Bash Scripting Cheat Sheet

Cheet says: Automate all the things!

Script Header

#!/bin/bash
set -euo pipefail    # Exit on error, undefined, pipe fail
Or more explicit:
set -e               # Exit on error
set -u               # Error on undefined variable
set -o pipefail      # Pipe fails if any command fails

Variables

name="world"
echo "Hello $name"
echo "Hello ${name}!"

Default values
echo ${var:-default}     # Use default if unset
echo ${var:=default}     # Set default if unset
echo ${var:+alternate}   # Use alternate if set
echo ${var:?error msg}   # Error if unset

String operations
${#var}                  # Length
${var^^}                 # Uppercase
${var,,}                 # Lowercase
${var:0:5}               # Substring (start:length)
${var/old/new}           # Replace first
${var//old/new}          # Replace all

Conditionals

If statement
if [[ $x -eq 1 ]]; then
    echo "one"
elif [[ $x -eq 2 ]]; then
    echo "two"
else
    echo "other"
fi

Comparisons
[[ $a == $b ]]     # String equal
[[ $a != $b ]]     # String not equal
[[ $a -eq $b ]]    # Integer equal
[[ $a -ne $b ]]    # Integer not equal
[[ $a -lt $b ]]    # Less than
[[ $a -gt $b ]]    # Greater than
[[ -z $a ]]        # Is empty
[[ -n $a ]]        # Is not empty

File tests
[[ -e file ]]      # Exists
[[ -f file ]]      # Is file
[[ -d dir ]]       # Is directory
[[ -r file ]]      # Is readable
[[ -w file ]]      # Is writable
[[ -x file ]]      # Is executable

Logical
[[ $a && $b ]]     # AND
[[ $a || $b ]]     # OR
[[ ! $a ]]         # NOT

Loops

For loop
for i in 1 2 3; do
    echo $i
done

for i in {1..10}; do
    echo $i
done

for file in *.txt; do
    echo "$file"
done

While loop
while [[ $count -lt 10 ]]; do
    ((count++))
done

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

Infinite loop
while true; do
    sleep 1
done

Functions

my_function() {
    local name=$1    # Local variable
    echo "Hello $name"
    return 0
}

Call it
my_function "world"

Get return value
result=$(my_function "test")

Arrays

Define
arr=(one two three)
arr[0]="first"

Access
echo ${arr[0]}           # First element
echo ${arr[@]}           # All elements
echo ${#arr[@]}          # Length
echo ${!arr[@]}          # All indices

Iterate
for item in "${arr[@]}"; do
    echo "$item"
done

Add element
arr+=("four")

Input/Output

Read input
read -p "Enter name: " name
read -s -p "Password: " pass   # Silent

Here document
cat <Redirectcmd > file      # Stdout to file
cmd >> file     # Append
cmd 2> file     # Stderr to file
cmd &> file     # Both to file
cmd 2>&1        # Stderr to stdout

Useful Patterns

Check command exists
if command -v git &>/dev/null; then
    echo "git installed"
fi

Parse arguments
while [[ $# -gt 0 ]]; do
    case $1 in
        -h|--help) show_help; exit 0 ;;
        -v|--verbose) VERBOSE=1 ;;
        *) echo "Unknown: $1"; exit 1 ;;
    esac
    shift
done

Trap signals
cleanup() { rm -f /tmp/lockfile; }
trap cleanup EXIT

Progress dots
for i in {1..10}; do
    echo -n "."
    sleep 0.1
done
echo " done!"

Parallel execution
for task in task1 task2 task3; do
    run_task "$task" &
done
wait  # Wait for all

Best Practices

Quote your variables
echo "$var"              # Good
echo $var                # Bad - word splitting

Use [[ ]] not [ ]
[[ $a == $b ]]           # Good
[ $a == $b ]             # Bad - less safe

Use $() not backticks
result=$(command)        # Good
result=`command`         # Bad - hard to nest

Check errors
if ! command; then
    echo "Failed"
    exit 1
fi