env command

Environment Variable	Description
BASH	Path of the BASH executable file
BASH_ENV	Path of the BASH environment file, which specifies BASH options
BASH_VERSION	Version of BASH
COLUMNS	Width, in characters, of console window
EUID	Effective user ID
HISTFILE	Path of the BASH command history file
HISTFILESIZE	Maximum number of lines recorded in history file
HISTSIZE	Maximum number of commands recorded in history file
HOME	Path of user’s home directory
HOSTNAME	Name of the host
IFS	Field separator (white space) characters
LINES	Length, in lines, of console window
LOGNAME	User’s log in name
LS_COLORS	Options for ls command
OSTYPE	Operating system name (”Linux”)
PATH	Program path
PPID	Process ID of the shell’s parent process
PS1	Command prompt string
PS2	Continuation prompt string
PS4	Execution trace string
PWD	Current working directory
SHELL	Path of the shell executable
SHLVL	Number of nested shell invocations
TERM	Terminal type
UID	User ID
USER	User name

Ref:http://www.linux-mag.com/id/803
http://linux.about.com/od/ttl_howto/a/hwtttl16t06.htm