Bash Scripting Cheatsheet: Variables, Conditionals, Loops & Functions

bash-scripting-cheatsheet.md

Bash Scripting Cheatsheet: Variables, Conditionals, Loops & Functions

A practical Bash reference for writing shell scripts: variables, conditionals, loops, functions, string and array handling, and the safety settings every script should start with. Copy-paste ready.

Script header & safety

#!/usr/bin/env bash
set -euo pipefail        # exit on error, unset var, and failed pipe
IFS=$'\n\t'              # safer word splitting

Flag	Effect
set -e	exit immediately if a command fails
set -u	error on undefined variables
set -o pipefail	a pipeline fails if any part fails
set -x	print each command (debug)

Variables

name="Ada"
echo "$name"             # always quote
greeting="Hello, $name"
count=$((3 + 4))         # arithmetic
today=$(date +%F)        # command substitution
readonly PI=3.14         # constant
export PATH="$PATH:/opt/bin"

Parameters & special vars

Var	Meaning
$0	script name
$1 $2 …	positional arguments
$#	number of arguments
$@	all arguments (as separate words)
$*	all arguments (as one string)
$?	exit code of last command
$$	current PID
$!	PID of last background job

"${1:-default}"          # use default if $1 unset
"${VAR:?must be set}"     # error out if unset

Conditionals

if [[ "$x" -gt 10 ]]; then
  echo "big"
elif [[ "$x" -eq 10 ]]; then
  echo "ten"
else
  echo "small"
fi

# string tests
[[ -z "$s" ]]   # empty
[[ -n "$s" ]]   # not empty
[[ "$a" == "$b" ]]
[[ "$s" == *.txt ]]      # glob match
[[ "$s" =~ ^[0-9]+$ ]]   # regex match

# file tests
[[ -f file ]]   # is a file
[[ -d dir ]]    # is a directory
[[ -e path ]]   # exists
[[ -x file ]]   # executable

Numeric operators: -eq -ne -lt -le -gt -ge

Loops

for i in 1 2 3; do echo "$i"; done
for f in *.txt; do echo "$f"; done
for i in {1..10}; do echo "$i"; done
for ((i=0; i<5; i++)); do echo "$i"; done

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

until [[ "$done" == "yes" ]]; do ...; done

Functions

greet() {
  local name="$1"        # local scope
  echo "Hello, $name"
  return 0
}
greet "Ada"
result=$(greet "Bob")    # capture output

Strings

${#str}                  # length
${str:0:3}               # substring (offset, length)
${str/foo/bar}           # replace first
${str//foo/bar}          # replace all
${str^^}                 # uppercase
${str,,}                 # lowercase
${file%.txt}             # strip suffix
${path##*/}              # basename
${path%/*}               # dirname

Arrays

arr=(a b c)
echo "${arr[0]}"         # first
echo "${arr[@]}"         # all
echo "${#arr[@]}"        # length
arr+=(d)                 # append
for x in "${arr[@]}"; do echo "$x"; done

declare -A map           # associative array
map[name]="Ada"
echo "${map[name]}"

Common idioms

command || echo "failed"            # run on failure
command && echo "ok"                # run on success
mkdir -p "$dir"                     # no error if exists
trap 'echo cleanup; rm -f "$tmp"' EXIT   # cleanup on exit
[[ $# -eq 0 ]] && { echo "usage: $0 <arg>"; exit 1; }

Quick reference table

Task	Snippet
Safe script header	set -euo pipefail
Default value	${1:-default}
Loop over files	for f in *.txt; do …; done
Read file line by line	while read -r l; do …; done < f
String length	${#s}
Replace all	${s//a/b}
Basename	${path##*/}
Regex test	[[ "$s" =~ ^[0-9]+$ ]]
Cleanup on exit	trap '…' EXIT

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