How to use defined function with xargs

Question

This is my code

#!/bin/bash

showword() {
  echo $1
}

echo This is a sample message | xargs -d' ' -t -n1 -P2 showword

So I have a function showword which echoes whatever string you pass as a parameter to the function.

Then I have xargs trying to call the function and pass one word at a time to the function, and run 2 copies of the function in parallel. The thing that is not working is xargs doesn't recognize the function. How can I achieve what I am trying to do, how can I make xargs work with the function showword?

Answer 1

Try exporting the function, then calling it in a subshell:

showword() {
  echo $1
}

export -f showword
echo This is a sample message | xargs -d' ' -t -n1 -P2 bash -c 'showword "$@"' _

This causes xargs to execute

bash -c 'showword "$@"' _ This
bash -c 'showword "$@"' _ is
bash -c 'showword "$@"' _ a
            ︙

The arguments passed to the bash command are, well, passed into the bash environment, but starting from 0.  So, inside the function,

• $0 is “_” and $1 is “This”
• $0 is “_” and $1 is “is”
• $0 is “_” and $1 is “a”
•       ︙

See Bash -c with positional parameters.

Note that export -f works only in Bash, and -Pn (--max-procs=max-procs) works only in GNU xargs.

Answer 2

Just adding an alternative solution using parallel which I have started using in place of xargs. The task is much easier with parallel

#!/bin/bash

showword() {
  echo $1
}
export -f showword

parallel -j2 showword {} ::: This is a sample message

• -j2 makes sure 2 copies of the function is run in parallel
• ::: anything after this is passed as separate arguments to parallel, separation is whitespace
• {} is replaced by the argument passed into parallel and fed into the showword function

---

If you are using zsh shell this solution will not work since zsh does not have any feature to export functions. You will need something like this:

#!/usr/bin/zsh

showword() {
  echo $1
}

# add the following to your .zshrc if you want env_parallel in your shell permanently
source /usr/bin/env_parallel.zsh

env_parallel -j2 --env showword showword {} ::: This is a sample message