A list of the commonly used variables in Linux

System Variable	Meaning	To View Variable Value Type
BASH_VERSION	Holds the version of this instance of bash.	echo $BASH_VERSION
HOSTNAME	The name of the your computer.	echo $HOSTNAME
CDPATH	The search path for the cd command.	echo $CDPATH
HISTFILE	The name of the file in which command history is saved.	echo $HISTFILE
HISTFILESIZE	The maximum number of lines contained in the history file.	echo $HISTFILESIZE
HISTSIZE	The number of commands to remember in the command history. The default value is 500.	echo $HISTSIZE
HOME	The home directory of the current user.	echo $HOME
IFS	The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. The default value is <space><tab><newline>.	echo $IFS
LANG	Used to determine the locale category for any category not specifically selected with a variable starting with LC_.	echo $LANG
PATH	The search path for commands. It is a colon-separated list of directories in which the shell looks for commands.	echo $PATH
PS1	Your prompt settings.	echo $PS1
TMOUT	The default timeout for the read builtin command. Also in an interactive shell, the value is interpreted as the number of seconds to wait for input after issuing the command. If not input provided it will logout user.	echo $TMOUT
TERM	Your login terminal type.	echo $TERM export TERM=vt100
SHELL	Set path to login shell.	echo $SHELL
DISPLAY	Set X display name	echo $DISPLAY export DISPLAY=:0.1
EDITOR	Set name of default text editor.	export EDITOR=/usr/bin/vim

set and env command

You can use the env / set command too:

 envenv | moresetset | more 

Sample outputs:

HOME=/home/vivekvivek@nas01:~$ envTERM=xterm-256colorSHELL=/bin/bashXDG_SESSION_COOKIE=9ee90112ba2cb349f07bfe2f00002e46-1381581541.324726-906214463SSH_CLIENT=192.168.1.6 60190 22SSH_TTY=/dev/pts/1USER=vivekMAIL=/var/mail/vivekPATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/gamesPWD=/home/vivekLANG=en_INSHLVL=1HOME=/home/vivekLANGUAGE=en_IN:enLOGNAME=vivekSSH_CONNECTION=192.168.1.6 60190 192.168.1.10 22_=/usr/bin/env

A note about env/set command