Linux Variables
Mental model
- A shell variable lives only in the current shell.
export promotes it to an environment variable so child processes inherit it. That's the whole distinction.
VAR=value → shell only. export VAR=value → inherited by anything you launch.- Inheritance is one-way (parent → child). A child can't change the parent's env — which is why a script can't
cd your shell for you.
Setting & inspecting
| Command |
What it does |
VAR=value |
set a shell variable (this shell only) |
export VAR=value |
set + export (children inherit) |
VAR=value cmd |
set for just that one command's run |
unset VAR |
remove it |
echo $VAR |
print one value |
printenv / env |
list exported (environment) variables |
set |
list all shell vars (incl. non-exported) |
declare -p VAR |
show a var + its attributes |
Common variables
| Variable |
Meaning |
$HOME |
your home directory |
$USER |
current username |
$PATH |
dirs searched for commands (colon-separated) |
$SHELL |
your login shell (not necessarily current one) |
$PWD / $OLDPWD |
current / previous directory |
$LANG |
locale (language/encoding) |
$EDITOR |
default editor (git, crontab, etc. use it) |
$TERM |
terminal type |
$DISPLAY |
which X display to draw to (see [[display-basics]]) |
$XDG_SESSION_TYPE |
x11 vs wayland (see [[display-basics]]) |
Shell special variables
| Variable |
Meaning |
$? |
exit code of last command (0 = ok) |
$$ |
PID of the current shell |
$! |
PID of the last backgrounded job |
$0 |
name of the script/shell |
$# |
number of args passed to a script |
$@ |
all args |
Persistence — where to define them
| File |
When it loads |
~/.bashrc |
every interactive non-login bash shell |
~/.bash_profile / ~/.profile |
login shells |
~/.zshrc |
interactive zsh |
/etc/environment |
system-wide, all users (simple KEY=val, no scripting) |
Login vs non-login is the classic "my var works in one terminal but not another" gotcha. A common fix is to source ~/.profile from ~/.bashrc.