Day 5: Shell Scripting and Automation

Objectives

• Understand the basics of shell scripting.
• Learn to write simple scripts to automate tasks.
• Use variables, conditionals, and loops in scripts.
• Schedule tasks using cron.

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1. Introduction to Shell Scripting

What is a Shell Script?

• A shell script is a text file containing a sequence of commands for a Unix-based operating system.
• It can automate repetitive tasks, manage system operations, and perform batch processing.

Creating and Running a Shell Script

1. Create a Script File

   • Use a text editor like nano or gedit to create a script file.

     $ nano myscript.sh

2. Write a Simple Script

   • Example content:

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

3. Make the Script Executable

   • Change the file permissions to make it executable:

     $ chmod +x myscript.sh

4. Run the Script

   • Execute the script:

     $ ./myscript.sh

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2. Shell Scripting Basics

Using Variables

• Assign values to variables:

  #!/bin/bash
  NAME="John"
  echo "Hello, $NAME"

Reading User Input

• Read input from the user:

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

Using Conditionals

• Use if statements to execute commands based on conditions:

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

Using Loops

• Use for and while loops to repeat commands:

  #!/bin/bash
  for i in 1 2 3 4 5; do
    echo "Looping ... number $i"
  done
  
  COUNT=1
  while [ $COUNT -le 5 ]; do
    echo "Count is $COUNT"
    COUNT=$((COUNT + 1))
  done

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3. Automating Tasks with cron

Introduction to cron

• cron is a time-based job scheduler in Unix-like operating systems.
• It allows users to schedule scripts or commands to run automatically at specified times and intervals.

Editing the Crontab

• Open the crontab file for editing:

  $ crontab -e

• Add a new cron job using the following format:

  * * * * * /path/to/script.sh

Cron Job Timing Format

• The cron timing format consists of five fields representing:
  • Minute (0 - 59)
  • Hour (0 - 23)
  • Day of the month (1 - 31)
  • Month (1 - 12)
  • Day of the week (0 - 7) (Sunday is both 0 and 7)

Example Cron Jobs

1. Run a script every day at 2 AM:

   0 2 * * * /path/to/script.sh

2. Run a script every Monday at 5 PM:

   0 17 * * 1 /path/to/script.sh

3. Run a script every hour:

   0 * * * * /path/to/script.sh

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Practical Exercises

Exercise 1: Create and Run a Basic Shell Script

1. Create a script file named greet.sh:

   $ nano greet.sh

2. Add the following content:

   #!/bin/bash
   echo "Hello, welcome to Linux shell scripting!"

3. Make the script executable:

   $ chmod +x greet.sh

4. Run the script:

   $ ./greet.sh

Exercise 2: Write a Script with Variables and User Input

1. Create a script file named userinfo.sh:

   $ nano userinfo.sh

2. Add the following content:

   #!/bin/bash
   echo "Enter your name:"
   read NAME
   echo "Enter your age:"
   read AGE
   echo "Hello, $NAME. You are $AGE years old."

3. Make the script executable:

   $ chmod +x userinfo.sh

4. Run the script:

   $ ./userinfo.sh

Exercise 3: Schedule a Cron Job

1. Create a script file named backup.sh:

   $ nano backup.sh

2. Add the following content to back up a directory:

   #!/bin/bash
   tar -czf /path/to/backup/backup.tar.gz /path/to/directory

3. Make the script executable:

   $ chmod +x backup.sh

4. Schedule the script to run every day at midnight:

   $ crontab -e

5. Add the following line to the crontab:

   0 0 * * * /path/to/backup.sh

By the end of Day 5, users will be capable of writing basic shell scripts, using variables, conditionals, and loops, and scheduling tasks with cron. These skills are essential for automating tasks and managing a Linux system efficiently.