Bash Scripting

Variables, arrays, control flow, error handling, trap, and production-grade scripting patterns.

Overview

Bash is the dominant shell for system automation. Production scripts must be written with defensive defaults.

Defensive Defaults

Always start scripts with:

-e — exit on any non-zero return
-u — treat unset variables as errors
-o pipefail — fail a pipeline if any command in it fails

Variables and Quoting

Always double-quote variable expansions: "$variable" not $variable — prevents word splitting and glob expansion. Use ${variable:-default} for a default when unset.

Exit Codes and trap

0 is success; anything else is failure. Use trap to register cleanup functions:

Process Substitution

Process substitution feeds command output as a file:

Diffs two sorted outputs without temporary files.

Key concepts

set -euo pipefail: Defensive defaults every production script should start with. -e exits on error, -u treats unset variables as an error, -o pipefail fails the whole pipeline on the first non-zero command.
Double quoting: Always quote variable expansions: "$var" not $var. Prevents word splitting and glob expansion.
trap: Register functions to run on shell exit or signals. trap cleanup EXIT ensures temp files are deleted even when the script exits early.
Exit codes: 0 is success; anything else is failure. Scripts should propagate exit codes correctly.
Process substitution: diff <(cmd1) <(cmd2) feeds command output as named pipes. Avoids temp files.

When to use it

Writing deployment scripts, cron jobs, and CI/CD pipeline steps
Automating repetitive system administration tasks
Designing error handling that cleans up temp files even on failure

Interview questions

Write a bash script that downloads a file, processes it, and cleans up temp files even if the script fails mid-way.
Explain the difference between $*, "$*", $@, and "$@" when passing arguments.