Environment Variables in Linux

An environment variable is a named value that the shell — and every program the shell launches — can read. They configure tools, hold credentials, set the locale, and point to important paths. Every Linux user works with them daily, even without realizing it.

See all environment variables

env             # list all environment variables
printenv        # same thing
printenv HOME   # show one variable

Read a single variable

echo $HOME      # /home/you
echo $USER      # you
echo $SHELL     # /bin/bash
echo $PWD       # current working directory

Set a variable for the current shell

# Shell-only variable (child processes don't see it)
MY_VAR="hello"
echo $MY_VAR

# Environment variable (child processes inherit it)
export MY_VAR="hello"

Without export, the variable is local to the current shell. With export, every command you run from this shell sees it.

Set a variable for one command only

DATABASE_URL="postgres://localhost/db" python myapp.py

That sets DATABASE_URL only for that one invocation of myapp.py. The shell’s own environment doesn’t change.

Unset a variable

unset MY_VAR

Make it permanent

For your user, add the export to one of:

• ~/.bashrc — runs for every interactive bash shell. Most common.
• ~/.bash_profile or ~/.profile — runs at login. Use for things only needed once per session.
• ~/.zshrc — for zsh users.

For all users system-wide:

• /etc/environment — simple key=value, no shell expansion.
• /etc/profile.d/*.sh — shell scripts that run at login.

The variables you’ll actually use

Variable	What it does
PATH	Where the shell looks for commands
HOME	Your home directory
USER	Your username
SHELL	Your default shell
EDITOR	Default editor (used by git, visudo, etc.)
LANG / LC_*	Locale (language, currency, time format)
TZ	Timezone
TERM	Terminal type (affects colors, control codes)
PS1	Your shell prompt

Set EDITOR so git stops opening nano

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

Use environment variables for secrets

Never hardcode API keys in scripts. Read them from environment:

# in your shell rc, or .env file
export OPENAI_API_KEY="sk-..."
export AWS_SECRET_ACCESS_KEY="..."

# in your script (bash)
curl -H "Authorization: Bearer $OPENAI_API_KEY" ...

For project-specific secrets, use a .env file (and .gitignore it!). Tools like direnv auto-load and unload .env files when you cd into project directories.

Common gotchas

• Changes to ~/.bashrc only apply to NEW shells. Run source ~/.bashrc to reload.
• sudo strips most environment variables for security. Use sudo -E to preserve them, or set them again after sudo.
• cron jobs don’t have your shell’s environment. Always use absolute paths or set PATH explicitly in the cron script.

What to learn next

Next: how to discover what any command does without searching the web — man, --help, tldr.