linux_xargs.md

• XArgs
  • -t
  • -I alias
  • -n number
  • -P maxprocs

XArgs (eXtended arguments)

Reads items from standard input (stdin) and builds command-line arguments from them.

stdin → argument list → command execution

• some shell commands can not take stdin and use that as a parameter, to do this you need another program to help with this and that is what xargs command does.
• xargs allows to take standard input and pass that along as an argument into another command.
  • stdin -> argument

$ seq 5 | echo  # echo can not take stdin as parameter

$ seq 5 | xargs echo  # xargs can help
1 2 3 4 5

-t

Echo the command to be executed to standard error immediately before it is executed.

$ seq 5 | xargs -t echo
echo 1 2 3 4 5
1 2 3 4 5

-I alias

specify a placeholder, basically an alias for the arguments.

$ ls | xargs -I {} echo /home/dt/{}
/home/dt/Applications
/home/dt/Desktop
/home/dt/Documents
/home/dt/Downloads
...

you can specify anything as the placeholder, as the alias

$ ls | xargs -I zzz echo /home/dt/zzz
/home/dt/Applications
/home/dt/Desktop
/home/dt/Documents
/home/dt/Downloads
...

the replstr of alias has max-length limit, default is 255, it the length of replstr is greater than its limit, the alias will not match, to solve this problem, you may specify the max-lenght

xargs -S 10240 -I {} ...

-n number

Set the maximum number of arguments taken from standard input for each invocation of utility.

$ ls | xargs
Applications Desktop Documents Downloads Google Drive Library Movies Music Pictures Public go

$ ls | xargs -n 1
Applications
Desktop
Documents
Downloads
...

-P maxprocs

Parallel mode: run at most maxprocs invocations of utility at once. If maxprocs is set to 0, xargs will run as many processes as possible.

$ seq 5 | xargs -n 1 -P 2 bash -c 'echo $0;sleep 1'
# execution will took 3 seconds

If maxprocs is set to 0, xargs will run as many processes as possible.

Very Important Option: -0

• Regular xargs splits on whitespace.
• If filenames contain spaces, it's dangerous.
• Correct way:

  find . -name "*.log" -print0 | xargs -0 rm

• Here:
  • -print0 separates results with null character
  • -0 tells xargs to split on null