Shell Scripting

Shell scripting allows you to automate tasks by writing sequences of commands in a file. Bash (Bourne Again Shell) is the default shell on most Linux distributions and the most widely used scripting shell.

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Your First Script

#!/bin/bash
# my_first_script.sh — a simple example

echo "Hello, World!"
echo "Today is $(date +%Y-%m-%d)"
echo "You are logged in as: $USER"

Running a Script

chmod +x my_first_script.sh     # make it executable
./my_first_script.sh            # run it
bash my_first_script.sh         # or run explicitly with bash

The Shebang Line

The first line #!/bin/bash (called the shebang) tells the system which interpreter to use:

Shebang	Interpreter
#!/bin/bash	Bash shell
#!/bin/sh	POSIX-compatible shell
#!/usr/bin/env python3	Python 3
#!/usr/bin/env node	Node.js

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Variables

# Assigning variables (no spaces around =)
name="Alice"
age=30
current_dir=$(pwd)
file_count=$(ls | wc -l)

# Using variables
echo "Name: $name"
echo "Age: $age"
echo "Directory: $current_dir"
echo "Files: $file_count"

# Curly braces for clarity
echo "Hello, ${name}!"
echo "File: ${name}_report.txt"

# Read-only variables
readonly PI=3.14159

# Unsetting variables
unset age

Special Variables

Variable	Meaning
$0	Script name
$1, $2, ...	Positional arguments
$#	Number of arguments
$@	All arguments (as separate words)
$*	All arguments (as a single string)
$?	Exit code of the last command
$$	PID of the current script
$!	PID of the last background process

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Conditionals

if / elif / else

#!/bin/bash

if [ -f "/etc/nginx/nginx.conf" ]; then
    echo "Nginx is configured"
elif [ -f "/etc/apache2/apache2.conf" ]; then
    echo "Apache is configured"
else
    echo "No web server found"
fi

Test Operators

File tests:

Operator	Test
-f file	File exists and is a regular file
-d dir	Directory exists
-e path	Path exists (file or directory)
-r file	File is readable
-w file	File is writable
-x file	File is executable
-s file	File exists and is not empty
-L file	File is a symbolic link

String tests:

Operator	Test
-z "$var"	String is empty
-n "$var"	String is not empty
"$a" = "$b"	Strings are equal
"$a" != "$b"	Strings are not equal

Numeric tests:

Operator	Test
$a -eq $b	Equal
$a -ne $b	Not equal
$a -gt $b	Greater than
$a -lt $b	Less than
$a -ge $b	Greater than or equal
$a -le $b	Less than or equal

Modern [[ ]] Syntax

# Supports && and || directly, pattern matching, and regex
if [[ "$name" == "Alice" && "$age" -gt 18 ]]; then
    echo "Adult Alice"
fi