Bash Scripting Cheat Sheet: Zero to Pro

Version note: associative arrays, ${v^^} / ${v,,} and mapfile need Bash 4 or newer; printf '%(...)T' and reading a negative index such as ${a[-1]} need Bash 4.2 or newer. macOS ships Bash 3.2; install a newer Bash before using those lines. Check with bash --version.

Running scripts#

#!/bin/bash                 # shebang: first line, first character
bash script.sh              # run with bash (no permission needed)
chmod +x script.sh          # make executable (once)
./script.sh                 # run from the current folder
bash -n script.sh           # syntax check only
bash -x script.sh           # trace every command
shellcheck script.sh        # lint for bugs

Variables and quoting#

name="web01"                # no spaces around =
echo "$name ${name}_old"    # read; braces when text follows
export APP_ENV=prod         # visible to child processes
readonly MAX=5              # constant
"double $expands"   'single $literal'   "escape \$ \""

Output and input#

echo "text"                 # print + newline
printf "%-10s %5d\n" a 42   # formatted (%s text, %d integer)
read -r -p "Name: " name    # prompt + read a line
read -r a b <<< "x y"       # split into variables
read -r -s -p "Pass: " pw   # hidden input

Exit status#

cmd; echo $?                # 0 = success, 1-255 = failure
status=$?                   # save immediately
cmd && echo ok || echo fail # quick reactions (not a true if/else)
exit 0 / exit 1 / exit 2    # success / error / usage error
${PIPESTATUS[@]}            # status of every command in the last pipe

Tests#

Files	Strings	Integers `[ ]`	Integers `(( ))`
`-e` exists	`==` / `=` equal	`-eq`	`==`
`-f` regular file	`!=` different	`-ne`	`!=`
`-d` directory	`-z` empty	`-gt`	`>`
`-r` readable	`-n` not empty	`-ge`	`>=`
`-w` writable	`== pat*` glob (in `[[ ]]`)	`-lt`	`<`
`-x` executable	`=~ regex` (in `[[ ]]`)	`-le`	`<=`
`-s` not empty

[ -f "$f" ]                 # POSIX test: quote everything
[[ -f $f && -r $f ]]        # bash test: && inside, no splitting
(( n > 5 ))                 # arithmetic test

Conditionals#

if CMD; then ...; elif CMD; then ...; else ...; fi

case "$x" in
  start|begin) ... ;;
  *.log)       ... ;;
  *)           ... ;;       # default
esac

Loops#

for s in a b c; do ...; done
for n in {1..5}; do ...; done          # range (no variables inside)
for f in ./*.log; do [[ -e $f ]] || continue; ...; done
for (( i = 0; i < n; i++ )); do ...; done
while (( i < 5 )); do ((i++)); done
until [[ -f ready ]]; do sleep 1; done
while IFS= read -r line || [[ -n $line ]]; do ...; done < file
break        continue        break 2

Substitution and arithmetic#

today=$(date +%F)            # command substitution
echo "$(( 17 / 5 )) $(( 17 % 5 )) $(( 2 ** 10 ))"   # 3 2 1024
(( count += 1 ))             # safe increment under set -e
pct=$(( used * 100 / total ))  # multiply before dividing

Functions#

greet() { local who=${1:-world}; echo "Hello, $who"; }
greet "Asha"                 # call: no parentheses
msg=$(greet)                 # capture echo output = data
is_ok() { [[ $1 == ok ]]; }  # status = yes/no answer
if is_ok "$s"; then ...; fi
return 0..255                # status only; echo for data

Script arguments#

$0 script name   $1..$9, ${10}   $# count   "$@" all (separate)   "$*" all (joined)
shift            # drop $1
: "${1:?usage: $0 NAME}"   # required argument

Arrays#

a=(x y z); a+=(w)            # create / append
"${a[0]}" "${a[-1]}"          # first / last
"${a[@]}"  ${#a[@]}  "${!a[@]}"  # all / count / indexes
"${a[@]:1:2}"                 # slice
mapfile -t lines < file       # file -> array
declare -A m=([k]=v); m[x]=1  # associative (bash 4+)
"${!m[@]}"                    # keys (unordered)
[[ -n ${m[k]+set} ]]          # key exists?

