bash-scripting.md

Bash Scripting Cheatsheet

Basics

#!/bin/bash              # Shebang
set -euo pipefail        # Strict mode

Variables

name="Sovon"             # No spaces around =
echo "$name"             # Use double quotes
echo "${name}_suffix"    # Braces for clarity
readonly PI=3.14         # Constant
unset name               # Delete variable

Special Variables

Variable	Meaning
$0	Script name
$1-$9	Arguments
$#	Number of arguments
$@	All arguments (separate)
$*	All arguments (single string)
$?	Last exit code
$$	Script PID
$!	Last background PID

Conditionals

if [[ condition ]]; then
    commands
elif [[ condition ]]; then
    commands
else
    commands
fi

Test Operators

Operator	Description
-z "$s"	String is empty
-n "$s"	String is not empty
"$a" == "$b"	Strings equal
"$a" != "$b"	Strings not equal
$a -eq $b	Numbers equal
$a -ne $b	Not equal
$a -lt $b	Less than
$a -gt $b	Greater than
-f file	File exists
-d dir	Directory exists
-r file	Readable
-w file	Writable
-x file	Executable

Loops

# For loop
for i in 1 2 3; do echo $i; done
for i in {1..10}; do echo $i; done
for file in *.txt; do echo "$file"; done
for ((i=0; i<10; i++)); do echo $i; done

# While loop
while [[ condition ]]; do commands; done
while read -r line; do echo "$line"; done < file.txt

# Until loop
until [[ condition ]]; do commands; done

Functions

greet() {
    local name=$1        # Local variable
    echo "Hello, $name!"
    return 0
}
greet "Sovon"
result=$(greet "World") # Capture output

Arrays

arr=(a b c d)
echo ${arr[0]}           # First element
echo ${arr[@]}           # All elements
echo ${#arr[@]}          # Length
arr+=(e)                 # Append
unset arr[1]             # Delete
for item in "${arr[@]}"; do echo "$item"; done

String Operations

str="Hello World"
echo ${#str}             # Length: 11
echo ${str:0:5}          # Substring: Hello
echo ${str/World/Linux}  # Replace: Hello Linux
echo ${str,,}            # Lowercase: hello world
echo ${str^^}            # Uppercase: HELLO WORLD

File/Path Operations

path="/home/sovon/file.tar.gz"
echo ${path##*/}         # Basename: file.tar.gz
echo ${path%/*}          # Directory: /home/sovon
echo ${path##*.}         # Extension: gz
echo ${path%.tar.gz}     # Remove ext: /home/sovon/file

Input/Output

read -p "Name: " name    # Prompt
read -s -p "Pass: " pass # Silent (passwords)
echo "text" > file       # Overwrite
echo "text" >> file      # Append
cmd 2>&1                 # Stderr to stdout
cmd &>/dev/null          # Silence all output

Common Patterns

# Default values
${var:-default}          # Use default if unset
${var:=default}          # Set and use default

# Command substitution
result=$(command)
today=$(date +%Y%m%d)

# Arithmetic
((count++))
result=$((a + b))

# Ternary
[[ $a -gt $b ]] && echo "yes" || echo "no"