• Shell scripting is a way to automate tasks by writing scripts (sequences of commands) in a text file, which the shell can execute.

Basic Shell Scripting Commands:

1. #!/bin/bash

• Purpose: This is called the shebang line. It specifies the interpreter for the script (in this case, Bash). The script will be executed by the Bash shell.

• Example: This script will print “Hello, World!” to the screen.

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

2. echo

• Purpose: The echo command is used to print a message or the value of a variable to the terminal.

• Usage :Example:

  • echo "Message" – Prints a string to the terminal.
  • echo $variable – Prints the value stored in a variable.

  echo "Hello, World!"
  name="Alice"
  echo "Hello, $name"
  

3. Variables

• Purpose: Variables are used to store data that can be reused later in the script.

• Usage: Example:

  • variable_name="value" – Declare and assign a value to a variable.
  • echo $variable_name – Access the value of a variable.

  name="Alice"
  echo "Hello, $name"  # Output: Hello, Alice
  

4. Conditionals (if/else)

• Purpose: Conditional statements allow the execution of commands based on whether a condition is true or false.

• Usage:

  if [ condition ]; then
      # command if condition is true
  else
      # command if condition is false
  fi
  
  #!/bin/bash
  
  number=5  # No spaces around '=' in variable assignment
  
  if [ "$number" -eq 5 ]; then
      echo "The number is 5 buddy"
  else
      echo "Teri lowwde"
  fi
  

  Example:

  if [ $name == "Alice" ]; then
      echo "Hello, Alice!"
  else
      echo "You're not Alice."
  fi
  

  This checks if the variable name is equal to “Alice” and prints a message accordingly.

5. Loops (for/while)

• Purpose: Loops allow repeated execution of commands. Useful for iterating over items or performing repetitive tasks.

• For Loop:

  for i in 1 2 3; do
      echo "Loop $i"
  done
  

  • This will loop through the numbers 1, 2, and 3, printing each one.

• While Loop:

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

  • This will increment count from 1 to 5 and print the value of count each time.

6. Reading Input

• Purpose: Accept user input during the execution of a script.

• Usage:

  read variable_name
  echo "You entered: $variable_name"
  

  Example:

  echo "Enter your name:"
  read name
  echo "Hello, $name!"
  

7. Functions

• Purpose: Functions allow you to group related commands together and reuse them within your script.

• Usage:

  my_function() {
      echo "This is a function"
  }
  my_function  # Call the function
  

  Example:

  greet() {
      echo "Hello, $1!"
  }
  greet "Alice"  # Output: Hello, Alice!
  

  • $1 refers to the first argument passed to the function.

8. Making a Script Executable

• Purpose: Allows you to run a script as if it were a command from the terminal.

• Steps:

  1. Make the script executable using chmod:

     chmod +x script_name.sh
     

  2. Run the script by calling it directly:

     ./script_name.sh
     

  This allows you to execute the script without explicitly invoking the interpreter (e.g., bash script_name.sh).

9. Comments

• Purpose: Comments are used to explain parts of your script and make it easier to understand.

• Usage:

  # This is a single-line comment
  

  Example:

  # This script greets the user
  echo "Hello, User!"
  

10. File Redirection and Pipes

• Purpose: Shell scripting allows you to redirect input/output or pipe the output of one command to another.

  • Redirection:
    • > – Redirects output to a file (overwrites).
    • >> – Redirects output to a file (appends).
  • Pipes:
    • | – Pipes the output of one command to another.

  Examples:

  echo "Hello, World!" > output.txt   # Redirects to a file
  cat output.txt                     # Display the contents of output.txt
  
  ls | grep "file"                   # Pipe the output of ls to grep
  

11. Exit Status

• Purpose: Every command in a script returns an exit status code, which indicates whether the command was successful.

  • 0 – Success
  • Non-zero – Error

  Usage:

  if [ $? -eq 0 ]; then
      echo "Last command was successful."
  else
      echo "Last command failed."
  fi