Bash Scripting Cheat Sheet

#!/bin/bash

Shebang (script interpreter)

set -euo pipefail

Exit on error, undefined vars, pipe fail

echo "Hello World"

Print line

read -p "Name: " name

Read user input with prompt

read -s password

Read silent input (passwords)

# This is a comment

Comment

name="Alice"

Assign variable (no spaces!)

echo "$name"

Read variable

echo "${name}!"

Brace syntax for clarity

readonly PI=3.14

Read-only constant

unset name

Remove variable

export PATH="$HOME/bin:$PATH"

Export to subshells

${var:-default}

Default if unset

${var:=default}

Default and assign if unset

${var:?error message}

Error if unset

${#var}

String length

${var:0:3}

Substring (offset 0, length 3)

${var/old/new}

Replace first occurrence

${var//old/new}

Replace all occurrences

if [ "$x" -eq 1 ]; then
  echo "one"
fi

If with numeric compare

if [[ "$s" == "hi" ]]; then ... fi

If with string compare ([[ ]])

if [ -f file.txt ]; then ... fi

If file exists

if [ -d /etc ]; then ... fi

If directory exists

if [ -z "$x" ]; then ... fi

If string is empty

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

If string is non-empty

-eq -ne -lt -le -gt -ge

Numeric operators

[[ "$s" =~ ^[0-9]+$ ]]

Regex match

case "$x" in
  1) ... ;;
  2|3) ... ;;
  *) ... ;;
esac

Case statement

[ -f file ] && echo exists || echo missing

Inline if/else

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

For over list

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

For over range

for i in $(seq 1 10); do ... done

For with seq

for f in *.txt; do ... done

For over files (glob)

for ((i=0; i<10; i++)); do ... done

C-style for loop

while [ "$n" -lt 10 ]; do
  n=$((n+1))
done

While loop

while read line; do echo "$line"; done < file.txt

Read file line by line

until [ condition ]; do ... done

Until loop (negated while)

break continue

Break/continue loop

arr=(a b c d)

Create array

echo "${arr[0]}"

Access by index

echo "${arr[@]}"

All elements

echo "${#arr[@]}"

Array length

arr+=(e f)

Append elements

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

Iterate array

declare -A map
map[key]="value"

Associative array (dict)

greet() {
  echo "Hi $1"
}
greet "Alice"

Define and call function

$1 $2 $3 ... $@ $#

Args: pos 1, 2, 3, all, count

local x=5

Function-local variable

return 0

Exit code from function

command > file.txt

Redirect stdout (overwrite)

command >> file.txt

Redirect stdout (append)

command 2> errors.log

Redirect stderr

command &> both.log

Redirect stdout + stderr

command < input.txt

Input from file

cmd1 | cmd2 | cmd3

Pipe between commands

$(command)

Command substitution

$((2 + 3))

Arithmetic expansion

cmd1 && cmd2 || cmd3

Run cmd2 if cmd1 succeeds, else cmd3

cmd &

Run in background

wait

Wait for background jobs

trap 'cleanup' EXIT

Run on script exit