Easy to follow Bash Scripting Cheatsheet

#aws #serverless #linux #devops

Bash scripting is essential if you do any kind of server management such as setting up a Linux server, setting up deployment, etc. You can write bash scripts/shell scripts to automate common tasks. I created this bash scripting cheatsheet that can provide you with fundamental knowledge about bash scripting

Basic Script Structure

#!/bin/bash
# Shebang line - tells the system to use bash interpreter

# Script header
SCRIPT_NAME="example_script"
VERSION="1.0"
AUTHOR="Your Name"
DATE=$(date +"%Y-%m-%d")

Variables

# Variable Declaration
name="John Doe"
age=30

# Read-only Variables
readonly PI=3.14159

# Environment Variables
echo $HOME
echo $USER
echo $PATH

# Command Substitution
current_date=$(date)
files=$(ls)

# Default Values
var=${parameter:-default_value}  # Use default if parameter is unset or null
var=${parameter:=default_value}  # Assign default if parameter is unset or null

Input and Output

# User Input
read -p "Enter your name: " username
read -sp "Enter password: " password  # Silent input

# Standard Output
echo "Hello, World!"
printf "Formatted output: %s is %d years old\n" "$name" "$age"

# Error Output
echo "Error message" >&2

# Redirecting Output
command > output.txt        # Overwrite
command >> output.txt       # Append
command 2> error.log        # Redirect stderr
command &> all_output.log   # Redirect both stdout and stderr

Conditional Statements

# If-Else Statements
if [ condition ]; then
    # commands
elif [ another_condition ]; then
    # commands
else
    # commands
fi

# Comparison Operators
# Numeric Comparisons
[ $a -eq $b ]  # Equal
[ $a -ne $b ]  # Not Equal
[ $a -gt $b ]  # Greater Than
[ $a -lt $b ]  # Less Than
[ $a -ge $b ]  # Greater or Equal
[ $a -le $b ]  # Less or Equal

# String Comparisons
[ "$str1" = "$str2" ]   # Equal
[ "$str1" != "$str2" ]  # Not Equal
[ -z "$str" ]           # Zero length
[ -n "$str" ]           # Non-zero length

# File Condition Tests
[ -f file ]     # Regular file exists
[ -d directory ]# Directory exists
[ -r file ]     # File is readable
[ -w file ]     # File is writable
[ -x file ]     # File is executable
[ -e file ]     # File exists

Loops

# For Loop
for variable in {1..5}; do
    echo $variable
done

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

# C-style For Loop
for (( i=0; i<5; i++ )); do
    echo $i
done

# While Loop
counter=0
while [ $counter -lt 5 ]; do
    echo $counter
    ((counter++))
done

# Until Loop
counter=0
until [ $counter -ge 5 ]; do
    echo $counter
    ((counter++))
done

# Break and Continue
while true; do
    # some condition
    break       # Exit loop
    continue    # Skip to next iteration
done

Functions

# Function Definition
function greet() {
    local name=$1
    echo "Hello, $name!"
}

# Alternative Syntax
greet() {
    echo "Hello, $1!"
}

# Function with Return Value
calculate_sum() {
    return $(($1 + $2))
}

# Calling Functions
greet "Alice"
calculate_sum 5 3
echo $?  # Prints the return value

Arrays

# Declaring Arrays
fruits=("Apple" "Banana" "Cherry")
numbers=(1 2 3 4 5)

# Access Array Elements
echo ${fruits[0]}
echo ${fruits[-1]}  # Last element

# All Array Elements
echo ${fruits[@]}
echo ${#fruits[@]}  # Array Length

# Modifying Arrays
fruits+=("Date")    # Append
unset fruits[1]     # Remove element

String Manipulation

# Substring Extraction
str="Hello, World!"
echo ${str:0:5}     # Substring from index 0, length 5
echo ${str:7}       # Substring from index 7 to end

# String Length
echo ${#str}

# String Replacement
text="Hello, World!"
echo ${text/World/Universe}  # Replace first occurrence
echo ${text//o/0}            # Replace all occurrences

# Case Conversion
echo ${str^^}       # Uppercase
echo ${str,,}       # Lowercase

Command-Line Arguments

# Accessing Arguments
echo $0         # Script name
echo $1         # First argument
echo $2         # Second argument
echo $#         # Number of arguments
echo $@         # All arguments

# Argument Processing
while [[ $# -gt 0 ]]; do
    case $1 in
        -h|--help)
            display_help
            shift
            ;;
        -v|--version)
            echo "Version 1.0"
            shift
            ;;
        *)
            echo "Unknown option: $1"
            shift
            ;;
    esac
done

Error Handling

# Exit Codes
exit 0    # Successful execution
exit 1    # Generic error

# Error Checking
command || { echo "Command failed"; exit 1; }

# Set Strict Mode
set -e  # Exit immediately on error
set -u  # Treat unset variables as error
set -o pipefail  # Fail on any command in a pipeline

File Handling

# File Existence
if [ -f "file.txt" ]; then
    echo "File exists"
fi

# Reading Files
while IFS= read -r line; do
    echo "$line"
done < file.txt

# Writing Files
echo "Content" > output.txt
echo "More content" >> output.txt

Arithmetic Operations

# Basic Arithmetic
a=10
b=20
c=$((a + b))
d=$((a * b))

# Arithmetic Expansion
((counter++))
((counter += 5))

# Bc for Complex Math
result=$(echo "scale=2; 10/3" | bc)

Miscellaneous Tips

# Debugging
bash -x script.sh  # Trace script execution
set -x            # Enable debugging
set +x            # Disable debugging

# Comments
# Single-line comment
: '
Multi-line
comment
block
'

# Sleep and Timing
sleep 5        # Wait 5 seconds
time command   # Measure execution time