Bash Scripting

Here are my notes on writing Bash scripts. It's opinionated and does not cover everything you need to know about scripting in Bash. I wrote this as an avid user of Bash, both as a shell and for scripting. Writing scripts in Bash can sometimes be challenging due to a somewhat arcane syntax, and there are several pitfalls to be aware of. Regardless, I enjoy writing scripts in Bash.

Style Guideline
Code Snippets

Files

Truncate a File to Zero Size

The trap Command

Create a Work Directory that is Automatically Deleted
Catch Ctrl-C to Prevent User from Killing Your Script
Return to the User's CWD on Exit

User Interaction

Wait for Key Press
Add Confirmation that a Script Should Run

References

Style Guideline

Following a Style Guideline help keeping scripts consistent, which improves readability and eases maintainability. A guideline also help enforce best pratices, ensuring safe and robust scripts. In a team, the guideline facilitates collaboration and assist in onboarding new developers.

Below are my minimal and opinionated list of rules to follow, when writing Bash scripts.

The first rule, that allows you to not follow the guidelines:

Align with the style used in any existing scripts

Formatting rules:

Use four spaces for each level of indentation
Keep a maximum line length of 80 characters

Preamble rules:

Start script with a shebang line and a one-line description
Enable options for strict mode

Example of a preable:

#!/usr/bin/env bash

# Send an email message if the disk is almost full

set -euo pipefail

Naming rules:

Files: lowercase, hyphen (-) to separate words, e.g., convert.sh
Variables: lowercase, underscores (_) to separate words
Functions: lowercase, underscores (_) to separate words

Documentation and Comment rules:

Linting rules:

Lint script using `shellcheck`
Disable checks using comments on the preceeding line

Example on disabling a check:

# shellcheck disable=SC1090 # we need a non-constant source here
source "$config_file"

Note that it's a good idea to comment why the check is disabled.

Code Snippets

Files

Truncate a File to Zero Size

The following will truncates the file file.txt to zero size:

: > file.txt

The Bash built-in : is a no-op with a return code of 0.

See https://superuser.com/a/90061

The trap Command

The trap command is a powerfull built-in.

Create a Work Directory that is Automatically Deleted

TMPDIR="$(mktemp -d)"; readonly TMPDIR; trap 'rm -rf -- "$TMPDIR"' EXIT

Catch Ctrl-C to Prevent User from Killing Your Script

Putting the following at the top of a script will prevent the user in killing a script by pressing Ctrl-C. It will also disable echoing of "^C" when Ctrl-C is pressed while the script runs.

trap '' INT
STTY=$(stty -g); readonly STTY; stty -ctlecho; trap 'stty $STTY' EXIT

Return to the User's CWD on Exit

Putting the following at the top of a script will ensure that the user will be returned to their current working directory (CWD) when the script exits. In other words, this makes it safe to change directory in the script.

CWD=$(cd "$(dirname "$0")" && pwd); readonly CWD; trap 'cd "$CWD"' EXIT

User Interaction

Wait for Key Press

read -rsp $'Press any key to continue...\n' -n1 _key

Add Confirmation that a Script Should Run

When running the follwowing, you'll be asked to confirm if it should run. If you need to run this from, e.g., CI, you can add the --yes option and it will run.

#!/usr/bin/env bash

rval=-1
case $# in
    0) echo "This is a demo!"
       read -r -p "Are you sure you want to continue? [y/N] " ans
       [[ ! ${ans,,} =~ ^(yes|y)$ ]] && msg="Bailing out..." && rval=0
       ;;
    1) [[ ! $1 =~ ^(--yes|-y)$ ]] && msg="Unknown option: $1" && rval=1
       ;;
    *) msg="Too many options: $#" && rval=1
       ;;
esac
[[ $rval -ne -1 ]] && echo "$msg" && exit $rval

echo "Doing stuff.."

References

Below are some Bash references that I find usefull.