Shell Scripting for DevOps - Automate Your Tasks

Lecture 8 in the DevOps journey dives deep into the essential topic of Shell Scripting. In this blog, we’ll cover what shell scripting is, its purpose, basic commands, script creation, automation using cron jobs, and more. Shell scripting is a crucial skill for DevOps engineers, as it allows for task automation and interaction with the OS kernel.

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What is Shell Scripting?

1. Definition: Shell scripting is essentially a file containing a series of shell commands, which can be executed sequentially.

2. Purpose: It’s used to automate repetitive tasks, which is valuable in DevOps for saving time and reducing human error.

3. Shell: A shell is a command-line interface that interacts with the OS kernel. There are multiple types of shells, including bash (Bourne Again Shell) and sh.

4. File Extension: Shell script files typically have the .sh extension.

5. Shebang: #!/bin/bash is a shebang that specifies the script is for the bash shell.

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Creating and Running Shell Scripts

To create a shell script, follow these steps:

1. Create Script: Use vim hello.sh to create a file.

2. Make Executable: Use chmod 774 hello.sh to make it executable.

3. Run Script: Use ./hello.sh or bash hello.sh to execute.

# Example of a simple shell script
#!/bin/bash

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

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Comments in Shell Scripts

1. Single-line Comment: Use # at the start of the line.

2. Multi-line Comment:

   • Use the syntax <<COMMENT and COMMENT to enclose multiple lines.

    <<COMMENT
    This is a multi-line comment example.
    COMMENT

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Assignment: Creating a Movie Dialog Script

Here’s a simple script simulating a dialog from the movie Salaar:

#!/bin/bash

# This is Salaar movie dialogue

<<Salaar
Vardha and Deva ki dost
Salaar

echo "Deva: Tu jab bhi bulayega mai aaunga"
echo "Vardha: Vaada kar!"
echo "Deva: Ye vaada hai mera."
echo "Vardha: Deva abb chal sath mai"
echo "Deva: Chal bhai."
echo "Vardha: Ruk jara date dekhlu tuje le jane ka: "
date

1. Make Executable: chmod 774 salaar.sh

2. Run Script: ./salaar.sh

3. 

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Scripting Concepts

Variables and Constants

1. Print Output: echo is used to print anything.

2. User Input: read gathers input from the user, and read -p adds a prompt.

    #!/bin/bash
    read -p "Enter your name: " name
    echo "My name is $name."
   

Installing Packages via Script

To automate package installation, here’s a script to install NGINX:

#!/bin/bash
echo "***************INSTALLING NGINX**************"
sudo apt update
sudo apt install nginx -y
sudo systemctl start nginx
sudo systemctl enable nginx
echo "**************INSTALLED NGINX******************"

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Installing Packages Using a Script

In this section, I learned how to create a script to install any package passed as an argument. Here's the shell script to install any package using a command-line argument:

Script: install_package.sh

#!/bin/bash

<< note
This script will install any package passed as an argument.

./install_package.sh <arg>

$1 is used to get the first argument (<arg>) which is passed by running the above command to install any package.
note

echo "**************INSTALLING $1**************"

# Updating system
sudo apt update

# Installing the package
sudo apt install $1 -y

# Start and enable the service
sudo systemctl start $1
sudo systemctl enable $1

echo "***************INSTALLED $1***************"

Steps to Execute:

1. Create the script using vim install_package.sh.

2. Change the file permissions:
   chmod 774 install_package.sh.

3. Run the script with the desired package as an argument. For example, to install SSH, run:
   ./install_package.sh ssh.

Output Image:

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Creating a Backup of Scripts

A backup script example:

#!/bin/bash

<< note

This scripts takes backup of any destination path given in argument
./backup.sh /home/ubuntu/scripts

note

timestamp=$(date '+%Y-%m-%d_%H-%M-%S')

backup_dir="${timestamp}_backup.zip"

zip -r $backup_dir $1

echo "Backup Completed!"

1. Execute: ./backup.sh /home/ubuntu/scripts

2. Result: This compresses all scripts in /home/ubuntu/scripts into a .zip file with a timestamp.

3. 

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Automating Execution with Cron Jobs

Cron is a tool that automates tasks on a schedule.

1. Editing Cron Jobs: Use crontab -e to access and edit the cron file.

2. Schedule Example: The following runs a backup script every minute.

    */1 * * * * bash /home/ubuntu/scripts/backup.sh /home/ubuntu/scripts
   

3. Updated backup.sh Script: This updated script stores backups in a backups directory with a timestamp.

    #!/bin/bash
   
    << note
   
    This scripts takes backup of any destination path given in argument
   
    ./backup.sh /home/ubuntu/scripts
   
    note
   
    function create_backup {
        timestamp=$(date '+%Y-%m-%d_%H-%M-%S')
   
        backup_dir="/home/ubuntu/backups/${timestamp}_backup.zip"
   
        zip -r $backup_dir $1
   
        echo "Backup Completed!"
   
    }          
    create_backup $1
   

4. To Stop Cron: Comment out or delete the scheduled job in crontab.

5. 

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Conclusion and Next Steps

This blog covered foundational shell scripting concepts, from creating scripts to automating backups using cron jobs. Shell scripting is a powerful tool in DevOps that enables engineers to automate complex tasks efficiently. In the upcoming blog, we’ll explore Backup Rotation—a critical practice for managing space and organizing backups systematically.

Stay tuned for the next blog on Backup with Rotation, where we’ll delve into creating a robust script to handle backups while managing storage space.

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