Bash Scripting & CLI One-Liners Cheat Sheet

Variables, parameter expansion, condition tests, loops, redirection, and text processing pipelines

Script Template & Safe Header

Best practice strict mode flags for robust shell scripts

Strict Bash HeaderExit on error (-e), unset variables (-u), and pipeline fails (-o pipefail)

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

Check command exit status

if [ $? -ne 0 ]; then
  echo "Command failed" >&2
  exit 1
fi

Default value if unsetUses default without modifying VAR

${VAR:-default_value}

String lengthNumber of characters in variable

${#VAR}

Substring extractionFirst 5 characters (offset:length)

${VAR:0:5}

Strip prefix patternShortest vs longest match from start

${FILE#*.}
${FILE##*.}

Strip suffix patternShortest vs longest match from end

${FILE%.*}
${FILE%%.*}

Search and replaceFirst match vs all matches

${VAR/old/new}
${VAR//old/new}

Check if file exists

if [[ -f "$FILE" ]]; then ... fi

Check if directory exists

if [[ -d "$DIR" ]]; then ... fi

Check if string is non-empty

if [[ -n "$VAR" ]]; then ... fi

Check if command exists in PATH

if command -v git &>/dev/null; then ... fi

Loop over files safely

for f in *.json; do
  [ -e "$f" ] || continue
  echo "Processing $f"
done

Read file line by line

while IFS= read -r line; do
  echo "$line"
done < input.txt

Redirect stdout and stderrCapture both stdout & stderr to file

command > output.log 2>&1
command &> output.log