Shell Scripting: Complete Course Overview

Introduction

• Audience: Beginners with zero knowledge of Shell Scripting.
• Course Includes: Basics to advanced, over 40 scripts, 5+ parts, real-life projects.
• Goal: Understanding of Shell Scripting through continuous practice and real-life applications.

What is a Shell?

• Shell: User interface for access to an operating system’s services.
• Function: Executes commands and scripts, interacts with kernel.
• Different Shell Types:
  • Bash (Bourne Again Shell): Most common and recommended.
  • Zsh (Z Shell): Default on macOS.

Default Shell Check

• Command: echo $0 to check the default shell in Linux terminal.
• Identify Installed Shells: Use cat /etc/shells to list all installed shells.
• Switch Shells: Possible but not deeply covered.

Basic Script Creation

• Example: Create a Hello World script.
  • Use #!/bin/bash at the start (shebang line).
  • Print command: echo "Hello World".
• Permissions: Use chmod +x script_name to make it executable.
• Execution: Run script with ./script_name.

Comments in Scripts

• Single-line comments: Use # at the start of the line.
• Multi-line comments: Use : ' your comment here ' :.

Variables in Shell Scripting

• Definition: Variables can store information to be used in scripts.
• Syntax: variable_name=value (No spaces around =).
• Usage: Access the variable using $variable_name.
• Example:

  name="Prashant"
  echo $name
  

Arrays in Shell Scripting

• Definition: Arrays store multiple values in a single variable.
• Syntax: array_name=(value1 value2 value3).
• Access Elements: ${array_name[index]}.
• Example:

  fruits=("Apple" "Banana" "Cherry")
  echo ${fruits[1]} # Outputs: Banana
  

• Operations:
  • Get all values: ${array_name[@]}.
  • Get length: ${#array_name[@]}.
  • Get range: ${array_name[@]:start:length}.
  • Append values: array_name+=("AnotherValue").

String Operations

• Length of a string: ${#var}.
• Convert to uppercase: ${var^^}.
• Convert to lowercase: ${var,,}.
• Substring extraction: ${var:position:length}.

User Interaction in Scripts

• Prompt for input: read variable_name.
• Example:

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

Arithmetic Operations

• Syntax: $((expression)) for performing arithmetic operations.
• Example:

  a=10
  b=20
  sum=$((a + b))
  echo $sum # Outputs: 30
  

Conditional Statements

• If-else: Use to perform actions based on conditional tests.
• Syntax:

  if [ condition ]; then
      # Commands
  elif [ condition ]; then
      # Commands
  else
      # Commands
  fi
  

• Example:

  if [ $a -gt $b ]; then
      echo "$a is greater than $b"
  else
      echo "$a is not greater than $b"
  fi
  

• Comparison Operators:
  • -eq: Equal
  • -ne: Not equal
  • -gt: Greater than
  • -ge: Greater than or equal to
  • -lt: Less than
  • -le: Less than or equal to

Logical Operators

• AND: && or -a
• OR: || or -o

Loops in Shell Scripting

• For Loop:

  for var in list; do
      # Commands
  done
  

• While Loop:

  while [ condition ]; do
      # Commands
  done
  

• Until Loop:

  until [ condition ]; do
      # Commands
  done
  

• Example of For Loop:

  for i in {1..5}; do
      echo "Number $i"
  done
  

Functions in Shell Scripting

• Definition: Grouping commands together to be reused.
• Syntax:

  function_name() {
      # Commands
  }
  

• Calling Functions: Just use the function name.
• Example:

  greet() {
      echo "Hello, $1"
  }
  greet "World" # Outputs: Hello, World
  

• Arguments: Access using $1, $2, etc., within the function.
• Return Values: Use return. Capture using $?.

Practical Projects

• Project 1: Monitor Free RAM Space
  • Concept: Check if RAM usage crosses a threshold and alert.
  • Commands Used: free, awk, if-else structure, mail command for alert.
• Project 2: Monitor Disk Usage and Send Email Alerts
  • Concept: Check disk usage, archive large/old files, send alerts.
  • Commands Used: df, find, tar, and mail.
• Project 3: Archive Old Log Files
  • Concept: Compress and move old log files to an archive directory.
  • Commands Used: find, gzip, and mv.

Automation and Scheduling

• Crontab: Schedule tasks to run at specified times.
• Syntax:

  * * * * * command-to-execute
  | | | | |
  | | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
  | | | ------- Month (1 - 12)
  | | -------- Day of month (1 - 31)
  | ---------- Hour (0 - 23)
  ------------ Minute (0 - 59)
  

• Example: 0 5 * * * /path/to/script (Runs script daily at 5 AM).