Part III · Shell and commands

27.1 Where environment variables come from

alsoenvenvironmentenvironment variablesenv varsexportprintenv/etc/environmentlocalelang

TL;DR

Every process gets a copy of its parent's environment: a list of NAME=value strings.
export NAME=value sets it for the current shell and the programs it starts, not for its parent or for other terminals.
To keep it, put the export in a startup file (27.2). For graphical apps on systemd desktops, use a file in ~/.config/environment.d/.

printenv LANG
export EDITOR=nvim
LANG=C sort names.txt    # set for one command only

On your machine

alice@laptopexample

$ atfq 27.1
  SHELL     /bin/bash
  HOME      /home/alice
  LANG      en_US.UTF-8
  EDITOR    nvim
  TERM      xterm-256color
  Login     /bin/zsh (SHELL says /bin/bash: changed since login?)
  In all    8 variables set; only well-known ones are shown here

An example: Alice's laptop, one of the test machines. In your terminal, this block shows your own machine, read live.

Pitfalls

A variable set without export stays in the shell: the programs it starts don't see it.
A change never reaches programs already running, nor other terminals: open a new one, or source ~/.bashrc.
systemd services don't read your shell files: give them variables with Environment= in a drop-in (see 22.3).
sudo resets most of the environment. sudo -E keeps it, when sudoers allows it.
Variables are visible to every process you run, in /proc/PID/environ: avoid keeping secrets there for long.

Deep dive 2 questions

Where does the first environment come from?

At login, PAM and the login program set HOME, USER, SHELL and a default PATH; pam_env adds what's in /etc/environment. The login shell then runs its startup files, which add more. Desktop sessions started by systemd also read environment.d files. Every program after that inherits the result.

What do LANG and LC_ALL do?

They choose the language of messages, the format of numbers and dates, the sort order and the character encoding. LC_ALL overrides everything, LANG is the fallback. locale shows the current values, and LC_ALL=C gives scripts stable, byte-based behaviour.

Commands: env printenv export
Paths: /etc/environment ~/.config/environment.d/
See also: 27.2 Shell startup files
23.3 PATH and command resolution