Understanding Environment and Shell Local Variables

Understanding Environment and Shell Local Variables

In Bash scripting, variables come in two distinct types: environment variables and shell local variables. Understanding the difference between them is crucial for writing effective scripts and managing your shell environment.

What Are Environment and Shell Local Variables?

When you start a terminal session, Bash initializes with a collection of predefined variables that customize your system’s behavior. These fall into two categories:

Environment Variables

• Scope: Available to the current shell and all child processes
• Lifetime: Persist until the session ends
• Purpose: Pass configuration data to programs
• Also known as: Exported variables

Shell Local Variables

• Scope: Available only to the current shell
• Lifetime: Exist only while the shell is running
• Purpose: Store temporary data for the current session
• Not inherited: Child processes don’t see them

Key Differences at a Glance

Viewing Variables

Environment Variables Only

env
# or
printenv

All Variables (Environment + Local)

set

Searching for Specific Variables

env | grep PATH
set | grep bash

Getting a Single Variable’s Value

printenv HOME
# or
echo $HOME

Practical Examples

Example 1: The PATH Variable

The PATH variable is a perfect example of an environment variable:

echo $PATH
# Output: /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

This variable tells the shell where to look for executable commands. It’s inherited by every program you run.

Example 2: User-Specific Information

echo "User: $USER"
echo "Home: $HOME"
echo "Shell: $SHELL"

These are all environment variables set when you log in.

Example 3: Shell Local Variables

# These are local to your current shell
echo $BASH_VERSION
echo $PPID  # Parent process ID

Try running env | grep BASH_VERSION - you won’t find it because it’s not exported.

Creating Variables

Creating Shell Local Variables

my_var="hello"
abc=123

These variables exist only in your current shell.

Creating Environment Variables

export MY_VAR="hello"
# or in two steps:
MY_VAR="hello"
export MY_VAR

Now MY_VAR is available to any child processes you spawn.

Making Variables Persistent

User-Specific Variables

Add to your shell configuration files:

~/.bashrc (for interactive shells):

export MY_CUSTOM_VAR="value"

~/.bash_profile (for login shells):

export MY_CUSTOM_VAR="value"

System-Wide Variables

For all users, edit system files (requires root):

/etc/environment:

MY_SYSTEM_VAR="value"

/etc/profile or /etc/bash.bashrc:

export MY_SYSTEM_VAR="value"

Real-World Use Cases

1. Adding Directories to PATH

# Add your scripts directory to PATH
export PATH="$PATH:$HOME/scripts"

2. Setting Default Editors

export EDITOR=vim
export VISUAL=vim

3. Custom Application Settings

export DATABASE_URL="postgresql://localhost/mydb"
export API_KEY="your-secret-key"

Important Commands

View All Environment Variables

env
printenv

View a Specific Variable

printenv HOME
echo $HOME

View All Variables (including local)

set

Clean Up set Output

set -o posix  # Removes functions from output
set           # Now shows only variables

Remove a Variable

unset MY_VAR      # Removes from current shell
export -n MY_VAR  # Removes from environment (keeps it local)

Common Environment Variables

Best Practices

1. Use UPPERCASE for environment variables by convention
2. Export only what’s needed - don’t pollute the environment
3. Document your variables - add comments in config files
4. Check before setting - avoid overwriting existing variables
5. Use descriptive names - MYAPP_DB_HOST not DBH

Summary

• Environment variables = Global, inherited, persistent
• Shell local variables = Local, temporary, isolated
• export = The key to making variables environment-wide
• env/printenv = View environment variables
• set = View everything
• Configuration files = Make variables persistent across sessions

What’s Next?

Now that you understand variables, you’re ready to learn about getting user input - an essential skill for creating interactive scripts!