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Variables and Data Types in Shell Scripting

In shell scripting, variables are used to store data, such as strings, numbers, and special values. Unlike other programming languages, shell scripting does not require explicit data type declarations. Everything is treated as a string by default.

1. Declaring and Using Variables

Syntax:

bash
variable_name=value

  • No spaces before or after =

  • Variables are case-sensitive (myVar and MYVAR are different)

Examples:

bash
name="John"
age=25
echo "My name is $name and I am $age years old."

🔹 Use $variable_name to access the value of a variable.

Reading User Input:

bash
echo "Enter your name:"
read user_name
echo "Hello, $user_name!"

2. Variable Types

a) Local Variables

These variables are defined in a script or terminal session and are not accessible outside it.

bash
my_var="Hello"
echo $my_var

b) Environment Variables

System-wide variables available to all processes. Examples: $HOME, $PATH, $USER

bash
echo "Home directory: $HOME"
echo "Current user: $USER"

To define an environment variable:

bash
export MY_ENV_VAR="This is an environment variable"

c) Special Variables

Shell provides built-in special variables:

Variable Description
$0 Script name
$1, $2, … Positional parameters (arguments passed to the script)
$# Number of arguments passed
$@ All arguments as separate words
$* All arguments as a single string
$? Exit status of the last command (0 = success, non-zero = error)
$$ Process ID (PID) of the current script
$! PID of the last background process

Example:

bash
echo "Script Name: $0"
echo "First Argument: $1"
echo "Number of Arguments: $#"

3. Constants (Read-Only Variables)

Use readonly or declare -r to prevent modification.

bash
readonly PI=3.14
declare -r MAX_USERS=100

4. Command Substitution in Variables

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

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

Or using backticks `command`:

bash
current_user=`whoami`
echo "Logged in as: $current_user"

5. Data Types (Emulated)

Since Bash treats everything as a string, data types must be handled explicitly.

a) String Variables

bash
greeting="Hello, World!"
echo $greeting

String operations:

bash
length=${#greeting} # Get length
echo "String Length: $length"

b) Integer Variables

To perform arithmetic, use (( )) or expr:

bash
num1=10
num2=5
sum=$((num1 + num2))
echo "Sum: $sum"

Or using expr:

bash
sum=`expr $num1 + $num2`
echo "Sum: $sum"

c) Boolean Variables

Bash does not have built-in boolean types, but 0 is used for “true” and non-zero for “false.”

bash
isValid=1 # 1 for true, 0 for false
if [ $isValid -eq 1 ]; then
echo "Valid"
else
echo "Not Valid"
fi

6. Unset and Default Values

  • Unset a variable:

    bash
    unset my_var
    echo $my_var # Outputs nothing

  • Set a default value if the variable is empty:

    bash
    echo ${var_name:-"Default Value"}

🚀 Summary

✅ Variables do not require type declaration.
✅ Everything is treated as a string unless explicitly handled as a number.
✅ Environment variables persist across sessions.
✅ Special variables help manage script execution.
✅ Use $(( )) or expr for arithmetic operations.

Here’s a hands-on example script that demonstrates different types of variables and operations in shell scripting.

📌 Shell Script: Working with Variables

Save this script as variables_demo.sh and run it using bash variables_demo.sh.

bash
#!/bin/bash

# Defining variables
name="Alice"
age=30
pi=3.14159

# Displaying variable values
echo "Hello, my name is $name and I am $age years old."
echo "Value of Pi: $pi"

# Reading user input
echo -n "Enter your city: "
read city
echo "You live in $city."

# Arithmetic operations
num1=10
num2=5
sum=$((num1 + num2))
diff=$((num1 - num2))
prod=$((num1 * num2))
quot=$((num1 / num2))

echo "Arithmetic Operations:"
echo "$num1 + $num2 = $sum"
echo "$num1 - $num2 = $diff"
echo "$num1 * $num2 = $prod"
echo "$num1 / $num2 = $quot"

# Using special variables
echo "Script Name: $0"
echo "First Argument: $1"
echo "Total Arguments: $#"

# Using a command inside a variable
current_date=$(date)
echo "Current Date: $current_date"

# Read-only variable
readonly constant_var="I am constant"
echo $constant_var
# constant_var="New value" # This will cause an error

# Unsetting a variable
unset name
echo "Name after unset: $name" # Output will be empty

# Setting default value if variable is empty
echo "User: ${username:-'Guest'}"

🔹 How to Run the Script

  1. Save the file as variables_demo.sh

  2. Give it execution permission:

    bash
    chmod +x variables_demo.sh

  3. Run the script:

    bash
    ./variables_demo.sh

🔹 Features Demonstrated in This Script

✅ Variable declaration and usage
✅ Reading user input
✅ Arithmetic operations
✅ Special variables like $0, $1, $#
✅ Using command substitution $(command)
✅ Read-only variables (readonly)
✅ Unsetting and default values (unset, ${var:-"default"})