Script

This comprehensive guide covers shell scripting fundamentals and advanced techniques in Linux.

📝 Basic Script Structure

Script Basics

#!/bin/bash                  # Shebang - defines shell to use
# Script description here
# Author: Your Name
# Date: YYYY-MM-DD

# Variables
NAME="John"
AGE=25

# Main script content
echo "Hello, $NAME!"

Running Scripts

# Make script executable
chmod +x script.sh

# Run script
./script.sh
bash script.sh
source script.sh            # Run in current shell

🔄 Variables and Data Types

Variable Declaration

# Basic variables
NAME="John"                 # String
AGE=25                     # Integer
PRICE=99.99                # Float
readonly CONSTANT="value"   # Constant

# Arrays
ARRAY=(item1 item2 item3)
echo "${ARRAY[0]}"         # Access first element
echo "${ARRAY[@]}"         # All elements
echo "${#ARRAY[@]}"        # Array length

Special Variables

$0      # Script name
$1-$9   # First 9 arguments
$#      # Number of arguments
$@      # All arguments
$?      # Last command exit status
$$      # Current process ID
$USER   # Current username
$HOME   # Home directory
$PWD    # Current directory

🔍 Control Structures

Conditional Statements

# If statement
if [ "$a" -eq "$b" ]; then
    echo "Equal"
elif [ "$a" -gt "$b" ]; then
    echo "Greater"
else
    echo "Less"
fi

# Case statement
case "$var" in
    pattern1)
        commands
        ;;
    pattern2)
        commands
        ;;
    *)
        default commands
        ;;
esac

Loops

# For loop
for i in {1..5}; do
    echo "$i"
done

# While loop
while [ "$count" -le 5 ]; do
    echo "$count"
    ((count++))
done

# Until loop
until [ "$count" -gt 5 ]; do
    echo "$count"
    ((count++))
done

📊 Functions

Function Declaration

# Basic function
function_name() {
    echo "Arguments: $@"
    return 0
}

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

# Call functions
function_name arg1 arg2
greet "John"

Function Libraries

# Source function library
source /path/to/library.sh

# Create library
lib_function() {
    # Function code
}

# Export functions
export -f lib_function

📥 Input/Output

User Input

# Read input
read -p "Enter name: " name
read -s -p "Password: " pass  # Silent input
read -n 1 -p "Continue? [Y/n]: " answer

# Select menu
select option in "Option 1" "Option 2" "Exit"; do
    case $option in
        "Exit")
            break
            ;;
        *)
            echo "Selected: $option"
            ;;
    esac
done

File Operations

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

# Write to file
echo "content" > file.txt    # Overwrite
echo "content" >> file.txt   # Append

🛠️ String Operations

String Manipulation

# String length
string="Hello"
echo "${#string}"

# Substring
echo "${string:0:2}"      # First 2 chars
echo "${string: -2}"      # Last 2 chars

# Replace
echo "${string/l/L}"      # First match
echo "${string//l/L}"     # All matches

Pattern Matching

# Remove pattern
echo "${string#pattern}"   # From beginning
echo "${string%pattern}"   # From end

# Case modification
echo "${string^^}"        # To uppercase
echo "${string,,}"        # To lowercase

🔢 Arithmetic Operations

Using expr

echo "Using expr"
expr 2 + 2
expr 2 - 2
expr 2 \* 2
expr 2 / 2
expr 2 % 2

Using $(( ))

echo "Using $(( ))"
echo $((2 + 2))
echo $((2 - 2))
echo $((2 * 2))
echo $((2 / 2))
echo $((2 % 2))

For float numbers use bc

echo "Using bc"
echo 4/3 | bc -l # bc is a calculator

Arithmetic operations with variable

A=2
B=3
echo "Using variables"
echo $(( A + B ))
echo $(( A - B ))
echo $(( A * B ))
echo $(( A / B ))
echo $(( A % B ))

Equals operators

Operator	Description
[ "abc" = "abc" ]	If string1 is exactly equal to string2 - true
[ "abc" != "abc" ]	If string1 is equal to string2 - false
[ 5 -eq 5 ]	If integer1 is equal to integer2 - true
[ 5 -ne 5 ]	If integer1 is equal to integer2 - false
[ 6 -gt 5 ]	If integer1 is greater than integer2 - true
[ 6 -lt 5 ]	If integer1 is less than integer2 - false
[[ "abcd" = bc ]]	If abcd contains bc - true
[[ "abc" = ab[cd] ]]	If 3rd character of abc is c or d - true
[[ "abe" = ab[cd] ]]	If 3rd character of abc is c or d - false
[[ "abc" < "bcd" ]]	If "abc" comes after "bcd" when sorted in alphabetical order - true
[[ "abc" > "bcd" ]]	If "abc" comes after "bcd" when sorted in alphabetical order - false

Conditional operator

Example	Description
[[ A -gt 4 && A -lt 9 ]]	If A is greater than 4 and less than 9
[[ A -gt 4 || A -lt 9 ]]	If A is greater than 4 or less than 9
-e FILE	If FILE exists
-d FILE	If FILE is a directory
-s FILE	If FILE is not empty
-x FILE	If FILE is executable
-w FILE	If FILE is writable

Basic Math

# Arithmetic expansion
result=$((5 + 3))
((result++))

# bc for floating point
result=$(echo "scale=2; 5.75 + 3.25" | bc)

# expr for integers
result=$(expr 5 + 3)

🐛 Debugging

Debug Options

# Enable debugging
set -x                    # Print commands
set -e                    # Exit on error
set -u                    # Error on undefined variables

# Debug specific section
set -x
commands
set +x

💡 Best Practices

1. Script Structure

   • Use meaningful variable names

   • Comment your code

   • Use functions for repeated code

   • Handle errors appropriately

2. Security

   # Quote variables
   file_name="$1"
   
   # Check arguments
   if [ $# -lt 1 ]; then
       echo "Error: Missing arguments"
       exit 1
   fi
   
   # Set secure permissions
   chmod 755 script.sh

3. Error Handling

   # Error function
   error() {
       echo "Error: $1" >&2
       exit 1
   }
   
   # Command error checking
   command || error "Command failed"

📋 Script Template

#!/bin/bash
#
# Script Name: script.sh
# Description: Brief description
# Author: Your Name
# Date: YYYY-MM-DD

# Exit on error
set -e

# Script variables
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly SCRIPT_NAME="$(basename "$0")"

# Functions
usage() {
    cat << EOF
Usage: $SCRIPT_NAME [options]
Options:
    -h, --help    Show this help message
    -v, --version Show version
EOF
}

# Main logic
main() {
    # Parse arguments
    while [[ $# -gt 0 ]]; do
        case "$1" in
            -h|--help)
                usage
                exit 0
                ;;
            *)
                echo "Unknown option: $1"
                usage
                exit 1
                ;;
        esac
        shift
    done

    # Main script logic here
}

# Run main if script is not sourced
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    main "$@"
fi