Shell Scripting

 

Introduction to Shell Scripting

What is Shell Scripting?

Shell scripting is the process of automating repetitive tasks in a Unix/Linux environment by writing a series of commands in a script file that can be executed within a shell. This eliminates the need for manual execution, improving efficiency and reducing human error.

Why Use Shell Scripting?

✔️ Automates day-to-day system administration tasks.
✔️ Saves time by executing multiple commands in a single script.
✔️ Enhances consistency and reduces manual errors.
✔️ Helps with troubleshooting, deployment, and server management.

---

Understanding Shell Script Basics

Shebang (#!) – Specifying the Interpreter

A shell script starts with a shebang (#!), which tells the system which shell interpreter should execute the script.

Common Shebang Lines:

bash

#!/bin/bash   # Default Bash Shell
#!/bin/sh      # POSIX-compliant Shell
#!/bin/dash    # Lightweight Shell
#!/bin/ksh     # KornShell

💡 Note: Different Linux distributions may link /bin/sh to different shell interpreters.

Basic Shell Commands

• man [command] → Displays the manual page for a command.
• echo "Hello, World!" → Prints text to the terminal.
• cat file.txt → Displays the contents of a file.
• chmod +x script.sh → Grants execute permission to a script.
• ./script.sh → Runs the script.

---

Control Flow in Shell Scripting

Conditional Statements: if-else

Used for decision-making in scripts.

bash

#!/bin/bash
read -p "Enter a number: " num
if [ $num -gt 10 ]; then
    echo "Number is greater than 10"
else
    echo "Number is 10 or less"
fi

Looping Constructs:

Loops allow repetitive execution of commands.

For Loop:

bash

for i in {1..5}; do
    echo "Iteration: $i"
done

While Loop:

bash

i=1
while [ $i -le 5 ]; do
    echo "Iteration: $i"
    ((i++))
done

Until Loop:

bash

i=1
until [ $i -gt 5 ]; do
    echo "Iteration: $i"
    ((i++))
done

---

Break vs Continue Statements

• break → Stops loop execution completely.
• continue → Skips the current iteration and proceeds to the next cycle.

Example:

bash

for i in {1..5}; do
    if [ $i -eq 3 ]; then
        continue  # Skips iteration when i=3
    fi
    echo "Iteration: $i"
done

---

Signal Handling with trap

The trap command helps in handling system signals, preventing abrupt script termination.

💡 Example: Preventing script exit on CTRL + C (SIGINT)

bash

trap "echo 'CTRL+C detected! Ignoring...'" SIGINT
while true; do
    echo "Running..."
    sleep 2
done

---

Shell Scripting in DevOps

Role of Shell Scripting in DevOps

Shell scripting is widely used in automation, infrastructure management, and CI/CD pipelines.

🔗 Shell → Git → Linux → Ansible Workflow:

1. Shell scripting helps automate tasks.
2. Git is used for version control.
3. Linux is the execution environment.
4. Ansible is used for configuration management.

Common DevOps Use Cases for Shell Scripting:

✔️ Automating deployments.
✔️ Managing log files and backups.
✔️ Configuring network settings.
✔️ Automating cloud infrastructure.

---

File Permissions in Shell Scripting

Using chmod to Change Permissions

chmod is used to set read, write, and execute permissions.

Permission Levels:

• 7 → Read (4) + Write (2) + Execute (1) = Full Access
• 4 → Read-only
• 2 → Write-only
• 1 → Execute-only

Example Usage:

bash

chmod 777 script.sh  # Full permission for all users
chmod 755 script.sh  # Read, write, execute for owner; read & execute for others

---

Debugging Shell Scripts

Enable Debug Mode (set -x)

Debugging helps track script execution and find issues.

bash

#!/bin/bash
set -x  # Enable debugging
echo "Executing Script..."
set +x  # Disable debugging

---

Running a Shell Script

Grant Execute Permission:

bash

chmod +x script.sh

Run the Script:

bash

./script.sh

or specify the interpreter explicitly:

bash

bash script.sh
sh script.sh

---

Advanced Shell Scripting Concepts

1️⃣ Script Metadata for Documentation

Always include metadata at the start of the script:

bash

#!/bin/bash
# Author: Your Name
# Date: YYYY-MM-DD
# Description: This script automates XYZ task
# Version: 1.0

2️⃣ Checking Available Storage (df Command)

Check available disk space in a human-readable format:

bash

df -h

3️⃣ Sorting a List of Names in a File (sort Command)

bash

sort names.txt

4️⃣ Network Troubleshooting with traceroute

bash

traceroute google.com

---

Conclusion

Shell scripting is a powerful tool for automating system administration, DevOps, and cloud tasks. By mastering scripting concepts such as loops, conditionals, functions, and debugging techniques, you can significantly enhance efficiency and reduce manual workloads.

Would you like me to cover real-world projects, cron jobs, or integrating shell scripts with cloud platforms? 🚀