LinuxAdvanced TopicsShell Scripting

Shell Scripting

Learn how to write and use shell scripts in Linux for automation and system administration.

Shell Scripting

Shell scripting is a powerful way to automate tasks and create system administration tools in Linux. This guide covers the basics and advanced concepts of shell scripting.

Shell Script Basics

Creating a Script

Basic script structure:

#!/bin/bash
# Script name: example.sh
# Description: Basic shell script example
 
# Variables
NAME="World"
 
# Main script
echo "Hello, $NAME!"

Script Permissions

chmod +x script.sh  # Make script executable
./script.sh        # Run the script

Variables and Data Types

Variable Declaration

# String variables
NAME="John"
# Numeric variables
AGE=25
# Array variables
FRUITS=("apple" "banana" "orange")

Variable Usage

echo $NAME           # Print variable
echo ${NAME}         # Print variable (braces)
echo "${NAME} is $AGE years old"  # String interpolation

Control Structures

Conditional Statements

# If statement
if [ $AGE -ge 18 ]; then
    echo "Adult"
else
    echo "Minor"
fi
 
# Case statement
case $FRUIT in
    "apple")
        echo "Red fruit"
        ;;
    "banana")
        echo "Yellow fruit"
        ;;
    *)
        echo "Unknown fruit"
        ;;
esac

Loops

# For loop
for i in {1..5}; do
    echo "Number: $i"
done
 
# While loop
while [ $COUNTER -lt 10 ]; do
    echo $COUNTER
    ((COUNTER++))
done

Functions

Function Definition

# Basic function
function greet() {
    echo "Hello, $1!"
}
 
# Function with return value
function add() {
    echo $(($1 + $2))
}

Function Usage

greet "John"        # Call function with argument
RESULT=$(add 5 3)   # Capture function output

Input and Output

Command Line Arguments

# Access arguments
echo $1    # First argument
echo $2    # Second argument
echo $#    # Number of arguments
echo $@    # All arguments

User Input

# Read user input
read -p "Enter your name: " NAME
read -s -p "Enter password: " PASSWORD

File Operations

File Testing

# Check if file exists
if [ -f "file.txt" ]; then
    echo "File exists"
fi
 
# Check if directory exists
if [ -d "directory" ]; then
    echo "Directory exists"
fi

File Reading

# Read file line by line
while read line; do
    echo "$line"
done < "file.txt"

Error Handling

Exit Codes

# Check command success
if command; then
    echo "Success"
else
    echo "Failed"
fi
 
# Exit with status
exit 1  # Exit with error

Error Trapping

# Trap errors
set -e  # Exit on error
trap 'echo "Error occurred"' ERR

Best Practices

  1. Script Organization

    • Use clear variable names
    • Add comments
    • Follow consistent formatting
    • Use functions for modularity

  2. Error Handling

    • Check for errors
    • Use proper exit codes
    • Implement error messages
    • Handle edge cases

  3. Security

    • Validate input
    • Use quotes for variables
    • Check file permissions
    • Avoid command injection

Advanced Topics

Regular Expressions

# Pattern matching
if [[ $STRING =~ ^[0-9]+$ ]]; then
    echo "Numeric string"
fi

Process Management

# Background processes
command &
 
# Wait for process
wait $PID
 
# Kill process
kill $PID

Next Steps

Continue learning about:

  • Advanced shell features
  • Script optimization
  • Debugging techniques
  • System administration scripts
  • Automation tools
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