A Bash Cheat Sheet

A quick reference for the most important Bash commands, syntax, and idioms.

File and Directory Operations

Command	Purpose
ls [-la]	List files (a=all, l=long format)
pwd	Print working directory
cd DIR	Change directory
mkdir [-p] DIR	Create directory (p=parents)
rm [-rf] FILE	Remove file or directory
cp [-r] SRC DEST	Copy file or directory
mv SRC DEST	Move or rename
touch FILE	Create empty file or update timestamp
find PATH -name PATTERN	Find files matching pattern
locate FILE	Find file by name (uses database)

Viewing File Content

Command	Purpose
cat FILE	Display entire file
less FILE	View file with paging
head [-n N] FILE	Show first N lines (default 10)
tail [-n N] FILE	Show last N lines
wc [-l] FILE	Count lines, words, characters
grep PATTERN FILE	Search for pattern in file
grep -i PATTERN FILE	Case-insensitive search
grep -r PATTERN DIR	Recursive search in directory

Text Processing and Transformation

Command	Purpose
sed 's/OLD/NEW/' FILE	Replace first occurrence per line
sed 's/OLD/NEW/g' FILE	Replace all occurrences
awk '{print $1}' FILE	Print first field of each line
awk 'NR > 1' FILE	Skip header row
sort FILE	Sort lines alphabetically
sort -n FILE	Sort numerically
uniq FILE	Remove consecutive duplicate lines
cut -d: -f1 FILE	Cut by delimiter, extract field
tr 'a-z' 'A-Z'	Translate characters (uppercase)

Permissions and Ownership

Command	Purpose
chmod 755 FILE	Set permissions (rwxr-xr-x)
chmod +x FILE	Add execute permission
chmod -w FILE	Remove write permission
chown USER FILE	Change file owner
chown USER:GROUP FILE	Change owner and group
chgrp GROUP FILE	Change group ownership
umask 022	Set default permissions mask

Process and System Information

Command	Purpose
ps	Show current processes
ps aux	Show all processes with details
top	Interactive process monitor
kill PID	Terminate process by ID
kill -9 PID	Force kill process
bg	Run job in background
fg	Bring job to foreground
jobs	List background jobs
&	Run command in background (append to command)
uname -a	System information
df -h	Disk space usage (human readable)
du -sh DIR	Directory size
free -h	Memory usage
whoami	Current user
id	User and group IDs

Bash Variables and Expansion

Syntax	Meaning
$VAR	Variable value
${VAR}	Variable value (explicit boundaries)
${VAR:-DEFAULT}	Use DEFAULT if VAR is unset
${VAR:=DEFAULT}	Set and use DEFAULT if unset
${VAR:+VALUE}	Use VALUE if VAR is set
${VAR#PATTERN}	Remove shortest prefix match
${VAR##PATTERN}	Remove longest prefix match
${VAR%PATTERN}	Remove shortest suffix match
${VAR%%PATTERN}	Remove longest suffix match
${VAR/OLD/NEW}	Replace first occurrence
${VAR//OLD/NEW}	Replace all occurrences
${!PREFIX*}	Names of variables starting with PREFIX
${#VAR}	Length of variable
$(COMMAND)	Command substitution
`COMMAND`	Command substitution (deprecated)
$((EXPR))	Arithmetic expansion

Conditional Testing

Test	Meaning
[ -f FILE ]	File exists and is regular file
[ -d DIR ]	Directory exists
[ -e FILE ]	File exists (any type)
[ -z STR ]	String is empty
[ -n STR ]	String is not empty
[ STR1 = STR2 ]	Strings are equal
[ STR1 != STR2 ]	Strings are not equal
[ NUM1 -eq NUM2 ]	Numbers are equal
[ NUM1 -ne NUM2 ]	Numbers are not equal
[ NUM1 -lt NUM2 ]	NUM1 less than NUM2
[ NUM1 -le NUM2 ]	NUM1 less than or equal
[ NUM1 -gt NUM2 ]	NUM1 greater than NUM2
[ NUM1 -ge NUM2 ]	NUM1 greater than or equal
[[ STR =~ REGEX ]]	String matches regex (Bash only)
[ COND1 -a COND2 ]	Logical AND
[ COND1 -o COND2 ]	Logical OR
[ ! COND ]	Logical NOT

Control Flow

# if-then-else
if CONDITION; then
  COMMANDS
elif CONDITION; then
  COMMANDS
else
  COMMANDS
fi

# case statement
case "$VAR" in
  PATTERN1) COMMANDS ;;
  PATTERN2) COMMANDS ;;
  *) DEFAULT_COMMANDS ;;
esac

# for loop
for VAR in LIST; do
  COMMANDS
done

# while loop
while CONDITION; do
  COMMANDS
done

# until loop
until CONDITION; do
  COMMANDS
done

Functions

function_name() {
  COMMANDS
  return EXIT_CODE
}

# Call function with arguments
function_name arg1 arg2 arg3

# Inside function, arguments are:
# $1 = arg1, $2 = arg2, $3 = arg3
# $@ = all arguments
# $# = number of arguments
# $0 = script name

Redirection and Pipes

Operator	Purpose
> FILE	Redirect stdout to file (overwrite)
>> FILE	Redirect stdout to file (append)
< FILE	Redirect stdin from file
2> FILE	Redirect stderr to file
2>&1	Redirect stderr to stdout
| COMMAND	Pipe stdout to command
|& COMMAND	Pipe stdout and stderr to command
&> FILE	Redirect both stdout and stderr

Special Variables

Variable	Meaning
$0	Script name
$1, $2, …	Positional parameters
$@	All positional parameters (quoted array)
$*	All positional parameters (unquoted)
$#	Number of positional parameters
$?	Exit code of last command
$$	Process ID of current shell
$!	Process ID of last background job
$-	Current shell options
$LINENO	Current line number
$SECONDS	Seconds since shell startup

String Operations

# Length
${#string}

# Substring from position 0, length 5
${string:0:5}

# Remove prefix
${string#prefix}
${string##prefix}  # longest match

# Remove suffix
${string%suffix}
${string%%suffix}  # longest match

# Replace first/all
${string/find/replace}
${string//find/replace}

# Uppercase/Lowercase (Bash 4+)
${string^^}  # uppercase
${string,,}  # lowercase

Arrays

# Indexed array
arr=(value1 value2 value3)
arr[0]="new_value"
arr+=("new_element")

# Associative array (requires declare -A)
declare -A map
map[key1]="value1"
map[key2]="value2"

# Access
${arr[0]}      # First element
${arr[@]}      # All elements
${arr[*]}      # All elements as string
${!arr[@]}     # All indices
${#arr[@]}     # Array length

Common Patterns

Loop through files

for file in *.txt; do
  echo "Processing $file"
done

Check if file exists

if [[ -f "$file" ]]; then
  echo "File exists"
fi

Default value

filename="${1:-.bashrc}"  # Use .bashrc if $1 not provided

Safe script header

#!/bin/bash
set -euo pipefail
trap 'echo "Error on line $LINENO"' ERR

Check for required arguments

if [[ $# -lt 2 ]]; then
  echo "Usage: script.sh ARG1 ARG2"
  exit 1
fi

Error handling

if ! command; then
  echo "Command failed" >&2
  exit 1
fi

Important Flags

Common command flags

• -v or --verbose - Verbose output

• -f or --force - Force operation

• -r or -R - Recursive

• -n or --dry-run - Show what would be done

• -h or --help - Show help

• -V or --version - Show version