Writing robust script

We should use the following options in our bash script for better quality:

set -eux
set -o pipefail

Meaning of these option:

  • -e: Exit immediately if a command fails.
  • -u: If a variable is not set, this is considered an error1.
  • -x: Expand variables in a command and print it before executing the command (better for debugging if command fails).
  • -o pipefail: If any command in a pipe fails, this pipe will fail.

For more options, see bash set command doc.

common check conditions for files

Some commonly used options for checking file conditions:

optionmeaning
[ -d FILE ]True if FILE exists and is a directory
[ -f FILE ]True if FILE exists and is a regular file
[ -r FILE ]True if FILE exists and is readable
[ -w FILE ]True if FILE exists and is writable
[ -x FILE ]True if FILE exists and is executable
[ -z STRING ]True if length of STRING is zero
[ -n STRING ]True if length of STRING is not zero

Note that to check if the opposite is true, use [ ! condition ].

Use [ ] or [[ ]] for condition check?

Both [ ] and [[ ]] can be used to check some conditions, but [ ] is POSIX compatible, and [[ ]] is not. Both bash, zsh support [[ ]].

[[ ]] is also more versatile and powerful than [ ]. For example, it supports &&, || and grouping command using () etc.

Compare string equality

We can use = and != to compare the equality of strings. Use of == is non-standard (see here).

How to set boolean variable?

In bash, there is not really a boolean type. We can just use literal string true to set a bool variable and check if variable is equal to string true. For example:

use_python=true
if [[ $use_python = true ]]; then
    # do something
else:
    # do another thing
fi

How to check if a command exists?

We can use command -v YOUCOMMAND to check if YOURCOMMAND exists. For example:

if [[ ! "$(command -v rg)" ]]; then
    # install ripgrep
else
    echo "ripgrep already exists. No action needed."
fi

According to here, which command is not a reliable way to check if a command exists since it may return an exit code of 0 even if no command is found.

References


  1. you will see unbound variable error message. ↩︎