For a complete list, see Bash Reference Manual - Bash Environment Variables. This is just my own memory-dump. To get the value of a variable, you can use:

echo $OLDPWD

OLDPWD

previous current directory

PWD

current directory

LANG

the language used by programs, check with locale -a for available locales. Can be set in .bash_profile with export LANG=”en_US.utf8″

LANGUAGE

the language used by programs, check with locale- a for available locales. Can be set in .bash_profile with export LANGUAGE=”en_US.utf8″

RSYNC_RSH

the shell used for rsync, usually /usr/bin/ssh