Bash Variables: The Foundation of Shell Scripting

Bash Variables: The Foundation of Shell Scripting

Variables are fundamental building blocks in any programming language, and Bash is no exception. In this comprehensive guide, you’ll learn everything you need to know about creating, managing, and naming variables in Bash scripts.

What Are Variables?

A variable is simply a named memory location that stores a value you can manipulate. Think of it as a labeled container where you can store data for later use in your scripts.

Creating Variables in Bash

Basic Syntax

variable_name=value

Key Rules:

• No spaces around the equals sign - this is critical!
• The variable name comes first
• The value follows immediately after the equals sign

Example:

os=linux

Common Mistake:

os = linux  # ❌ WRONG - this will fail!

When you add spaces, Bash interprets os as a command and tries to execute it, resulting in a “command not found” error.

Handling Values with Spaces

When your variable value contains spaces, you must enclose it in quotes:

distro="Kali Linux"  # ✅ Correct
distro=Kali Linux    # ❌ Wrong - will cause errors

Variable Types in Bash

Bash is a weakly typed language, meaning:

• You don’t need to declare data types
• The type is determined by the value assigned
• Bash automatically handles the conversion

String Variables:

name="John Doe"
os="linux"

Integer Variables:

age=30
count=100

Important Limitation:

Bash does not support floating-point numbers. You can only work with integers.

Variable Expansion and Combining Variables

You can create variables that reference other variables:

os=linux
distro=Ubuntu
mydistro="$os $distro"  # Results in "linux Ubuntu"

Managing Variables

Viewing All Variables

To see all shell variables and functions:

set

Since this produces a lot of output, use grep to search:

set | grep distro

Removing Variables

To delete a variable:

unset variable_name

Example:

echo $distro      # Shows current value
unset distro
echo $distro      # Returns empty (variable is gone)

Read-Only Variables (Constants)

Sometimes you need to ensure a variable’s value never changes. Use the declare command with the -r flag:

declare -r LOG_DIR="/var/log"

Characteristics of Read-Only Variables:

• Cannot be modified after declaration
• Cannot be unset
• Useful for constants that should remain fixed

Example:

declare -r SECONDS_PER_HOUR=3600
SECONDS_PER_HOUR=7200  # ❌ Error: is read only
unset SECONDS_PER_HOUR  # ❌ Error: cannot unset

Variable Naming Conventions

Best Practices:

For User Variables:

• Use lowercase letters
• Use underscores to separate words
• Make names descriptive
• Examples:

  user_name="john"
  file_count=42
  log_directory="/var/log"
  

For Constants and Environment Variables:

• Use UPPERCASE letters
• Use underscores to separate words
• Declare at the top of your script
• Examples:

  declare -r MAX_RETRIES=3
  PATH="/usr/bin:/bin"
  

Invalid Variable Names:

2users=true        # ❌ Starts with a number
file.name=100      # ❌ Contains a dot
user@name=100      # ❌ Contains @ symbol

Valid Variable Names:

user4=false        # ✅
server_name="apex" # ✅
A=67               # ✅
_underscore=100    # ✅ Can start with underscore

Summary: Key Takeaways

• Syntax: variable_name=value (no spaces around =)
• Strings with spaces: Always use quotes: variable="value with spaces"
• No type declarations: Bash handles types automatically
• No floating-point: Only integers are supported
• Use set: To view all variables
• Use unset: To remove variables
• Use declare -r: To create read-only constants
• Naming: Lowercase for variables, UPPERCASE for constants

What’s Next?

Now that you understand variables, you’re ready to explore variable expansion and quoting - two essential concepts that will help you handle complex string operations and avoid common pitfalls in your scripts.