Variables in Bash are used to store values such as text, numbers, or even the result of a command. They allow you to reuse and manipulate data throughout your script. Bash variables are flexible, but there are a few important rules and types to understand: user-declared, environment, local/global scope, and how to perform operations like concatenation or arithmetic.

Bash Variable Examples

1. Declare and use a simple variable:

Bash variables are created by assigning a value with = and used by placing a $ before the variable name.

greeting="Hello"
name="John"
echo "$greeting, $name!"

2. Environment Variables

Environment variables are special system-wide variables used to configure the shell or software.

echo "Your home directory is: $HOME"

This prints the value of the built-in HOME environment variable. You can also create your own environment variable:

export MY_VAR="Visible Everywhere"

3. Local vs. Global Variables

Variables declared inside functions are local by default if specified with local. Otherwise, they are global within the script.

myvar="Global"

my_function() {
  local myvar="Local"
  echo "Inside function: $myvar"
}

my_function
echo "Outside function: $myvar"

This shows how local limits the scope of a variable to inside the function.

4. Common Variable Operations

• Concatenation (Combining Strings)

You can join strings by simply placing them next to each other:

first="Bash"
second="Scripting"
echo "$first $second"

• Arithmetic with Numbers

Use (( )) or let for performing math operations with numeric variables:

x=5
y=3
sum=$((x + y))
echo "Sum: $sum"

Or using let:

let result=x*y
echo "Multiplication: $result"

Important Points

• No spaces around the equal sign when assigning variables.
• Variables are case-sensitive. NAME is different from name.
• Use export to make a variable available to child processes.
• Use local inside functions to limit a variable’s scope.

Try It Out!

Create a script that combines two strings into one message. Try to store two numbers and print their product using (( )) too.

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