Shell Scripting

Introduction to Shell Scripting

Shell scripting is the process of writing a series of commands for the shell (command-line interpreter) to execute. A shell script is a text file containing these commands, and it can automate repetitive tasks, manage systems, and even develop simple applications.

Why Use Shell Scripting?

• Automation: Perform repetitive tasks like backups and system monitoring.
• Efficiency: Combine multiple commands into a single script.
• Flexibility: Manage files, process data, and interact with the system.
• Portability: Shell scripts can run on any Unix-based system with minimal modification.

Writing a Shell Script

1. Create a New Script File

Use a text editor to create a new file with a .sh extension. For example:

nano script_name.sh

2. Add the Shebang

The first line in the script specifies the shell to be used.

Example:

#!/bin/bash

3. Write Commands

Add shell commands line by line. For instance:

#!/bin/bash
echo "Hello, World!"

4. Make the Script Executable

Change the file permissions to make it executable:

chmod +x script_name.sh

5. Run the Script

Execute the script using:

./script_name.sh

Basic Elements of Shell Scripting

1. Variables

Variables store data that can be reused.

Example:

#!/bin/bash
name="Alice"
echo "Hello, $name!"

2. Conditional Statements

Execute commands based on conditions.

Example:

#!/bin/bash
if [ $1 -gt 10 ]; then
  echo "The number is greater than 10."
else
  echo "The number is 10 or less."
fi

3. Loops

Perform repetitive tasks.

For Loop Example:

#!/bin/bash
for i in {1..5}; do
  echo "Iteration $i"
done

While Loop Example:

#!/bin/bash
count=1
while [ $count -le 5 ]; do
  echo "Count is $count"
  count=$((count + 1))
done

4. Functions

Group commands into reusable blocks.

Example:

#!/bin/bash
greet() {
  echo "Hello, $1!"
}

greet "Alice"

5. User Input

Read user-provided input.

Example:

#!/bin/bash
echo "Enter your name:"
read name
echo "Hello, $name!"

Shell Scripting Best Practices

1. Use Comments: Add comments to explain the code.

   # This is a comment
   echo "Hello, World!"
   

2. Error Handling: Check the exit status of commands.

   if [ $? -eq 0 ]; then
     echo "Command succeeded."
   else
     echo "Command failed."
   fi
   

3. Use Meaningful Variable Names: Avoid generic names like var1.

4. Test Scripts: Test in a safe environment before running in production.

5. Quote Variables: Prevent issues with spaces or special characters.

   file_name="My File.txt"
   echo "$file_name"
   

Advanced Topics

1. Script Arguments

Pass arguments to scripts via the command line.

Example:

#!/bin/bash
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"

Run the script:

./script_name.sh arg1 arg2

2. Redirecting Output

Send output to a file or another command.

Examples:

echo "Hello" > output.txt   # Write to file
cat file.txt | grep "text"  # Pipe output

3. Cron Jobs

Schedule scripts to run at specific times using cron.

Edit the crontab:

crontab -e

Add a cron job:

0 5 * * * /path/to/script.sh  # Run at 5:00 AM daily

4. Debugging

Use set -x to debug scripts.

#!/bin/bash
set -x
echo "Debugging this script."

Example Shell Script

Here’s an example of a backup script:

#!/bin/bash

# Backup script

# Variables
source_dir="/path/to/source"
backup_dir="/path/to/backup"
timestamp=$(date +%Y%m%d_%H%M%S)
backup_file="$backup_dir/backup_$timestamp.tar.gz"

# Create a backup
echo "Creating backup..."
tar -czf $backup_file $source_dir

if [ $? -eq 0 ]; then
  echo "Backup created successfully: $backup_file"
else
  echo "Backup failed!"
fi

Conclusion

Shell scripting is a versatile tool for automating tasks, managing systems, and simplifying workflows. Mastering shell scripting opens the door to advanced system administration and DevOps practices.

Let me know if you’d like examples of specific use cases or more advanced topics!