SHELL SCRIPTING

Shell Scripting

Process of automating command execution instead of running them one by one

Shell

A command-line interface that acts as a bridge between the user and the kernel

Kernel

Core part of the operating system that manages system resources

CLI (Command-Line Interface)

A text-based interface used to interact with the system

Script

A file containing a sequence of commands executed automatically

Bash

Bourne Again Shell; a widely used command-line shell and scripting language

.sh file

A file extension used for Bash scripts

Script Execution

Running a script from top to bottom as written

Text Editors

Tools like vim gedit and emacs used to create script files

#!/bin/sh

A line that indicates the start of a shell script

Comment (#)

Used to add notes in a script that are not executed

echo

A command used to display text or variable output

Case Sensitivity

File names that differ in uppercase and lowercase are treated as different

Valid File Characters

Letters numbers dot and underscore allowed in file names

Invalid File Characters

Characters like > < | : & should be avoided in file names

Variable

A storage location used to hold data values in a script

Local Variable

A variable that exists only within the current shell session

Shell Variable

A special variable set by the shell for system use

Environment Variable

A variable accessible to child processes of the shell

Assignment Operator (=)

Used to assign a value to a variable

$ Symbol

Used to access the value of a variable

$BASH_VERSION

A shell variable that shows the current Bash version

Special Variables

Predefined variables used to access script information

$0

Represents the filename of the current script

$1 $2 $n

Represents positional arguments passed to the script

$#

Represents the number of arguments passed to the script

$*

Represents all arguments as a single string

$@

Represents all arguments as separate quoted strings

$?

Represents the exit status of the last executed command

$$

Represents the process ID of the current shell

$!

Represents the process ID of the last background process

read

A command used to accept user input into a variable

Arithmetic Operators

Operators used for performing mathematical calculations

Addition (+)

Adds two values

Subtraction (-)

Subtracts one value from another

Multiplication (*)

Multiplies two values

Division (/)

Divides one value by another

Modulus (%)

Returns the remainder after division

Equality (==)

Checks if two values are equal

Inequality (!=)

Checks if two values are not equal

let

A built-in command used to perform arithmetic operations

Conditional Expression

A statement that evaluates to true or false

Square Brackets [ ]

Used to enclose conditions in shell scripting

Relational Operators

Operators used to compare numeric values

-eq

Checks if two values are equal

-ne

Checks if two values are not equal

-gt

Checks if one value is greater than another

-lt

Checks if one value is less than another

-ge

Checks if one value is greater than or equal to another

-le

Checks if one value is less than or equal to another

Conditional Statements

Used to perform different actions based on conditions

if statement

A control structure used to test conditions

else statement

Executes when the if condition is false

case statement

A control structure used to match values against patterns

esac

Marks the end of a case statement

;;

Ends each case option

*

Represents the default case option

Loop

A structure used to repeat a block of code multiple times

for loop

Used to iterate over a list of items

while loop

Repeats code while a condition is true

until loop

Repeats code until a condition becomes true

for in do done

Keywords used in a for loop

while do done

Keywords used in a while loop

until do done

Keywords used in an until loop

Iteration

One complete cycle of a loop execution

Automation

The use of scripts to perform tasks automatically