Bash Scripting 31 🐧 Cheatsheet — Quick Reference

A compact reference for everything covered in the Bash Scripting series. Tables only, no prose. Use it during script writing and debugging.

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Script Headers

Pattern	Effect
#!/bin/bash	Shebang, Bash interpreter
#!/usr/bin/env bash	Portable shebang
set -e	Exit on failing command
set -u	Exit on unset variable
set -o pipefail	Pipeline fails on any stage failure
set -euo pipefail	Strict mode (all three)
set -x	Trace commands to stderr
set +x	Disable tracing
IFS=$'\n\t'	Safer word splitting

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Variables

Syntax	Purpose
var=value	Assign (no spaces around =)
local var=value	Function-local variable
readonly var=value	Immutable variable
export var=value	Export to child processes
${var}	Expand variable
${var:-default}	Default if unset/empty
${var-default}	Default if unset
${var:=default}	Assign default if unset
${var:?message}	Error if unset
${var:+alt}	Alt if set
${#var}	Length
${var:offset:length}	Substring
${var#pattern}	Remove shortest prefix
${var##pattern}	Remove longest prefix
${var%pattern}	Remove shortest suffix
${var%%pattern}	Remove longest suffix
${var/old/new}	Replace first match
${var//old/new}	Replace all matches
${var^^}	Uppercase
${var,,}	Lowercase

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Special Variables

Variable	Value
$0	Script name
$1–$9	Positional arguments
${10}	Tenth argument and beyond
$#	Argument count
$@	All arguments (quoted = separate words)
$*	All arguments (quoted = one word)
$?	Last exit status
$$	Shell PID
$BASHPID	Current Bash PID (subshell-aware)
$!	Last background PID
$LINENO	Current line number
$BASH_COMMAND	Command being executed
$FUNCNAME	Current function name
$RANDOM	Random 0–32767
$SECONDS	Seconds since shell started
$PIPESTATUS	Array of pipeline exit codes

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Conditionals

Syntax	Meaning
[[ -f file ]]	Regular file exists
[[ -d dir ]]	Directory exists
[[ -e path ]]	Anything exists
[[ -L path ]]	Symbolic link
[[ -r file ]]	Readable
[[ -w file ]]	Writable
[[ -x file ]]	Executable
[[ -s file ]]	Non-empty
[[ -O file ]]	Owned by user
[[ -G file ]]	Owned by group
[[ -N file ]]	Modified since read
[[ $a -eq $b ]]	Numeric equal
[[ $a -ne $b ]]	Numeric not equal
[[ $a -lt $b ]]	Numeric less than
[[ $a -gt $b ]]	Numeric greater than
[[ $a -le $b ]]	Numeric less or equal
[[ $a -ge $b ]]	Numeric greater or equal
[[ $a == $b ]]	String equal (pattern match)
[[ $a != $b ]]	String not equal
[[ $a < $b ]]	String less than (lexical)
[[ $a > $b ]]	String greater than
[[ $a =~ regex ]]	Regex match
[[ -z $a ]]	Empty string
[[ -n $a ]]	Non-empty string
[[ $a && $b ]]	Logical AND
[[ $a || $b ]]	Logical OR
[[ ! $a ]]	Logical NOT

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String Comparison vs Numeric

Type	Operators
String	=, !=, <, >, -z, -n
Numeric	-eq, -ne, -lt, -le, -gt, -ge
Arithmetic	(( )) for all math

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Arithmetic

Syntax	Purpose
$((a + b))	Addition
$((a - b))	Subtraction
$((a * b))	Multiplication
$((a / b))	Integer division
$((a % b))	Modulo
$((a ** b))	Exponentiation
((a++))	Increment
((a--))	Decrement
((a += 5))	Compound assignment
let "a = 1 + 2"	Alternative syntax

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Loops

Pattern	Purpose
for i in 1 2 3; do ...; done	List iteration
for ((i=0; i<10; i++)); do ...; done	C-style loop
for i in {1..10}; do ...; done	Brace range
for f in *.txt; do ...; done	Glob iteration
for ((i=0; i<${#arr[@]}; i++)); do ...; done	Array by index
while [[ $i -lt 10 ]]; do ...; done	While loop
while read -r line; do ...; done < file	Read file line by line
until [[ $i -ge 10 ]]; do ...; done	Until loop
break	Exit loop
continue	Skip to next iteration
break 2	Exit two nested loops

