Bash · Cheat sheet

Bash scripting cheat sheet

Bash scripting cheat sheet: safety flags, variables, loops, functions, arrays, parameter expansion, and traps.

64 commands·9 sections·Updated Sep 26, 2026·By SecOpsLog

Script basics & safety

Beginner

#!/usr/bin/env bash: Shebang — pick bash from PATH.
set -euo pipefail: Exit on most errors, unset vars, pipe failures (-e is off in if/&&/|| tests).
IFS=$'\n\t': Safer word-splitting (newline/tab only).
chmod +x script.sh && ./script.sh: Make executable and run.
bash -n script.sh: Syntax-check without executing.
bash -x script.sh: Trace every command as it runs.Example output
+ name=prod + echo prod prod

Beginner

name="prod": Assign — no spaces around =.
echo "$name": Always quote expansions to avoid splitting.
out=$(date +%F): Command substitution into a variable.Example output
2025-06-12
out=${ date +%F; }: Bash 5.3+: substitution that runs in the current shell (no subshell), so side effects persist.
readonly PI=3.14: Constant — cannot be reassigned.
export PATH="$HOME/bin:$PATH": Export to child processes.
echo "${name:-default}": Use default if name is unset/empty.
echo "${name:?must be set}": Error out with a message if unset or empty.
echo $((3 + 4 * 2)): Integer arithmetic.Example output
11

Intermediate

Intermediate

for f in *.log; do gzip "$f"; done: Loop over glob matches.
for i in {1..5}; do echo "$i"; done: Brace-expansion range.Example output
1 2 3 4 5
for ((i=0;i<n;i++)); do ...; done: C-style counting loop.
while IFS= read -r line; do ...; done < file: Read a file line by line, keeping leading/trailing spaces.
until [[ -f ready.flag ]]; do sleep 1; done: Loop until the condition becomes true.
break / continue: Exit a loop / skip to the next iteration.

Intermediate

Advanced

${#s}: Length of a string.
${s:2:4}: Substring: offset 2, length 4.
${s^^} / ${s,,}: Upper-case / lower-case the whole string.Example output
PROD / prod
${file%.txt}: Strip a suffix (shortest match with %).
${path##*/}: Basename — strip longest leading match.Example output
app.js # from /srv/app.js
${path%/*}: Dirname — strip the trailing /file.
${s/foo/bar}: Replace first foo (// replaces all).
${arr[@]:1:2}: Slice an array from index 1, length 2.

Advanced

arr=(a b c): Index array literal.
echo "${arr[1]}": Element by index (0-based).Example output
b
echo "${arr[@]}": All elements as separate words.
echo "${#arr[@]}": Number of elements.
arr+=(d): Append an element.
mapfile -t lines < file: Read every line of a file into an array (-t drops newlines).
declare -A m; m[key]=val: Associative array (map).
for k in "${!m[@]}"; do ...; done: Iterate map keys.

Advanced

Advanced

trap 'rm -f "$tmp"' EXIT: Run cleanup whenever the script exits.
trap 'echo "err on line $LINENO"' ERR: Report the line an error occurred on (set -E to cover functions).
tmp=$(mktemp): Create a safe temporary file.
PS4='+ ${BASH_SOURCE}:${LINENO}: ': Richer trace prefix for set -x.
set -x / set +x: Turn command tracing on / off around a block.
declare -p var: Print a variable’s type and value for debugging.
shellcheck script.sh: Static analysis — catches quoting and logic bugs.

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