Bash

Command	Description
`pwd`	Print working directory
`cd dir`	Change directory
`cd ..`	Go up one level
`cd ~`	Go to home directory
`cd -`	Go to previous directory
`ls`	List files
`ls -la`	List all files with details
`ls -lh`	Human-readable file sizes
`tree`	Directory tree view
`mkdir dir`	Create directory
`mkdir -p a/b/c`	Create nested directories
`rm file`	Delete file
`rm -rf dir`	Delete directory recursively
`cp src dst`	Copy file
`cp -r src dst`	Copy directory
`mv src dst`	Move or rename
`touch file`	Create empty file / update timestamp
`ln -s target link`	Create symbolic link

Command	Description
`cat file`	Print file contents
`less file`	Page through file (`q` to quit)
`head -n 20 file`	First 20 lines
`tail -n 20 file`	Last 20 lines
`tail -f file`	Follow file (live updates)
`wc -l file`	Count lines
`wc -w file`	Count words
`sort file`	Sort lines alphabetically
`sort -n file`	Sort numerically
`uniq file`	Remove duplicate adjacent lines
`sort file \| uniq -c`	Count occurrences of each line
`cut -d: -f1 file`	Cut field 1 using `:` delimiter
`tr 'a-z' 'A-Z'`	Translate characters (lowercase to upper)
`diff file1 file2`	Show differences between files

Command	Description
`grep pattern file`	Search for pattern in file
`grep -r pattern dir`	Recursive search in directory
`grep -i pattern file`	Case-insensitive search
`grep -n pattern file`	Show line numbers
`grep -v pattern file`	Invert match (exclude lines)
`grep -l pattern dir`	List files with matches
`grep -E 'a\|b' file`	Extended regex (OR)
`find . -name "*.txt"`	Find files by name
`find . -type f -mtime -7`	Files modified in last 7 days
`find . -size +10M`	Files larger than 10MB
`locate filename`	Fast file search (uses index)
`which cmd`	Show path of command
`type cmd`	Show how a command is resolved

Syntax	Description
`cmd1 \| cmd2`	Pipe stdout of cmd1 to cmd2
`cmd > file`	Redirect stdout to file (overwrite)
`cmd >> file`	Redirect stdout to file (append)
`cmd < file`	Read stdin from file
`cmd 2> file`	Redirect stderr to file
`cmd &> file`	Redirect both stdout and stderr
`cmd 2>&1`	Redirect stderr to stdout
`cmd \| tee file`	Write to file and also stdout
`cmd1 && cmd2`	Run cmd2 only if cmd1 succeeds
`cmd1 \|\| cmd2`	Run cmd2 only if cmd1 fails
`cmd1 ; cmd2`	Run cmd2 regardless of cmd1
`$(cmd)`	Command substitution
`cmd`	Command substitution (older syntax)

Variables

name="Alice"
echo $name
echo "${name}!"       # Use braces for clarity
readonly PI=3.14      # Constant
unset name            # Delete variable

Special variables

Variable	Description
`$0`	Script name
`$1` … `$9`	Positional arguments
`$@`	All arguments (array)
`$#`	Number of arguments
`$?`	Exit status of last command
`$$`	PID of current shell
`$!`	PID of last background process
`$HOME`	Home directory
`$PATH`	Executable search path
`$PWD`	Current directory

String operations

str="Hello World"
echo ${#str}          # Length: 11
echo ${str:6}         # Substring: World
echo ${str:0:5}       # Substring: Hello
echo ${str/World/Bash}  # Replace: Hello Bash
echo ${str,,}         # Lowercase: hello world
echo ${str^^}         # Uppercase: HELLO WORLD
echo ${str% World}    # Strip suffix: Hello
echo ${str#Hello }    # Strip prefix: World

# Arithmetic expansion
echo $((3 + 4))
echo $((10 % 3))

# let
let x=5+3

# expr (older)
expr 5 + 3

# bc for floats
echo "scale=2; 10/3" | bc

Operator	Description
`+` `-` `*` `/`	Basic arithmetic
`%`	Modulo
`**`	Exponentiation
`++x` / `x++`	Pre/post increment
`--x` / `x--`	Pre/post decrement

if / elif / else

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

Test operators

Operator	Description
`-eq` `-ne` `-lt` `-le` `-gt` `-ge`	Numeric comparison
`=` `!=`	String equality
`-z str`	String is empty
`-n str`	String is non-empty
`-f file`	File exists and is regular
`-d file`	Directory exists
`-e file`	File or directory exists
`-r` `-w` `-x`	Readable / writable / executable

for loop

for i in 1 2 3; do
  echo $i
done

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

for ((i=0; i<5; i++)); do
  echo $i
done

while / until

while [ "$count" -lt 5 ]; do
  echo $count
  ((count++))
done

until [ "$count" -ge 5 ]; do
  ((count++))
done

case

case "$var" in
  start) echo "Starting" ;;
  stop)  echo "Stopping" ;;
  *)     echo "Unknown" ;;
esac

greet() {
  local name="$1"          # local variable
  echo "Hello, $name!"
  return 0                 # exit status
}

greet "Alice"
result=$(greet "Bob")      # capture output

Feature	Syntax
Define function	`name() { ... }`
Call function	`name arg1 arg2`
Local variable	`local var=value`
Return status	`return N` (0=success)
Access args	`$1`, `$2`, `$@`

# Indexed array
arr=(one two three)
echo ${arr[0]}          # one
echo ${arr[@]}          # all elements
echo ${#arr[@]}         # length: 3
arr+=(four)             # append
unset arr[1]            # remove element

# Associative array (Bash 4+)
declare -A map
map[key]="value"
echo ${map[key]}
echo ${!map[@]}         # all keys
echo ${map[@]}          # all values

Template

#!/usr/bin/env bash
set -euo pipefail       # strict mode
IFS=$'\n\t'

main() {
  echo "Args: $@"
}

main "$@"

set options

Option	Description
`set -e`	Exit on error
`set -u`	Error on undefined variable
`set -o pipefail`	Catch errors in pipes
`set -x`	Print each command (debug)
`set +x`	Disable debug

Input

read -p "Enter name: " name
read -s -p "Password: " pass   # silent input
read -t 10 input                # timeout

Exit codes

exit 0    # success
exit 1    # general error
exit 2    # misuse of command
echo $?   # check last exit code

Command	Description
`ps aux`	List all running processes
`top` / `htop`	Interactive process viewer
`kill PID`	Send SIGTERM to process
`kill -9 PID`	Force kill (SIGKILL)
`pkill name`	Kill by process name
`jobs`	List background jobs
`bg %N`	Resume job N in background
`fg %N`	Bring job N to foreground
`cmd &`	Run command in background
`nohup cmd &`	Run immune to hangup
`wait`	Wait for all background jobs
`sleep N`	Pause for N seconds
`time cmd`	Measure execution time

export VAR=value         # export to child processes
printenv                 # print all env variables
env VAR=x cmd            # run cmd with modified env
source ~/.bashrc         # reload config
. script.sh              # source script (same shell)

# PATH management
export PATH="$HOME/.local/bin:$PATH"

# Aliases
alias ll='ls -la'
alias gs='git status'
unalias ll

.bashrc essentials

# Prompt
export PS1="\u@\h:\w\$ "

# History
export HISTSIZE=10000
export HISTFILESIZE=20000
export HISTCONTROL=ignoredups:erasedups
shopt -s histappend

# Auto-complete
bind 'set show-all-if-ambiguous on'

Bash¶