Environment Variables

The state every program reads

Environment variables are key/value pairs every running process can read. $HOME, $PATH, $USER, $LANG — all of them shape how programs behave. printenv prints them all; echo $VAR shows one.

Set, export, unset

VAR=value — local to this shell only.
export VAR=value — passed to every child process.
unset VAR — remove it.
VAR=value cmd — set just for one invocation. HTTPS_PROXY=... curl ... is the canonical example.

Why export matters

Without export, the value lives only in the current shell. If you spawn python script.py and the script reads os.environ['VAR'], it sees nothing. Always export for tools to pick it up.

The variables you'll touch

$PATH — executable lookup.
$HOME — your home dir.
$EDITOR — what git commit opens (set to nvim, code -w, cursor -w).
$PAGER — what man uses (set to less -R).
$LANG / $LC_* — locale and character set.
$TERM — terminal type.

Where to set them

Interactive use → ~/.zshrc. Login-time only → ~/.zprofile. The full breakdown is the next lesson; for now remember that one of those files is the right place.

An environment variable is copied to children

An exported value reaches processes created afterward; it does not flow backward into an existing parent or sideways into a sibling. Use that direction to explain why a new terminal and a long-running application see different values.

Separate secret delivery from secret storage

Environment values can leak through child processes, diagnostics, or crash reports. Keep tokens out of plaintext startup files and inject them from a credential store at the narrowest runtime boundary. A convenient transport is not a safe store.

Code

Inspect and set·bash

printenv | head
echo $EDITOR $PAGER $SHELL
# Set for this shell only
FOO=bar
echo $FOO
# Set for children
export FOO=bar
python3 -c 'import os; print(os.environ.get("FOO"))'

Per-command environment·bash

HTTPS_PROXY=http://proxy:8080 curl https://example.com
DEBUG=1 ./run.sh
# Doesn't pollute the shell — verify
echo $DEBUG