Bashref

[ ] is the POSIX test command (also known as test), available in all POSIX-compatible shells. [[ ]] is Bash's extended test construct, which adds pattern matching with ==, regex matching with =~, and logical operators && and || inside the brackets. With [ ], you must quote variables to prevent word splitting; with [[ ]], quoting is less critical. Prefer [[ ]] in Bash scripts for safer and more powerful conditionals.

set -e causes the script to exit immediately when any command returns a non-zero exit code. set -u treats references to undefined variables as errors (instead of silently using empty string). set -o pipefail makes a pipeline fail if any command in the pipe fails (without it, only the last command's exit code matters). Combining these as set -euo pipefail at the top of your script is best practice for writing robust, production-quality shell scripts.

${VAR:-default} returns the value of VAR if it is set and non-empty, otherwise returns "default" without modifying VAR. ${VAR:=default} does the same but also assigns "default" to VAR if it was empty. ${VAR:+alt} returns "alt" if VAR is set and non-empty, otherwise returns empty. ${VAR:?error message} exits with an error message if VAR is unset or empty.

Declare an associative array with declare -A mymap. Set values with mymap[key]="value". Access values with ${mymap[key]}. List all keys with ${!mymap[@]} and all values with ${mymap[@]}. Note that associative arrays require Bash 4.0 or later. On macOS, the default /bin/bash is version 3; install Bash 5 via Homebrew and use #!/usr/local/bin/bash or #!/opt/homebrew/bin/bash.

Bash parameter expansion handles this without external commands. ${f##*/} removes everything up to the last slash (gets filename from path). ${f%.*} removes the shortest suffix matching .* (removes last extension). ${f%%.*} removes the longest suffix matching .* (removes all extensions). ${f#*/} removes the shortest prefix matching */ (removes first path component). These are faster than using basename, dirname, or sed.

trap allows you to register a command or function to run when your script receives a signal or exits. Common usage: trap "rm -f /tmp/lockfile" EXIT runs cleanup when the script exits for any reason. trap "echo Interrupted" INT catches Ctrl+C. trap "cleanup_func" TERM catches kill signals. trap "" SIGPIPE ignores broken pipe signals. Always use trap for cleanup of temporary files, lock files, or background processes in production scripts.

Both represent all positional parameters (script arguments). The critical difference is in quoting: "$@" expands to separate quoted strings — "$1" "$2" "$3" — preserving arguments with spaces. "$*" expands to a single string with all arguments joined by the first character of IFS (usually a space): "$1 $2 $3". Always use "$@" when passing arguments to other commands to correctly handle arguments that contain spaces.

Append & to run a command in the background: task1 & task2 &. Use wait to block until all background jobs complete: wait. Use wait $! to wait for the most recently launched background job. Capture PIDs for selective waiting: task1 & pid1=$!; task2 & pid2=$!; wait $pid1; wait $pid2. Use nohup command & to keep a job running after you log out (output goes to nohup.out by default).

Some of the most practical bash one-liners: (1) Find and delete files older than N days: find /path -mtime +7 -delete. (2) Count lines matching a pattern: grep -c "error" logfile. (3) Kill a process on a specific port: kill -9 $(lsof -t -i:3000). (4) Batch rename files: for f in *.JPG; do mv "$f" "${f%.JPG}.jpg"; done. (5) Sum a CSV column: awk -F, '{sum+=$2} END {print sum}' data.csv. (6) Monitor a file in real time: tail -f /var/log/syslog. (7) Compress a directory: tar -czf archive.tar.gz dir/. This bash cheat sheet includes all these patterns and more.