Part 2: Bash Basics – Variables, Input/Output, and Exit Codes

 Introduction

Now that you’ve written your first Bash script in Part 1, it’s time to dive deeper into the core building blocks of Bash scripting — variables, input/output operations, and exit codes.

These concepts are essential for writing dynamic and interactive scripts that can make decisions, store data, and handle user input.

Understanding Variables in Bash

Variables in Bash are like containers that store information — text, numbers, filenames, or command results.

🔹 Declaring a Variable

#!/bin/bash
name="Mishri"
echo "Hello, $name!"

🟢 Output:

Hello, Mishri!

💡 No spaces are allowed around the equal sign (=).
❌ name = "Mishri" → This will cause an error.

🔹 Accessing Variables

Use a dollar sign ($) before the variable name to access its value:

echo $name

🔹 Using Command Substitution

You can store the output of a command in a variable:

current_date=$(date)
echo "Today's date is: $current_date"

🟢 Output Example:

Today's date is: Sun Nov 10 09:30:42 IST 2025

🔹 Environment Variables

These are system-wide variables available to all programs.

Example:

echo $HOME
echo $USER

Common environment variables:

🔹 Readonly Variables

Prevent accidental modification:

readonly app="MyScript"
app="NewScript" # ❌ This will throw an error

🔹 Unset a Variable

Remove a variable from memory:

unset name
echo $name  # Output will be empty

 Reading User Input

You can make scripts interactive using the read command.

Example:

#!/bin/bash
echo "Enter your name:"
read username
echo "Welcome, $username!"

🟢 Output:

Enter your name:
Mishri
Welcome, Mishri!

Using read with Options

Example:

read -p "Enter your username: " user
read -s -p "Enter your password: " pass
echo
echo "Hello $user, login successful!"

🧾 Output in Bash

🔹 Using echo

Prints text or variable values.

echo "Hello, World!"
echo "User: $USER"

🔹 Using printf (More Controlled)

printf "Name: %s\nAge: %d\n" "Mishri" 25

🟢 Output:

Name: Mishri
Age: 25

✅ printf gives more formatting control than echo.

🔁 Redirecting Output and Input

🔹 Output Redirection

Example:

echo "System Check: OK" > status.log
cat status.log

🟢 Output:

System Check: OK

⚠️ Exit Codes and Error Handling

Every Bash command returns an exit status (or exit code):

• 0 = success ✅
• Non-zero = error ❌

You can check the exit code of the last executed command using $?.

Example:

ls /tmp
echo $?   # Should print 0 if success

ls /nonexistent_folder
echo $?   # Will print non-zero (error)

🔹 Using Exit Codes in Scripts

#!/bin/bash
if ls /nonexistent_folder 2>/dev/null
then
  echo "Directory found!"
else
  echo "Error: Directory not found!"
  exit 1
fi

Output:

Error: Directory not found!

🧩 Pro Tip: Always use meaningful exit codes in larger scripts (like exit 1, exit 2) for debugging.

Combining Everything: Mini Script Example

Here’s a real example combining variables, input, and exit codes.

#!/bin/bash
# Simple Login Script

read -p "Enter username: " user
read -s -p "Enter password: " pass
echo

if [[ $user == "admin" && $pass == "1234" ]]; then
  echo "Login successful. Welcome, $user!"
  exit 0
else
  echo "Invalid credentials. Access denied."
  exit 1
fi

Output:

Enter username: admin
Enter password:
Login successful. Welcome, admin!

Summary

In this section, you learned:

✅ How to define and use variables
✅ How to read user input interactively
✅ How to output data using echo and printf
✅ How to redirect input/output
✅ How to handle and interpret exit codes

🔜 Next in Series

👉 Part 3: Control Structures in Bash (if, for, while, case)
We’ll cover how to make your scripts smart — with conditions, loops, and branching.