Variables

Variables are containers for storing data in Bash. They're fundamental to any programming or scripting language, and Bash is no exception. In this lesson, you'll learn how to create variables, assign values, follow naming conventions, and work with variables effectively. Unlike many languages, Bash doesn't require variable declarations - just assign a value and use it!

Variable Assignment

In Bash, you create a variable simply by assigning a value to it. The syntax is straightforward: variable_name=value. Note that there are no spaces around the equals sign!

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Accessing Variables

To use a variable's value, prefix it with the dollar sign $. Bash will replace $variable_name with the actual value stored in the variable.

# Assign a value
name="Alice"
age=25

# Access the value
echo "My name is $name"
echo "I am $age years old"

# Can also use braces for clarity
echo "Hello, ${name}!"
echo "Age: ${age}"

Variable Naming Rules

Bash has specific rules for variable names. Understanding these rules helps you write valid code and avoid errors.

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Strings vs Numbers

Bash treats all variables as strings by default. Even numbers are stored as strings, but Bash can perform arithmetic when needed.

# Both are strings
text="Hello"
number="42"

# But Bash can do arithmetic
count=10
count=$((count + 1))  # Arithmetic expansion
echo $count  # 11

# Or use let
let count=count+1
echo $count  # 12

Unsetting Variables

To remove a variable (make it undefined), use the unset command. This is useful for cleaning up or resetting values.

# Create a variable
temp="temporary data"
echo $temp  # temporary data

# Unset it
unset temp
echo $temp  # (empty - variable no longer exists)

# Check if variable is set
if [ -z "${temp:-}" ]; then
    echo "Variable is unset or empty"
fi

Common Mistakes

# Wrong - spaces cause error
name = "Alice"  # Error: command not found

# Correct - no spaces
name="Alice"

# Wrong - prints literal "name"
name="Alice"
echo name  # Output: name

# Correct - use $ to access value
echo $name  # Output: Alice

# Wrong - hyphens not allowed
my-var="value"  # Error: command not found

# Correct - use underscore
my_var="value"

# Wrong - can't start with number
2nd_place="silver"  # Error: command not found

# Correct - start with letter or underscore
second_place="silver"
_2nd_place="silver"  # Also valid

# Ambiguous - Bash looks for "name_file" variable
name="test"
echo $name_file  # Wrong: tries to find variable "name_file"

# Correct - use braces for clarity
echo ${name}_file  # Output: test_file

Exercise: Work with Variables

#!/bin/bash
# Personal information script

# Create variables
first_name="Alex"
last_name="Johnson"
age=28
city="San Francisco"

# Display information
echo "Hello! My name is $first_name $last_name"
echo "I am $age years old"
echo "I live in $city"
echo ""

# Unset a variable
unset city
echo "After unsetting city:"
echo "City variable: '${city:-not set}'"

Summary

What's Next?

Great progress! Now that you understand basic variables, let's explore Variable Expansion - powerful techniques for working with variables, including default values, error handling, and string manipulation. You'll learn patterns like ${var:-default} that make your scripts more robust!