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Arrays

Syntax	Purpose
arr=()	Empty array
arr=(a b c)	Initialize
arr+=(d)	Append
${arr[0]}	First element
${arr[-1]}	Last element
${arr[@]}	All elements (separate)
${arr[*]}	All elements (joined)
${#arr[@]}	Element count
${!arr[@]}	All indices
${arr[@]:1:2}	Slice (offset 1, length 2)
unset arr[1]	Remove element
declare -A map	Associative array
map[key]=value	Set key
${map[key]}	Get key
${!map[@]}	All keys
${map[@]}	All values

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Functions

Syntax	Purpose
name() { ...; }	Define function
function name { ...; }	Alternative syntax
local var=value	Local variable
return N	Return status (0–255)
$1, $@, $#	Function arguments
${FUNCNAME[0]}	Current function name
name arg1 arg2	Call function

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Pipelines and Redirection

Syntax	Purpose
cmd > file	Redirect stdout (overwrite)
cmd >> file	Redirect stdout (append)
cmd 2> file	Redirect stderr
cmd &> file	Redirect both
cmd 2>&1	Merge stderr into stdout
cmd < file	Redirect stdin
cmd <<< "string"	Here-string
cmd <<EOF ... EOF	Here-document
cmd1 | cmd2	Pipe stdout to stdin
cmd1 |& cmd2	Pipe stdout+stderr
cmd >(other)	Process substitution (output)
cmd <(other)	Process substitution (input)
tee file	Write to file and stdout
exec 3> file	Open FD 3 for writing
exec 3>&-	Close FD 3

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Job Control

Command	Purpose
cmd &	Run in background
jobs	List jobs
jobs -l	List with PIDs
fg %1	Foreground job 1
bg %1	Background job 1
Ctrl+Z	Suspend foreground job
kill %1	Kill job by number
kill $pid	Kill by PID
kill -9 $pid	SIGKILL
kill -0 $pid	Check liveness
wait	Wait for all background jobs
wait $pid	Wait for specific PID
disown	Detach from job table
nohup cmd &	Ignore SIGHUP

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Signals and Traps

Signal	Number	Meaning
SIGHUP	1	Terminal closed
SIGINT	2	Ctrl+C
SIGQUIT	3	Ctrl+\
SIGKILL	9	Forced kill (uncatchable)
SIGTERM	15	Polite termination
SIGTSTP	20	Ctrl+Z
SIGCONT	18	Resume
EXIT	—	Shell exits
ERR	—	set -e failure
DEBUG	—	Before each command

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Trap Patterns

Pattern	Purpose
trap 'cleanup' EXIT	Run cleanup on any exit
trap 'echo interrupted; exit 130' INT	Handle Ctrl+C
trap 'echo terminated; exit 143' TERM	Handle SIGTERM
trap 'echo error line $LINENO' ERR	Report failures
trap - INT	Reset to default
trap -p	Show registered traps
trap -l	List signal names

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Text Processing

Command	Purpose
grep pattern file	Search lines
grep -E 'regex' file	Extended regex
grep -c	Count matches
grep -v	Invert match
grep -i	Case-insensitive
grep -o	Output only match
sed 's/old/new/' file	Replace first per line
sed 's/old/new/g'	Replace all
sed -i	Edit in place
sed -n '5,10p'	Print lines 5–10
awk '{print $1}'	Print first field
awk -F: '{print $1}'	Custom delimiter
awk '$3 > 100'	Filter by field
cut -d: -f1	Cut field
sort file	Sort lines
sort -n	Numeric sort
sort -u	Unique sort
uniq	Remove adjacent duplicates
uniq -c	Count occurrences
tr 'a-z' 'A-Z'	Translate characters
tr -d '\n'	Delete characters
head -n 10	First 10 lines
tail -n 10	Last 10 lines
tail -f	Follow file
wc -l	Count lines
wc -w	Count words
wc -c	Count bytes
xargs	Build command from stdin

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Regular Expressions

Pattern	Meaning
.	Any character
*	Zero or more
+	One or more
?	Zero or one
^	Start of line
$	End of line
[abc]	Character class
[^abc]	Negated class
[a-z]	Range
(abc)	Group
a|b	Alternation (ERE)
{n}	Exactly n (ERE)
{n,m}	Between n and m (ERE)
\d	Digit (PCRE)
\w	Word char (PCRE)
\s	Whitespace (PCRE)
[[:digit:]]	POSIX digit
[[:alpha:]]	POSIX letter
[[:space:]]	POSIX whitespace