String manipulation#

${#v}            length            ${v:2:3}   substring
${v#pat}  ${v##pat}   trim shortest/longest from START
${v%pat}  ${v%%pat}   trim shortest/longest from END
${v/old/new}  ${v//old/new}   replace first / all
${v^^}  ${v,,}   upper / lower
${v:-def}  ${v:=def}  ${v:?err}   default / assign default / require
${p##*/} file name   ${p%/*} folder   ${f%.*} drop extension   ${f##*.} extension

Redirection and pipes#

cmd > f      # stdout to file (overwrite)      cmd >> f    # append
cmd 2> f     # stderr to file                  cmd > f 2>&1   # both (order matters)
cmd &> f     # both (bash)                     cmd > /dev/null 2>&1   # silence
cmd < f      # stdin from file                 a | b | c   # pipeline
cmd | tee -a log   # screen + file             echo "err" >&2   # write to stderr

Here-docs#

cat <<EOF > file      # expands $vars
port=$port
EOF
cat <<'EOF'           # literal, no expansion
$HOME stays as text
EOF
cmd <<< "string"      # here-string

Pro toolkit#

set -euo pipefail                     # strict mode
trap cleanup EXIT                     # always clean up
trap 'exit 130' INT TERM              # Ctrl+C / kill
trap 'echo "line $LINENO failed" >&2' ERR
tmp=$(mktemp); dir=$(mktemp -d)       # safe temp files
mv "$tmp" final.conf                  # atomic replace
mkdir -p d; grep -qxF "$l" f || echo "$l" >> f   # idempotent
mkdir "$lock" 2>/dev/null || exit 0   # simple lock (or flock -n)
log() { printf '%s [%s] %s\n' "$(date '+%F %T')" "$1" "${*:2}" >&2; }
die() { log ERROR "$*"; exit 2; }
PS4='+ ${LINENO}: '; set -x ... set +x   # trace a section

getopts template#

while getopts ":n:vh" opt; do
  case $opt in
    n) name=$OPTARG ;;
    v) verbose=1 ;;
    h) usage; exit 0 ;;
    :) echo "-$OPTARG needs a value" >&2; exit 2 ;;
    \?) echo "unknown -$OPTARG" >&2; exit 2 ;;
  esac
done
shift $((OPTIND - 1))

cron#

# minute hour day-of-month month day-of-week  command
*/5 * * * *    /opt/scripts/health.sh >> /var/log/health.log 2>&1
0 2 * * *      /opt/scripts/backup.sh
30 9 * * 1-5   /opt/scripts/report.sh
crontab -e  (edit)   crontab -l  (list)   escape % as \%   set PATH in the script

Exit codes worth knowing#

Code	Meaning
0	success
1	general error
2	misuse / bad arguments (convention)
126	found but not executable
127	command not found
128+N	killed by signal N (130 = Ctrl+C, 143 = TERM)

Production script template#

A verified skeleton combining strict mode, logging, traps, getopts and a main function. Copy it as the starting point for new scripts. File: template-pro-skeleton.sh

#!/bin/bash
# template.sh - a starting point for production scripts (copy and rename me)
set -euo pipefail

readonly SCRIPT=${0##*/}
verbose=0
name="world"
work_dir=""

usage() { echo "Usage: $SCRIPT [-n NAME] [-v] [-h]"; }
log()   { printf '%s [%s] %s\n' "$(date '+%F %T')" "$1" "${*:2}" >&2; }
die()   { log ERROR "$*"; exit 2; }

# cleanup is called by the EXIT trap
# shellcheck disable=SC2329
cleanup() { if [[ -n $work_dir ]]; then rm -rf "$work_dir"; fi; }
trap cleanup EXIT
trap 'die "interrupted"' INT TERM

parse_args() {
  local opt
  while getopts ":n:vh" opt; do
    case $opt in
      n) name=$OPTARG ;;
      v) verbose=1 ;;
      h) usage; exit 0 ;;
      :) die "-$OPTARG needs a value" ;;
      \?) usage >&2; die "unknown option -$OPTARG" ;;
    esac
  done
  shift $((OPTIND - 1))
  [[ -n $name ]] || die "name must not be empty"
}

main() {
  parse_args "$@"
  work_dir=$(mktemp -d)
  if (( verbose )); then log INFO "working in a private temp folder"; fi
  echo "Hello, $name!" > "$work_dir/greeting.txt"
  cat "$work_dir/greeting.txt"
}

main "$@"

Run it:

bash template-pro-skeleton.sh -n Pushpjeet -v

Sample output (yours will differ: the timestamp changes every run):

2026-10-08 18:55:34 [INFO] working in a private temp folder
Hello, Pushpjeet!

Keep going#

Full tutorial with 135 examples, 25 exercises and the capstone: Bash Scripting: Zero to Pro
Test yourself: Take the Bash quiz (25 questions)
All scripts, including template-pro-skeleton.sh: Download all 162 scripts (zip)
New to the terminal? Start with Linux from Scratch and its shell scripting module (M14).