  errorfile txt  Thus  program output is not cluttered with errors  I hope you got your answer

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As an addition to other answers  unix considers everything as a file system  Your keyboard is a file that is read only from the perspective of the kernel  The screen is a write only file  Similarly  folders  input-output devices etc are also considered to be files  Whenever a file is opened  say when the device drivers for device files  requests an open    or a process opens an user file the kernel allocates a file descriptor  an integer that specifies the access to that file such it being read only  write only etc   for reference   https   en wikipedia org wiki Everything is a file

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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  quot test txt quot   use the file descriptor as show below  FILE  1   give the name of file in the command line exec 5 lt  gt  FILE    5  here act as the file descriptor    Reading from the file line by line using file descriptor while read LINE  do     echo  quot  LINE quot  done  lt  amp 5    Writing to the file using descriptor echo  quot Adding the date   date  quot   gt  amp 5  exec 5 lt  amp -   Closing a file descriptor

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Hear it from the Horse s Mouth   APUE  Richard Stevens   To the kernel  all open files are referred to by File Descriptors  A file descriptor is a non-negative number     When we open an existing file or create a new file  the kernel returns a file descriptor to the process   The kernel maintains a table of all open file descriptors  which are in use  The allotment of file descriptors is generally sequential and they are allotted to the file as the next free file descriptor from the pool of free file descriptors  When we closes the file  the file descriptor gets freed and is available for further allotment  See this image for more details        When we want to read or write a file  we identify the file with the file descriptor that was returned by open   or create   function call  and use it as an argument to either read   or write    It is by convention that  UNIX System shells associates the file descriptor 0 with Standard Input of a process  file descriptor 1 with Standard Output  and file descriptor 2 with Standard Error  File descriptor ranges from 0 to OPEN MAX  File descriptor max value can be obtained with ulimit -n  For more information  go through 3rd chapter of APUE Book

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In simple words  when you open a file  the operating system creates an entry to represent that file and store the information about that opened file  So if there are 100 files opened in your OS then there will be 100 entries in OS  somewhere in kernel   These entries are represented by integers like     100  101  102       This entry number is the file descriptor  So it is just an integer number that uniquely represents an opened file in operating system  If your process opens 10 files then your Process table will have 10 entries for file descriptors   Similarly when you open a network socket  it is also represented by an integer and it is called Socket Descriptor  I hope you understand

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More points regarding File Descriptor    File Descriptors  FD  are non-negative integers  0  1  2       that are associated with files that are opened  0  1  2 are standard FD s that corresponds to STDIN FILENO  STDOUT FILENO and STDERR FILENO  defined in unistd h  opened by default on behalf of shell when the program starts  FD s are allocated in the sequential order  meaning the lowest possible unallocated integer value  FD s for a particular process can be seen in  proc  pid fd  on Unix based systems

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File Descriptors are the descriptors to a file  They give links to a file  With the help of them we can read  write and open a file

[unix] What are file descriptors, explained in simple terms?

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