Introduction to Shell Scripting

Introduction

• Shell scripting is an essential skill for anyone working in Unix or Linux environments.
• A shell script is a program written for the shell, or command-line interpreter, of an operating system.
• It automates repetitive tasks and can combine various commands to execute complex processes.
• There are different types of shell scripting, variable types, operators, and data structures that can be leveraged to write effective scripts.

Types of Shell Scripting

1. Bash (Bourne Again Shell):
   • One of the most common shell types used in Linux environments.
   • It’s an extension of the Bourne shell and provides features like command-line editing and job control.
2. Sh (Bourne Shell):
   • The original Unix shell developed by Stephen Bourne, and it serves as the foundation for many other shells like Bash and Korn shell.
3. Zsh (Z Shell):
   • A powerful shell similar to Bash but with additional features such as built-in spell checking and improved scripting capabilities.
4. Csh (C Shell):
   • The C shell’s syntax is similar to the C programming language, and it provides features like command history and job control.
5. Ksh (Korn Shell): -A high-performance shell developed by David Korn, it blends features of the Bourne and C shells.

Variables

• Variables in shell scripts are containers for storing data, and they do not require a specific data type declaration(default - string).

#!/bin/bash
name="Scripting"
echo "The topic is $name"

Operators

• Operators allow you to perform operations like arithmetic, comparisons, assignments, and more. Shell scripting supports a wide variety of operators.

1. Arithmetic Operators
   • Arithmetic operations can be done using the expr command or $((…)) syntax.

#!/bin/bash
a=10
b=5
sum=$((a + b))
echo "Sum: $sum"

• Common arithmetic operators:
  • • (Addition)
  • • (Subtraction)
  • • (Multiplication)
  • / (Division)
  • % (Modulus)

1. Comparison Operators
   • Comparison operators are used to compare values, typically integers, in a script. These comparisons are often used in if statements.

#!/bin/bash
a=10
b=20

if [ $a -eq $b ]; then
    echo "a is equal to b"
else
    echo "a is not equal to b"
fi

• Common comparison operators:
  • -eq (Equal to)
  • -ne (Not equal to)
  • -gt (Greater than)
  • -lt (Less than)
  • -ge (Greater than or equal to)
  • -le (Less than or equal to)

1. Assignment Operators
   • Assignment operators assign values to variables.

#!/bin/bash
a=10
a+=5  # Equivalent to a = a + 5
echo "a: $a"
Operators like +=, -=, *=, /=, and %= perform the respective operations and then assign the result to the variable.

1. Logical Operators
   • Logical operators are used to combine conditions.

#!/bin/bash
a=10
b=20

if [[ $a -lt 15 && $b -gt 15 ]]; then
    echo "Both conditions are true"
fi

• Common logical operators:
  • && (Logical AND)

  • (Logical OR)

  • ! (Logical NOT)

1. Bitwise Operators
   • Bitwise operators perform bit-level operations.

#!/bin/bash
a=5  # Binary: 101
b=3  # Binary: 011
result=$((a & b))  # Bitwise AND
echo "Result of AND: $result"

• Bitwise operators:
  • & (AND) | (OR) ^ (XOR) « (Left shift)

    (Right shift)

    1. Identity Operators Identity operators are used to compare whether two objects refer to the same object.

bash

#!/bin/bash a=”hello” b=”hello”

if [ “$a” == “$b” ]; then echo “a and b are identical” fi