Simple commands
date Current date and time
cal Calendar
who List users on this machine
clear Clear the window
finger <name> Display information on user <name>
wc <filename> Count lines, words, and characters.
head <filename> List first few lines.
tail <filename> List last few lines.
grep <string> <filename> List lines containing <string>
sort <filename> Sort lines.
Online documentation
man <command> Display documentation on <command>
Mail utility
mail <username> Mail to a user.
mail or mail -f Read mail.