Linux · Bash · Shell Scripting · Automation
Bash Scripting
Cheatsheet
A practical reference for Bash shell scripting — covering variables, loops, conditionals, functions, arrays, string operations, and real-world script patterns.
What is Bash?
Bash (Bourne Again SHell) is the default command-line interpreter on most Linux distributions and macOS. It is both an interactive shell (for running commands) and a scripting language (for writing automation scripts saved in .sh files).
Why use it? Bash scripts automate repetitive tasks, system administration, deployments, log parsing, backups, and CI/CD pipelines. A single Bash script can replace hours of manual work.
Real-world use: DevOps engineers, sysadmins, security analysts, and developers use Bash daily. It is core to Linux system administration, Ansible playbooks, Docker entrypoints, and GitHub Actions workflows.
Always start scripts with the shebang line:
#!/bin/bash
This tells the OS to use /bin/bash to interpret the script. Save with .sh extension and make executable: chmod +x script.sh
1. Getting Started
#!/bin/bash # This is a comment # Print output echo "Hello, World!" echo "Today: $(date)" # command substitution # Exit codes exit 0 # 0 = success exit 1 # non-zero = error echo $? # print last exit code # Make script executable and run # chmod +x script.sh && ./script.sh
2. Variables
# Assign (no spaces around =) NAME="Kenneth" AGE=25 DIR="/var/www" # Use variable echo "Hello, $NAME" echo "Age: ${AGE}" # braces for clarity # Command substitution HOSTNAME=$(hostname) FILES=$(ls /etc | wc -l) # Read-only variable readonly PI=3.14159 # Unset variable unset NAME # Special variables echo $0 # script name echo $1 # first argument echo $# # number of arguments echo $@ # all arguments echo $$ # current PID echo $? # last exit code
3. User Input
# Read from user read -p "Enter your name: " NAME echo "Hello, $NAME!" # Silent input (for passwords) read -sp "Password: " PASS echo # newline after hidden input # Read with timeout read -t 10 -p "Input (10s): " INPUT # Command-line arguments #!/bin/bash echo "Script: $0" echo "Arg 1: $1" echo "Arg 2: $2" echo "All args: $@" # Run: ./script.sh hello world
4. Conditionals
if / elif / else
if [ $AGE -ge 18 ]; then echo "Adult" elif [ $AGE -ge 13 ]; then echo "Teen" else echo "Child" fi # String comparison if [ "$NAME" == "root" ]; then echo "Root user" fi
Test operators
# Numeric -eq equal to -ne not equal -lt less than -le less or equal -gt greater than -ge greater or equal # String == equal != not equal -z empty string -n not empty # File tests -f is a file -d is a directory -e exists -r readable -x executable
# File tests in practice if [ -f "/etc/passwd" ]; then echo "File exists" fi if [ ! -d "/tmp/mydir" ]; then mkdir -p /tmp/mydir fi # Compound conditions if [[ $AGE -gt 18 && "$NAME" != "" ]]; then echo "Valid adult" fi # case statement case $OS in Ubuntu) echo "Debian-based" ;; CentOS) echo "RPM-based" ;; *) echo "Unknown" ;; esac
5. Loops
# for loop — iterate list for i in 1 2 3 4 5; do echo "Number: $i" done # for loop — range for i in {1..10}; do echo $i done # for loop — C-style for (( i=0; i<5; i++ )); do echo "i=$i" done # for loop — iterate files for file in /var/log/*.log; do echo "Processing: $file" done # while loop COUNT=0 while [ $COUNT -lt 5 ]; do echo "Count: $COUNT" COUNT=$((COUNT + 1)) done # Read file line by line while IFS= read -r line; do echo "$line" done < /etc/hosts # until loop until [ $COUNT -ge 10 ]; do COUNT=$((COUNT + 1)) done # Loop control break # exit loop continue # skip to next iteration
6. Functions
# Define a function greet() { local NAME=$1 # local variable (scoped) echo "Hello, $NAME!" } # Call the function greet "Kenneth" # Function with return value add() { local RESULT=$(( $1 + $2 )) echo $RESULT # return via stdout } SUM=$(add 5 3) echo "Sum: $SUM" # Function with exit code check_root() { if [ $EUID -ne 0 ]; then echo "Must be root" >&2 return 1 fi } check_root || exit 1 # exit if not root
7. Arrays
# Declare array FRUITS=("apple" "banana" "cherry") NUMBERS=(1 2 3 4 5) # Access elements echo ${FRUITS[0]} # apple echo ${FRUITS[1]} # banana echo ${FRUITS[-1]} # last element (cherry) # All elements echo ${FRUITS[@]} # all elements echo ${#FRUITS[@]} # array length # Loop over array for fruit in ${FRUITS[@]}; do echo $fruit done # Add / remove elements FRUITS+=("date") # append unset FRUITS[0] # remove index 0 # Associative arrays (dictionaries) declare -A PORTS PORTS["HTTP"]=80 PORTS["HTTPS"]=443 PORTS["SSH"]=22 echo ${PORTS["HTTPS"]} # 443
8. String Operations
STR="Hello World" # Length echo ${#STR} # 11 # Uppercase / lowercase echo ${STR^^} # HELLO WORLD echo ${STR,,} # hello world # Substring echo ${STR:0:5} # Hello (pos 0, length 5) echo ${STR:6} # World (from pos 6) # Replace echo ${STR/World/Bash} # Hello Bash (first) echo ${STR//l/L} # HeLLo WorLd (all) # Strip prefix / suffix FILE="report.txt" echo ${FILE%.txt} # report (strip suffix) echo ${FILE#report} # .txt (strip prefix) # Default values echo ${VAR:-"default"} # use default if VAR unset echo ${VAR:="default"} # set and use default if unset
9. Arithmetic
# Arithmetic expansion RESULT=$(( 5 + 3 )) # 8 RESULT=$(( 10 / 2 )) # 5 RESULT=$(( 7 % 3 )) # 1 (modulus) RESULT=$(( 2 ** 8 )) # 256 (power) # Increment / decrement COUNT=0 (( COUNT++ )) # post-increment (( COUNT += 5 )) # add 5 COUNT=$(( COUNT - 1 )) # subtract 1 # Float arithmetic (use bc) echo "scale=2; 10/3" | bc # 3.33 AREA=$(echo "scale=2; 3.14 * 5 * 5" | bc)
10. Real-World Script Patterns
#!/bin/bash # Robust script template set -euo pipefail # exit on error, unset var, pipe fail IFS=$'\n\t' # safer word splitting # Log function log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" ; } # Error handler die() { echo "ERROR: $*" >&2; exit 1; } # Check dependency require() { command -v "$1" >/dev/null || die "$1 not found" } require curl require jq # Trap cleanup on exit cleanup() { log "Cleaning up..." rm -f /tmp/tmpfile.$$ } trap cleanup EXIT log "Starting script..."