Bash Environment Variables

Environment variables are dynamic values that affect the behavior of processes and programs on a computer system. In Bash, they play a crucial role in storing and managing information used by the shell and other applications.

What Are Environment Variables?

Environment variables are name-value pairs that exist in a shell's environment. They store information such as system configuration, user preferences, and temporary data. These variables are accessible to the shell and any child processes it spawns.

Viewing Environment Variables

To view all environment variables in your current Bash session, use the env or printenv command:

env

To view a specific environment variable, use the echo command with the variable name prefixed by a dollar sign:

echo $HOME

Setting Environment Variables

You can set environment variables using the following syntax:

VARIABLE_NAME=value

For example, to set a custom path:

CUSTOM_PATH=/usr/local/bin:/usr/bin:/bin
echo $CUSTOM_PATH

Exporting Environment Variables

To make an environment variable available to child processes, use the export command:

export VARIABLE_NAME=value

This ensures that the variable is passed to any subshells or scripts that are executed.

Common Environment Variables

• PATH: Specifies directories to search for executable files
• HOME: User's home directory
• USER: Current user's username
• SHELL: Path to the current shell
• PWD: Current working directory

Persistent Environment Variables

To make environment variables persistent across sessions, add them to your shell's configuration file. For Bash, this is typically ~/.bashrc or ~/.bash_profile:

echo 'export CUSTOM_VAR="my value"' >> ~/.bashrc
source ~/.bashrc

Using Environment Variables in Scripts

Environment variables are commonly used in Bash scripts to make them more flexible and portable. Here's an example:

#!/bin/bash
echo "Hello, $USER!"
echo "Your home directory is: $HOME"
echo "Your current shell is: $SHELL"

Best Practices

• Use uppercase letters for environment variable names by convention
• Avoid spaces around the equals sign when setting variables
• Quote variable values if they contain spaces or special characters
• Use local variables within functions to avoid polluting the global namespace

Understanding and effectively using environment variables is crucial for efficient Bash command-line interface usage and script development. They provide a powerful way to customize your shell environment and pass information between processes.