Note the older format of test CONDITION is replaced by [ CONDITION ] and in some shell flavors extra functionality with using [[ CONDITION ]].

Ignore

CMD || true

Optionally hide output.

CMD > /dev/null || true

Or just

set -e

# ...

set +e
CMD
set -e

Check file

If a file exists:

if [[ -f .env ]]; then
  cat .env
fi

If it does not exist:

if [[ ! -f .env ]]; then
  echo 'File is missing: .env'
fi

See Bash cheatsheet for more info.

Variable not set

Abort the script if a check evaluates to false. Here we see if a variable is set.

$ [[ -z "$MY_VAR" ]] && (echo 'MY_VAR must be set' ; exit 1)

Note the brackets are needed, otherwise on a true evaluation of the first condition, the exit will still run.

Using exit is good for a script and not for direct terminal use otherwise you will close the terminal tab.

Or in multiple lines.

if [[ -z "$MY_VAR" ]]; do
  echo 'MY_VAR must be set'
  exit 1
fi

One-liner status check

Check if the status of the previous command was a pass or fail. This can help if the command is long and you don’t want to fit it on one line.

Skip error

This will keep going and not abort the script.

$ [[ $? -eq 0 ]] && echo 'Passed!' || echo 'Failed!'

Example use:

  • Use the true commands to give a zero (pass) exit status. 
    $ true
$ [[ $? -eq 0 ]] && echo 'Passed!' || echo 'Failed!'
Passed!
  • Use the false commands to give a non-zero (fail) exit status. 
      $ false
  $ [[ $? -eq 0 ]] && echo 'Passed!' || echo 'Failed!'
  Failed!

Exit on error

Note brackets are needed.

$ false
$ [[ $? -eq 0 ]] && echo 'Passed!' || (echo 'Failed!'; exit 1)

If statement

This is a multi-line if statement, which is useful for more complex statements or if readability is important.

false

if [[ $? -eq 0 ]]; then
  echo 'Passed!'
else
  echo 'Failed!'
  exit 1
fi

Show message on failure only:

false

if [[ $? -ne 0 ]]; then
  echo 'Failed!';
  exit 1
fi

Check if root

if [ $UID -ne 0 ]; then
  echo 'I am not root'
fi