A quick-reference cheatsheet for Bash scripting and the command line. Bash is both an interactive shell and a scripting language available on virtually every Unix-like system. Scripts start with #!/usr/bin/env bash and should be made executable with chmod +x script.sh.
Variables are untyped strings by default. No spaces around = when assigning. Always double-quote variable expansions ("$var") to prevent word splitting and glob expansion on values that contain spaces or special characters.
name="Alice"
echo "$name" # always quote expansions
echo "${name}_suffix" # braces disambiguate the variable name${var:-default} # use default if var is unset or empty
${var:=default} # assign default if var is unset or empty
# without the colon: only triggers when var is unset, not emptyreadonly PI=3.14
export PATH="$HOME/bin:$PATH"$0 # script name
$1..$9 # positional args
$@ # all args (as array)
$# # arg count
$? # last exit code
$$ # current PIDcount=$((count + 1))
((i++))
# $((...)) is POSIX; use it instead of let or exprBash provides built-in string manipulation via parameter expansion — no need to invoke external tools like sed or awk for simple operations. Command substitution captures the output of a command into a variable.
${#var}${var:offset:length}
# e.g. ${str:2:4} — starts at index 2, takes 4 chars${var/find/replace} # first match
${var//find/replace} # all matches${var#prefix} # strip shortest prefix match
${var##longest_prefix} # strip longest prefix match
${var%suffix} # strip shortest suffix match
${var%%longest_suffix} # strip longest suffix match${var^^} # uppercase
${var,,} # lowercasefiles=$(ls -1 *.txt)
today=$(date +%Y-%m-%d)
echo "Backed up on $today"Bash conditions test exit codes — 0 means true, non-zero means false. Use [[ ... ]] (Bash built-in) instead of [ ... ] (POSIX test) — it handles empty strings and spaces safely without quoting tricks, and supports &&, ||, and regex matching.
if [[ "$1" == "start" ]]; then
echo "starting"
elif [[ "$1" == "stop" ]]; then
echo "stopping"
else
echo "unknown command"
fi-z "$str" # empty string
-n "$str" # non-empty string
-f file # is a regular file
-d dir # is a directory
-e path # path exists
-r / -w / -x # readable / writable / executablefor file in *.txt; do
echo "Processing $file"
done
for i in {1..5}; do echo $i; done
for ((i=0; i<10; i++)); do echo $i; donewhile IFS= read -r line; do
echo "$line"
done < input.txtcase "$env" in
prod) echo "production" ;;
staging) echo "staging" ;;
dev|*) echo "development" ;;
esacFunctions in Bash are just named command groups. Arguments are accessed as $1, $2, etc. — there is no parameter list. Use local to scope variables to the function; without it, all variables are global by default.
greet() {
local name="${1:-World}"
echo "Hello, $name!"
}
greet "Alice" # Hello, Alice!
greet # Hello, World!is_even() {
(( $1 % 2 == 0 )) # exit code 0 = true
}
if is_even 4; then echo "even"; fi
# capture output instead
get_user() { echo "alice"; }
user=$(get_user)Bash supports indexed and associative arrays. Always quote array expansions with "${arr[@]}" to preserve elements that contain spaces. "${arr[*]}" joins all elements into one string.
fruits=("apple" "banana" "cherry")
echo "${fruits[0]}" # apple
echo "${fruits[@]}" # all elements
echo "${#fruits[@]}" # length: 3
fruits+=("date") # append
for f in "${fruits[@]}"; do echo "$f"; donedeclare -A colors
colors[red]="#ff0000"
colors[green]="#00ff00"
echo "${colors[red]}"
for key in "${!colors[@]}"; do
echo "$key = ${colors[$key]}"
doneEvery process has three standard file descriptors: stdin (0), stdout (1), and stderr (2). Redirection operators route these streams to files or other commands. Pipes chain commands so stdout of one becomes stdin of the next.
cmd > out.txt # overwrite
cmd >> out.txt # append
cmd 2> err.txt # stderr only
cmd > out.txt 2>&1 # stdout + stderr togethercmd > /dev/null 2>&1ps aux | grep nginx | awk '{print $2}'
cat access.log | sort | uniq -c | sort -rn | head -20cat <<EOF
line one
line two
EOF
# indented here-doc (Bash 4+, strips leading tabs)
cat <<-EOF
line one
line two
EOFread -rp "Enter name: " name
read -rs -p "Password: " pass # -s = silent, no echoBash can run commands in the background, wait for them, and check their exit codes. The trap built-in lets you handle signals and run cleanup code when a script exits.
long_task &
pid=$!
wait $pid # wait for specific PIDcmd && echo "ok" || echo "failed"
cmd; echo "exit: $?"tmpfile=$(mktemp)
trap "rm -f $tmpfile" EXIT
# also handle Ctrl-C
trap "echo 'interrupted'; exit 1" INT TERMA few idioms that make scripts more robust and maintainable. set -euo pipefail should be at the top of almost every script — it prevents silent failures that are hard to debug later.
#!/usr/bin/env bash
set -euo pipefail
# -e exit on error
# -u error on unset variables
# -o pipefail pipe fails if any command failsDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
# use $DIR to reference files relative to the scriptif ! command -v docker &> /dev/null; then
echo "docker is required but not installed" >&2
exit 1
fiif [[ "$EUID" -ne 0 ]]; then
echo "Run as root" >&2
exit 1
fiwhile getopts "vf:" opt; do
case $opt in
v) verbose=true ;;
f) file="$OPTARG" ;;
*) echo "Usage: $0 [-v] [-f file]" >&2; exit 1 ;;
esac
done
shift $((OPTIND - 1)) # remaining positional args