Understanding Environment Variables

Environment variables are named values that affect how processes run. They store configuration, paths, and settings that programs need.

What Are Environment Variables?

Environment variables are:

• Key-value pairs stored in the shell
• Inherited by child processes
• Used to configure programs
• Available system-wide or per-session

Viewing Variables

See All Variables

env
printenv

See Specific Variable

echo $HOME
echo $USER
echo $PATH
printenv HOME

Common Environment Variables

Variable	Description
HOME	User's home directory
USER	Current username
PATH	Directories to search for commands
PWD	Current working directory
SHELL	Current shell program
TERM	Terminal type
LANG	Language setting
EDITOR	Default text editor

Exercise: View an Environment Variable

Display the value of the HOME variable:

Variable Syntax

$VARIABLE           # Access value
${VARIABLE}         # Access value (safer in strings)
${VARIABLE:-default}  # Default if unset
${VARIABLE:=default}  # Set default if unset

Examples:

echo "Home is $HOME"
echo "User: ${USER}"
echo "${UNDEFINED:-not set}"

Types of Variables

Environment Variables

Available to child processes:

export MY_VAR="value"

Shell Variables

Only in current shell:

my_var="value"

Where Variables Are Set

File	When Loaded
/etc/environment	System-wide, all users
/etc/profile	Login shells, all users
~/.bashrc	Interactive non-login shells
~/.profile	Login shells
~/.bash_profile	Bash login shells

Checking if Variable Exists

# Check if set
if [ -z "$VARIABLE" ]; then
    echo "Not set"
fi

# Check if not empty
if [ -n "$VARIABLE" ]; then
    echo "Has value"
fi

Using Variables in Commands

cd $HOME
ls $HOME/documents
cat ${HOME}/.bashrc

Exercise: List All Variables

Use env to see all environment variables:

Key Takeaways

• Environment variables store configuration values
• Access with $VARIABLE or ${VARIABLE}
• Common ones: HOME, PATH, USER
• Use env or printenv to list all
• Shell vs environment variables have different scope