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