Understanding Linux Environment Variables

What Are Environment Variables?

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Environment variables are key-value pairs that configure the behavior of processes on a Linux system. They’re inherited by child processes, which means setting a variable in your shell makes it available to every command you run from that shell.

echo $HOME
# /home/mike

Programs use environment variables for configuration without hardcoding values — paths, credentials, locale settings, and runtime options are all commonly passed this way.

Viewing Environment Variables

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List all variables

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# All exported environment variables
env

# Or equivalently
printenv

Print a specific variable

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printenv PATH
echo $HOME

Show all variables including shell-local ones

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# Includes unexported shell variables and functions
set

Setting Variables

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Temporary (current session only)

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# Set and export in one step
export MY_APP_ENV=production

# Or set first, then export
MY_APP_ENV=production
export MY_APP_ENV

For a single command

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Prefix the command with the variable assignment:

DEBUG=true ./my-script.sh

This sets DEBUG only for that command — it doesn’t affect your current shell.

Unsetting a variable

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unset MY_APP_ENV

Shell Variables vs Environment Variables

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Not every shell variable is an environment variable:

# Shell variable (local to this shell, NOT inherited by child processes)
MY_VAR="hello"

# Environment variable (inherited by child processes)
export MY_VAR="hello"

To check if a variable is exported:

# Shows only exported variables
export -p | grep MY_VAR

Persistent Variables

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Session variables disappear when you close the terminal. To make them persist, add them to shell startup files.

Where to put them

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Example: add a persistent variable for your user

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echo 'export EDITOR=vim' >> ~/.bashrc

Reload without logging out:

source ~/.bashrc

Example: add a system-wide variable

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# /etc/environment uses simple KEY=value syntax (no export, no shell commands)
sudo sh -c 'echo "JAVA_HOME=/usr/lib/jvm/java-17" >> /etc/environment'

Common Environment Variables

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The PATH Variable

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PATH is the most important environment variable. It tells the shell where to find executable programs.

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

The shell searches directories left to right. The first match wins.

Adding to PATH

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# Prepend (your directory is searched first)
export PATH="$HOME/.local/bin:$PATH"

# Append (searched last)
export PATH="$PATH:/opt/myapp/bin"

Make it persistent:

echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.bashrc

Check where a command resolves from

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which python3
# /usr/bin/python3

type -a python3
# Shows ALL matches in PATH

Variable Expansion

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The shell expands variables before executing commands:

APP_DIR=/opt/myapp
echo "$APP_DIR/bin"
# /opt/myapp/bin

Braces for disambiguation

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VERSION=3
echo "${VERSION}_final"
# 3_final

# Without braces, shell looks for $VERSION_final (undefined)
echo "$VERSION_final"
# (empty)

Default values

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# Use default if variable is unset or empty
echo "${DB_HOST:-localhost}"

# Assign default if unset
: "${DB_PORT:=5432}"
echo $DB_PORT
# 5432

Environment Variables in Scripts

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Scripts inherit the environment of the parent shell. You can also load variables from a file:

.env file pattern

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# .env
DB_HOST=localhost
DB_PORT=5432
DB_NAME=myapp

Load it into your shell:

export $(grep -v '^#' .env | xargs)

Or source it if it uses export:

# .env (with export)
export DB_HOST=localhost
export DB_PORT=5432

source .env

Passing variables to child processes

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# Only exported variables are inherited
export API_KEY="secret123"
./my-script.sh  # can access $API_KEY

# Non-exported variables are NOT inherited
LOCAL_VAR="hello"
./my-script.sh  # cannot access $LOCAL_VAR

Inspecting a Process’s Environment

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View the environment of a running process:

# By PID
cat /proc/<pid>/environ | tr '\0' '\n'

# For a specific process
cat /proc/$(pgrep nginx | head -1)/environ | tr '\0' '\n'

Best Practices

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• Keep secrets out of shell history — use read -s or load from files with restricted permissions instead of typing export SECRET=value directly
• Use /etc/environment for system-wide variables that all users and services need
• Use ~/.bashrc for user-specific interactive settings and ~/.bash_profile for login-time setup
• Always quote variable expansions ("$VAR") to prevent word splitting and globbing
• Use ${VAR:-default} syntax in scripts to handle missing variables gracefully
• Don’t put sensitive values in /etc/environment or profile scripts — use secret management tools for production credentials
• Prepend to PATH with $HOME/.local/bin:$PATH rather than overwriting it entirely