Bash Scripting Cheat Sheet: Variables, Tests & Loops

Bash syntax for variables, conditionals, loops, and safe scripting in one place.

The bash constructs you reach for when writing CI and automation scripts.

Strict mode & variables

script.sh

#!/usr/bin/env bash
set -euo pipefail

name="${1:-default}"        # default if unset
count=${#array[@]}          # array length
echo "${PATH%%:*}"          # first path entry

Parameter expansion

Form	Result
${var:-x}	x if var unset/empty
${var:=x}	Assign x if unset
${var:?msg}	Error msg if unset
${var#pat}	Strip shortest prefix
${var%pat}	Strip shortest suffix
${var/old/new}	Replace first match
${#var}	String length

Test conditions

Test	True when
[[ -f f ]]	File exists
[[ -d d ]]	Directory exists
[[ -z s ]]	String empty
[[ -n s ]]	String non-empty
[[ a == b ]]	String equal
(( a > b ))	Arithmetic compare

Loops

loops

for f in *.txt; do echo "$f"; done
while read -r line; do echo "$line"; done < file
case "$x" in
  start) run ;;
  *) echo "unknown" ;;
esac

Key takeaways

set -euo pipefail catches errors, unset vars, and broken pipes.
Always quote "${var}" to survive spaces and globbing.
[[ ]] is the safer modern test; (( )) for arithmetic.

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