Examples of environment variable names

HOME

this is the pathname of your "home" directory when you login and to which cd changes when invoked with no argument.

PATH

defines the directories searched when the shell is looking for the file to execute when processing a command.

MAIL

if set to the name of a mail file the shell will inform you of the arrival of new mail.

PS1

contains the shell prompt string.

PS2

is the continuation prompt when a command spans more than one line.

IFS (Internal Field Separator)

The set of characters used for blank interpretation in the shell. Normally space, TAB and newline.