file / type / which / whereis / locate / find

file     /usr/bin/passwd          #file type
type [-tpa] name

which    ifconfig                 #command
whereis [-bmsu] ifconfig          #-b:binary, -m:manual, -s:source, -u:others
locate  [-ir] keyword             #-i:ignore big/low case, -r:regular
find [PATH] [option] [action]     #find /etc -newer /etc/passwd