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Bash Regex

Syntax	Meaning
[[ $x =~ pattern ]]	Regex match
[[ $x =~ $var ]]	Pattern from variable (unquoted)
[[ $x =~ "pattern" ]]	Literal (quoted)
${BASH_REMATCH[0]}	Full match
${BASH_REMATCH[1]}	First group

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getopts

Syntax	Meaning
getopts "ab:c" opt	Parse: -a flag, -b arg, -c flag
getopts ":ab:" opt	Silent mode (leading colon)
$opt	Current option letter
$OPTARG	Option argument
$OPTIND	Next argument index
while getopts ...; do case ...; esac; done	Standard loop
shift $((OPTIND - 1))	Remove parsed options

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Date and Time

Format	Output
%Y-%m-%d	2026-11-05
%H:%M:%S	14:30:00
%Y-%m-%dT%H:%M:%S%z	2026-11-05T14:30:00+0100
%s	Unix epoch
%A	Weekday name
%B	Month name
%j	Day of year
date -u	UTC
date -d "yesterday"	Relative date
date +%s	Epoch seconds

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File Operations

Command	Purpose
touch file	Create or update timestamp
mkdir -p dir	Create with parents
cp -r src dst	Copy recursively
mv src dst	Move/rename
rm -rf dir	Remove recursively
ln -s target link	Symlink
ln target link	Hard link
chmod 755 file	Set permissions
chown user:group file	Set ownership
mktemp	Temporary file
mktemp -d	Temporary directory
find . -name "*.log"	Find files
find . -type f -mtime +7	Find older files
find . -exec cmd {} \;	Execute on results
basename /a/b/c.txt	c.txt
dirname /a/b/c.txt	/a/b
realpath file	Absolute path

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Process Management

Command	Purpose
ps aux	List processes
ps -ef	Alternative format
pgrep name	Find PID by name
pkill name	Kill by name
top	Interactive process viewer
htop	Better process viewer
lsof -p pid	Open files
ss -tlnp	Listening ports
systemctl status svc	Service status
systemctl restart svc	Restart service

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File Locking

Pattern	Purpose
flock -n file cmd	Non-blocking lock
flock -w 10 file cmd	Wait up to 10s
( flock -n 200 || exit 1; ... ) 200>file	In-script lock
exec env FLOCKER="$0" flock -en "$0" "$0" "$@"	Self-locking

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SSH

Command	Purpose
ssh user@host cmd	Run command
ssh -o BatchMode=yes host cmd	No prompts
ssh -o ConnectTimeout=10 host cmd	Timeout
ssh -o ControlMaster=yes -fN host	Master connection
ssh -o ControlPath=sock host cmd	Reuse master
ssh -O exit host	Close master
ssh-keygen -t ed25519 -f key -N ""	Generate key
ssh-copy-id -i key.pub user@host	Copy public key

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Debugging

Command	Purpose
set -x	Trace commands
set +x	Stop tracing
bash -n script.sh	Check syntax
bash -x script.sh	Run with tracing
shellcheck script.sh	Static analysis
trap '...' DEBUG	Trap before each command
declare -p var	Show variable type and value
type cmd	Where command is defined
command -v cmd	Find command path

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Common Patterns

Pattern	Purpose
cmd || true	Ignore failure
cmd || { echo "fail"; exit 1; }	Fail with message
[[ -d dir ]] || mkdir -p dir	Create if missing
while read -r line; do ...; done < file	Read file
while IFS= read -r line; do ...; done < file	Preserve whitespace
read -p "Prompt: " var	Read user input
read -s -p "Password: " var	Read password silently
printf '%s\n' "${arr[@]}"	Print array
mapfile -t arr < file	Read file into array
exec 3>&1	Save stdout
exec > logfile	Redirect all output
exec >&3	Restore stdout

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Quick Rules

Rule	Reason
Quote variables: "$var"	Prevent word splitting
Use [[ ]] over [ ]	Safer, supports regex
Use $(...) over backticks	Nested, readable
Use printf over echo	Portable, predictable
Use local in functions	Prevent scope leaks
Use set -euo pipefail	Fail fast
Use -r with read	Prevent backslash interpretation
Use -- to end options	Handle filenames starting with -
Prefer arrays over space-separated strings	Handle spaces correctly

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