1) man = manual provides in depth information about the requested command

man mkdir – gives the details how to use mkdir command together with the syntax

man -k reboot – quickly search for manuals containing reboot within them

man -h keyword – Print a one-line help message and exit.


2) apropos = Its useful if you  know only a part of the name or description of a command. Say for example, you want to add a user, but you do not know the specific command, you can try apropos user. It will list out the database of commands that related to user.


3) info = readable online documentation.

for example, info useradd will give a detail explanation of this syntax


4) whatis = it search the database for the keyword, and print it in a single line. it is the same as apropos except you must type the correct keyword else it will not show any result

Example: whatis finger


5) help = display a brief help on a command (it doesnt contain all commands)

usage = keyword –help

Ecample: reboot –help

Many of the syntax are not available for example useradd –help will not give any result


6) file = determine file type 


file * : will list all files in the directory with the file type information

file report2007.txt :  report2007.txt: ASCII text