Linux Shell Scripting Cheat Sheet

name='alice'

# Set a shell variable in the current shell.

export APP_ENV=production

# Expose a variable to child processes.

echo "${PORT:-8080}"

# Expand parameter with a fallback.

echo "${DATABASE_URL:?DATABASE_URL is required}"

# Fail fast if a variable is missing.

files=$(find . -type f | wc -l)

# Assign command output to a variable.

printf '%s
' "$name"

# Print variable preserving spaces.

if [[ -f /etc/app.conf ]]; then echo 'exists'; fi

# Use a shell test in an if statement.

if [[ -d releases ]]; then ls releases; fi

# Test for directory presence.

for f in *.log; do echo "$f"; done

# Iterate through words or filenames.

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

# Process lines safely using while-read.

case "$1" in start) echo start ;; stop) echo stop ;; *) echo usage ;; esac

# Match input against multiple patterns.

greet() { echo "hello $1"; }

# Create a reusable shell function.

set -euo pipefail

# Fail script on errors and unset vars.

trap 'rm -f "$tmpfile"' EXIT

# Register shell cleanup logic.

echo 'hello' > greeting.txt

# Write output to a file, overwriting it.

echo 'next line' >> app.log

# Append output to an existing file.

command 2> errors.log

# Send errors to a separate file.

command > command.log 2>&1

# Capture all output in one file.

ps aux | grep nginx

# Send output of one command as input to another.

diff <(sort old.txt) <(sort new.txt)

# Pass command output as a pseudo-file.

read -rp 'Enter environment: ' env

# Read a value interactively.

printf 'user=%s id=%s
' "$USER" "$UID"

# Use printf instead of echo for predictable formatting.

#!/usr/bin/env bash

# Start a Bash script with an explicit interpreter.

chmod +x deploy.sh

# Add execute bit to a script.

./deploy.sh

# Execute a script from the current directory.

bash -x ./deploy.sh

# Print commands as they execute.

while getopts ':f:n:' opt; do case "$opt" in f) file="$OPTARG" ;; n) count="$OPTARG" ;; esac; done

# Handle flags in shell scripts.

shellcheck deploy.sh

# Static-analyze shell scripts for bugs.