00. Linux Shells and Shell Scripting

What is a Shell?¶

A shell is a command-line interface (CLI) that allows users to interact with the operating system. It acts as a bridge between the user and the kernel, interpreting commands and executing them.

User enters command → Shell interprets → Kernel executes → Output returned

Check Current Shell¶

echo $SHELL

Example Output¶

/bin/bash

Explanation¶

$SHELL is an environment variable.
It stores the path of the current default shell.

List All Available Shells¶

cat /etc/shells

Example Output¶

/bin/bash
/usr/bin/zsh
/usr/bin/fish

Explanation¶

This file contains all shells installed on the system.
Useful when changing your default shell.

Switch Shell (Temporary)¶

zsh

Explanation¶

Starts a new shell session (Zsh here).
Does not change default shell.

To return:

exit

Change Default Shell (Permanent)¶

chsh -s /usr/bin/zsh

Explanation¶

chsh = change shell
-s specifies the shell path
Takes effect after logout/login

Popular Shells Comparison¶

Feature	Bash	Zsh	Fish
Default in Linux	Yes	No	No
Scripting	Strong	Strong	Limited
Auto-completion	Basic	Advanced	Very advanced
Customization	Moderate	Very high	High
Plugins	Limited	Extensive (Oh My Zsh)	Built-in features

What is a Shell Script?¶

A shell script is a file containing a sequence of commands executed by the shell.

File extension: .sh
Used for automation, system tasks, and scripting workflows

Creating and Running a Script¶

Step 1: Create File¶

nano script.sh

Step 2: Add Code¶

#!/bin/bash
echo "Hello World"

Explanation¶

#!/bin/bash → Shebang (defines interpreter)
echo → Prints output

Step 3: Make Executable¶

chmod +x script.sh

Explanation¶

Adds execute permission

Step 4: Run Script¶

./script.sh

Variables in Shell¶

#!/bin/bash

echo "Enter your name:"
read name

echo "Hello, $name"

Explanation¶

read name → takes user input
$name → accesses variable value

Loops¶

For Loop¶

#!/bin/bash

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

Explanation¶

{1..5} → sequence generator
Loop runs 5 times
$i → current iteration value

While Loop¶

#!/bin/bash

count=1

while [ $count -le 5 ]
do
  echo $count
  ((count++))
done

Explanation¶

Runs while condition is true
-le means "less than or equal"
((count++)) increments value

Conditional Statements¶

#!/bin/bash

echo "Enter username:"
read user

if [ "$user" = "admin" ]; then
  echo "Access granted"
else
  echo "Access denied"
fi

Explanation¶

if [ condition ] → evaluates condition
= → string comparison
then → executes if true
else → executes if false
fi → ends block

Command Line Arguments¶

#!/bin/bash

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"

Run:¶

./script.sh hello world

Output:¶

Script name: ./script.sh
First argument: hello
Second argument: world

Explanation¶

$0 → script name
$1, $2 → arguments passed

Functions in Shell¶

#!/bin/bash

greet() {
  echo "Hello, $1"
}

greet Subrat

Explanation¶

greet() defines function
$1 inside function → argument

Comments¶

# This is a single-line comment

Best Practice¶

Use comments to explain logic
Improve readability and maintainability

File Operations Example¶

#!/bin/bash

file="test.txt"

if [ -f "$file" ]; then
  echo "File exists"
else
  echo "File not found"
fi

Explanation¶

-f → checks if file exists
Useful in automation scripts

Exit Status¶

#!/bin/bash

ls file.txt
echo $?

Explanation¶

$? → last command status
0 → success
non-zero → error

Best Practices¶

Always use shebang (#!/bin/bash)
Use meaningful variable names
Quote variables: "$var"
Add comments for complex logic
Test scripts before deployment

Summary¶

Concept	Description
Shell	Interface between user and OS
Script	File with executable commands
Variables	Store values
Loops	Repeat execution
Conditionals	Decision making
Functions	Reusable blocks
Permissions	Required to execute scripts