Shells allows easy forwarding of output form one program to input of another program. It is also available in other languages. General term for such phenomena is piping or pipelining.

Simple pipes:

# Print all "a" from random generator
cat /dev/random | grep 'a'

# Delete all local branches from git, except current:
git branch | grep -v '*' | xargs git branch -D
# Or print instead of deleting
git branch | grep -v '*' | xargs echo git branch -D

Error detection

Simply result of last command

By default error code stored in $? variable is result of last command used in pipie. If previous command fails, we need to use more sophisticated method.

not-existing-command
# result is different than 0
echo "$?"

not-existing-command | cat "test" | grep "t"
# result is 0, which is false positive
echo "$?"

PIPESTATUS variable

For complex cases shells have dedicated variable:

  • PIPESTATUS - bash, indexed form 0
  • pipestatus - zsh, indexed form 1

BASH

not-existing-command | cat "test" | grep "t"
echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}"
# result: "127 0 0"

ZSH

not-existing-command | cat "test" | grep "t"
echo "${pipestatus[1]} ${pipestatus[2]}"
# result: "127 0 0"
https://unix.stackexchange.com/questions/14270/get-exit-status-of-process-thats-piped-to-another
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