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.

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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.

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3) info = readable online documentation.

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

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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

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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

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6) file = determine file type 

Examples:

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

file report2007.txt :  report2007.txt: ASCII text

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