What is dev null 2 1

Question

I found this piece of code in  etc cron daily apf         bin bash    etc apf apf -f  gt  gt   dev null 2 gt  amp 1    etc apf apf -s  gt  gt   dev null 2 gt  amp 1     It s flushing and reloading the firewall  I don t understand the  gt  gt   dev null 2 gt  amp 1 part     What is the purpose of having this in the cron  It s overriding my firewall rules   Can I safely remove this cron job

User · Answer

This is the way to execute a program quietly, and hide all its output.

/dev/null is a special filesystem object that discards everything written into it. Redirecting a stream into it means hiding your program's output.

The 2>&1 part means "redirect the error stream into the output stream", so when you redirect the output stream, error stream gets redirected as well. Even if your program writes to stderr now, that output would be discarded as well.

User · Answer

gt  gt   dev null redirects standard output  stdout  to  dev null  which discards it    The  gt  gt  seems sort of superfluous  since  gt  gt  means append while  gt  means truncate and write  and either appending to or writing to  dev null has the same net effect  I usually just use  gt  for that reason    2 gt  amp 1 redirects standard error  2  to standard output  1   which then discards it as well since standard output has already been redirected

User · Answer

dev null is a standard file that discards all you write to it  but reports that the write operation succeeded   1 is standard output and 2 is standard error   2 gt  amp 1 redirects standard error to standard output   amp 1 indicates file descriptor  standard output   otherwise  if you use just 1  you will redirect standard error to a file named 1   any command   gt  gt  dev null 2 gt  amp 1 redirects all standard error to standard output  and writes all of that to  dev null

User · Answer

Edit  etc conf apf  Set DEVEL MODE  0   DEVEL MODE set to 1 will add a cron job to stop apf after 5 minutes

User · Answer

Let s break  gt  gt   dev null 2 gt  amp 1 statement into parts     Part 1   gt  gt  output redirection  This is used to redirect the program output and append the output at the end of the file  More       Part 2   dev null special file  This is a Pseudo-devices special file   Command ls -l  dev null will give you details of this file   crw-rw-rw-  1 root root 1  3 Mar 20 18 37  dev null   Did you observe crw  Which means it is a pseudo-device file which is of character-special-file type that provides serial access       dev null accepts and discards all input  produces no output  always returns an end-of-file indication on a read   Reference  Wikipedia     Part 3  2 gt  amp 1 file descriptor  Whenever you execute a program  the operating system always opens three files  standard input  standard output  and standard error as we know whenever a file is opened  the operating system  from kernel  returns a non-negative integer called a file descriptor  The file descriptor for these files are 0  1  and 2  respectively   So 2 gt  amp 1 simply says redirect standard error to standard output       amp  means whatever follows is a file descriptor  not a filename    In short  by using this command you are telling your program not to shout while executing   What is the importance of using 2 gt  amp 1   If you don t want to produce any output  even in case of some error produced in the terminal  To explain more clearly  let s consider the following example     ls -l  gt   dev null   For the above command  no output was printed in the terminal  but what if this command produces an error     ls -l file doesnot exists  gt   dev null ls  cannot access file doesnot exists  No such file or directory   Despite I m redirecting output to  dev null  it is printed in the terminal  It is because we are not redirecting error output to  dev null  so in order to redirect error output as well  it is required to add 2 gt  amp 1     ls -l file doesnot exists  gt   dev null 2 gt  amp 1

User · Answer

As described by the others  writing to  dev null eliminates the output of a program  Usually cron sends an email for every output from the process started with a cronjob  So by writing the output to  dev null you prevent being spammed if you have specified your adress in cron

User · Answer

I use  gt  gt   dev null 2 gt  amp 1 for a silent cronjob  A cronjob will do the job  but not send a report to my email   As far as I know  don t remove  dev null  It s useful  especially when you run cPanel  it can be used for throw-away cronjob reports

User · Answer

Let me explain a bit by bit  0 1 2 0  standard input 1  standard output 2  standard error      gt  gt  in command  gt  gt   dev null 2 gt  amp 1 appends the command output to  dev null  command     dev null 2  1  After command   command   gt  1 output on the terminal screen   gt  2 output on the terminal screen   After redirect   command  gt  gt   dev null   gt  1 output to  dev null   gt  2 output on the terminal screen   After  dev null 2 gt  amp 1  command  gt  gt   dev null 2 gt  amp 1   gt  1 output to  dev null   gt  2 output is redirected to 1 which is now to  dev null

[shell] What is /dev/null 2>&1?

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