CODE.md

Code: Bash Best Practices

An attempt to bring order in good advice on writing Bash scripts I collected from several sources.

Some sources

• https://github.com/dylanaraps/pure-bash-bible
• https://bertvv.github.io/cheat-sheets/Bash.html
• https://github.com/anordal/shellharden/blob/master/how_to_do_things_safely_in_bash.md
• https://stackoverflow.com/questions/10953833/passing-multiple-distinct-arrays-to-a-shell-function
• https://www.reddit.com/r/commandline/comments/lha15t/bash_execution_tips_the_difference_between_and/

Suggested tools

• VSCode with shellcheck and Bash IDE extensions.
• WSL or WSL2 on Windows 10/11 environments.

Code: General

The principles of Clean Code apply to Bash as well Always use long parameter notation when available. This makes the script more readable, especially for lesser known/used commands that you don’t remember all the options for.

Avoid:

rm -rf -- "${dir}"

Good:

rm --recursive --force -- "${dir}"

Very Bad:

rm --recursive --force -- "/{dir}" #If $dir is empty, it will delete everything!

Good:

rm --recursive --force -- "/${dir:?}" #If $dir is empty, the command will fail

Don’t use:

cd "${foo}" [...] cd ..

but

( cd "${foo}" [...] )

pushd and popd may also be useful:

pushd "${foo}" [...] popd

Use nohup foo | cat & if foo must be started from a terminal and run in the background.

Code: Variables

• Prefer local variables within functions over global variables.
• If you need global variables, make them read-only.
• Variables should always be referred to in the ${var} form (as opposed to $var).
• Variables should always be quoted, especially if their value may contain a whitespace or separator character: "${var}".
• Capitalization Environment (exported) variables: ${ALL_CAPS} Local variables: ${lower_case}
• Positional parameters of the script should be checked, those of functions should not.
• Some loops happen in subprocesses, so don’t be surprised when setting variabless does nothing after them. Use stdout and greping to communicate status.

Variable expansion

Good: "$my_var" Bad: $my_var

Command substitution

Good: "$(cmd)" Bad: $(cmd)

There are exceptions where quoting is not necessary, but because it never hurts to quote.

The exceptions only matter in discussions of style – feel welcome to ignore them. For the sake of style neutrality, Shellharden does honor a few exceptions:

• variables of invariably numeric content: $?, $$, $!, $# and array length ${#array[@]}
• assignments: a=$b
• the magical case command: case $var in … esac
• the magical context between double-brackets ([[ and ]]) – this is a language of its own.

Should I use backticks? Command substitutions also come in this form:

Correct: "cmd" Bad: cmd

Code: Arrays

arr=() Create an empty array arr=(1 2 3) Initialize array ${arr[2]} Retrieve third element ${arr[@]} Retrieve all elements ${!arr[@]} Retrieve array indices ${#arr[@]} Calculate array size arr[0]=3 Overwrite 1st element arr+=(4) Append value(s) str=$(ls) Save ls output as a string arr=( $(ls) ) Save ls output as an array of files ${arr[@]:s:n} Retrieve n elements starting at index s