Shell Scripting Guide

Introduction

Shell scripting is a powerful way to automate tasks, simplify complex workflows, and improve efficiency in daily operations. A shell script is a text file containing a sequence of commands that the shell interprets and executes. Shell scripting is essential in areas such as system administration, DevOps, and CI/CD pipelines, making it a valuable skill for engineers working with Linux systems. By learning shell scripting, engineers can automate repetitive tasks, manage system operations, and create robust workflows with minimal manual intervention.

Key Concepts in Shell Scripting

Shebang (``): The shebang is the first line of a shell script and specifies the interpreter to be used for executing the script. For example:
```
#!/bin/bash
```
This tells the system to use the Bash shell to run the script.
Variables: Variables store data that can be used and manipulated within a script.
```
# Defining variables
name="John Doe"
echo "Hello, $name"
```

Conditionals: Shell scripts can use if, elif, else, and case statements to make decisions.

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

Loops: Loops allow repetitive execution of commands.

For loop:

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

While loop:

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

Functions: Functions enable code reuse and better organization.
```
greet() {
    echo "Hello, $1!"
}
greet "Alice"
```

Examples of Common Use Cases

Automating Backups: Automate file backups using tar:

#!/bin/bash
source_dir="/path/to/source"
backup_dir="/path/to/backup"
tar -czf "$backup_dir/backup_$(date +%Y%m%d).tar.gz" $source_dir
echo "Backup completed."

Working with Files: Renaming all .txt files to .bak:

for file in *.txt; do
    mv "$file" "${file%.txt}.bak"
done

Parsing Data: Extract specific lines from a log file:
```
grep "ERROR" /var/log/syslog > error_logs.txt
```

Tips and Best Practices

Use Descriptive Variable Names: This improves script readability and maintainability.
```
user_name="Alice"
```
Add Comments: Document the purpose of commands for clarity.
```
# This loop processes all .txt files
```

Error Handling: Check for potential errors and handle them gracefully.

if [ ! -d "$backup_dir" ]; then
    echo "Backup directory does not exist."
    exit 1
fi

Test Incrementally: Test your scripts in parts before running them fully.
Use ShellCheck: A linting tool that checks for errors and potential issues in shell scripts.

Real-World Applications

DevOps: Automate deployment pipelines and infrastructure provisioning. Example: Starting or stopping Docker containers based on conditions.
```
if docker ps | grep -q my_app; then
    docker stop my_app
else
    docker start my_app
fi
```

System Administration: Monitor disk usage and send alerts.

usage=$(df / | grep / | awk '{print $5}' | sed 's/%//')
if [ $usage -gt 80 ]; then
    echo "Disk usage is critically high: $usage%"
fi

CI/CD Pipelines: Automate build and test scripts.

#!/bin/bash
echo "Running tests..."
./run_tests.sh && echo "All tests passed!" || echo "Tests failed."

Key Takeaways

Shell scripting is a versatile tool for automation and efficiency.
Key elements include variables, conditionals, loops, and functions.
Use best practices to write readable, maintainable, and error-free scripts.
Practical applications span across DevOps, system administration, and software development pipelines.

Mastering shell scripting will empower you to streamline your tasks, enhance productivity, and become a more proficient engineer. Dive into hands-on projects to solidify your understanding and keep exploring the endless possibilities with shell scripts.