What are the most important environment variables in Linux?

Question

Accepted Answer

*The shell pre-populates variables describing your session — $HOME`, `$USER, $PATH`, `$PWD — plus special ones like $?` (last exit code) and `%%MATH_BLOCK_0%%`, `$!) are set by bash itself and report runtime state; $? in particular is the workhorse for checking whether the previous command succeeded.

Variable
Description
Example

$HOME
User's home directory
/home/student

$USER
Current username
student

$HOSTNAME
System hostname
server01

$SHELL
Default shell
/bin/bash

$PATH
Command search path
/usr/bin:/bin

$PWD
Current directory
/home/student

$LOGNAME
Login name
student

$?
Last command exit code
0

$$
Current shell PID
12345

$!
Last background PID
12346

View all environment variables:
env          # All exported variables
set          # All variables (including local)
echo $PATH   # Specific variable

Export to child processes:
export MY_VAR="value"

Variable	Description	Example
`$HOME`	User's home directory	`/home/student`
`$USER`	Current username	`student`
`$HOSTNAME`	System hostname	`server01`
`$SHELL`	Default shell	`/bin/bash`
`$PATH`	Command search path	`/usr/bin:/bin`
`$PWD`	Current directory	`/home/student`
`$LOGNAME`	Login name	`student`
`$?`	Last command exit code	`0`
`$$`	Current shell PID	`12345`
`$!`	Last background PID	`12346`