Shell provides an interface between the user and the kernel and executes programs called commands.

Different shells:

• sh => Bourne shell
• ksh => korn shell
• bash => Bourne again shell
• csh => c shell
• zsh = z shell

Most popular one is Bash and it is used in shell scripting.

Script File

1. Shell script file should end with .sh or it should have shebang #!.

   #!/bin/bash
   
   echo "Hello world"
   

   Shebang defines the interpreter to use to interpret the script.

2. Script is exectued line by line.

3. It also have programming concepts like conditional statement, loops, etc.

4. To execute the script you can use either ways

   1. Make it executable and execute like binary

      chmod +x script.sh
      ./script.sh
      

   2. Execute directly using interpreter

      bash script.sh
      

---

 

• Variables

  Variable store some values

  #!/bin/bash
  
  name="tester"
  echo "hello $name"
  
  read -p "Enter you age: " age
  echo "your age is $age"
  

  • read: It is used for user input, age is variable in which we are storing that user input.
  • $ is use to access variable content.

 

• Arguments

  Arguments are values which is provided at runtime

  #!/bin/bash
  
  echo "File name is: $0"
  echo "Hello $1"
  echo "Creating file $2" && touch $2 
  

  Execute

  ./script tester myfile2
  

  Output

  File name is: /tmp/script
  Hello tester
  Creating file myfile2
  

  • $1,$2,.. $n This is how we access arguments.
  • $0 is the first argument, which is the name of file.
  • $# This returns the count of arguments.
  • $@ This represents all the arguments.

   

---

 

• Loops

  Execute until a specific condition is met.

  Example:

  #!/bin/bash
  
  dir=/home/user/scripts/*.sh
  for files in $dir
  do
      echo $files
  done
  

  This script print all .sh files in that directory.

  Example 2: Script to add multiple users.

  #!/bin/bash
  
  read -p "Enter the number of users: " user_num
  
  for (( i=1; i<=$user_num; i++ ))
  do
      read -p "Enter username: " user
      read -s -p "Enter pass: " pass
      sudo useradd -m $user
      echo "$user:$pass" | sudo chpasswd
      echo "Done"
  done
  
  cat /etc/passwd | grep $user
  

 

• If else statement

  Execute something on some condition

  #!/bin/bash
  
  name="tester"
  
  if [ $name == "tester" ]
  then
          echo "yes, tester logged in"
  else 
          echo "Invalid user"
  fi
  

 

Example script.

1. Install nginx and serve page using script.

   # Update system
   sudo apt update
   
   # Install nginx
   sudo apt install nginx -y
   
   # Start & enable nginx service
   sudo systemctl enable --now nginx
   
   # Serve the page
   sudo rm /var/www/html/*.html && echo "Hello world" | sudo tee /var/www/html/index.html 
   

2. Backup script

   #!/bin/bash
   
   src=/home/user/devops/script
   tgt=/home/user/backups
   filename=$(date +'%d-%m-%Y').tar.gz
   tar -cvzf $tgt/$filename $src 
   
   echo "Completed!"
   

   You can create crontab for this.

   crontab -e
   

   * * * * * /path/to/backup/script
   

That's it.