Bash One-Liners Cheatsheet

Overview

Bash carries a lot of weight in day-to-day plumbing. This card collects the one-liners worth memorizing, grouped by task. The full rules (set -euo pipefail, quoting, error handling) live in shell; this page is the lookup table for “what was that flag again?“. All examples assume Bash 5 with set -euo pipefail.

Find files

Find before rename, find before delete, find before pipe.

One-liner	What it does
find . -type f -name '*.md'	All markdown files, recursive.
find . -type f -newer reference.txt	Files modified after reference.txt.
find . -type f -size +10M	Files larger than 10 MB.
find . -type f -mtime -7	Files modified in the last 7 days.
find . -type f -name '*.log' -delete	Delete matched files in one pass.
find . -type f -name '*.tmp' -exec rm {} +	Same, batched for fewer rm calls.
find . -type d -empty -delete	Prune empty directories.
grep -rIn "pattern" .	Recursive grep, line numbers, skip binaries.
rg -n "pattern"	ripgrep; faster and respects .gitignore.

Use find ... -print0 \| xargs -0 when filenames may contain spaces or newlines.

Find and replace

In-place edits should be staged in git first.

One-liner	What it does
sed -i 's/old/new/g' file.txt	GNU sed in-place replacement (Linux).
sed -i '' 's/old/new/g' file.txt	BSD sed in-place replacement (macOS).
find . -name '*.md' -exec sed -i 's/old/new/g' {} +	Replace across many files.
grep -rl "old" . | xargs sed -i 's/old/new/g'	Replace only in files that contain “old”.
tr 'A-Z' 'a-z' < file > lower.txt	Lowercase a file.
awk '{print $2, $1}' file	Swap columns 1 and 2.

sed -i differs between BSD and GNU. Write sed -i.bak and keep the backup for portability.

Parameter expansion

The shell offers cheap string ops; reach for these before sed.

Expansion	Result for f=path/to/file.tar.gz
${f##*/}	file.tar.gz (basename).
${f%/*}	path/to (dirname).
${f%.*}	path/to/file.tar (drop last extension).
${f%%.*}	path/to/file (drop all extensions).
${f#*.}	tar.gz (drop everything up to the first dot).
${f//\//-}	path-to-file.tar.gz (replace all / with -).
${name:-default}	Use default if name unset or empty.
${name:?error message}	Exit with an error if name is unset.
${#name}	Length of name.

Pure-bash string ops avoid forking a subprocess; in tight loops the win is real.

Jobs and traps

Background work needs cleanup; traps make cleanup unconditional.

One-liner	What it does
cmd &	Run in background.
jobs	List background jobs in this shell.
wait	Block until all background jobs finish.
wait $PID	Block until one specific PID exits.
disown %1	Detach job 1 from the shell; survives logout.
trap 'rm -rf "$TMP"' EXIT	Cleanup runs on normal and abnormal exit.
trap 'echo interrupted; exit 130' INT	Custom handler for Ctrl-C.
nohup cmd > out.log 2>&1 &	Detach from controlling terminal.

Always trap EXIT for temp-directory cleanup. Use mktemp -d to create the directory.

Arithmetic and tests

Bash arithmetic is integer-only; use bc or awk for floats.

One-liner	What it does
(( count++ ))	Increment in arithmetic context.
echo $(( 3 * (4 + 5) ))	Inline arithmetic; prints 27.
echo "scale=2; 7/3" | bc	Float division; prints 2.33.
[[ -f file ]]	True if file exists and is regular.
[[ -d dir ]]	True if directory exists.
[[ -z $var ]]	True if variable is empty or unset.
[[ $a == "yes" ]]	String equality.
[[ $s =~ ^[0-9]+$ ]]	Regex match; numeric string.

Prefer [[ ... ]] over [ ... ]. It does not split words, does pattern and regex matching, and does not need quoting around variables.

Common gotchas

• Unquoted variables word-split. Write "$var" everywhere unless you intentionally want splitting.
• cd /tmp/$dir fails silently if cd fails. Write cd "$dir" \|\| exit 1 or run with set -e.
• Pipelines hide exit codes from earlier commands. set -o pipefail fixes it.
• for f in *.md returns the literal *.md if nothing matches. Set shopt -s nullglob first.
• read strips leading and trailing whitespace and interprets backslashes. Use read -r line for raw input.
• $(...) and backticks both subshell out. Prefer $(...); it nests and quotes cleanly.

Related

• shell
• git-commands
• regex-patterns
• dotfiles
• Cheatsheets