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Unix is an operating system, like Windows XP, but unlike Windows, there is less GUI here.
Its an open source operating system, which has given rise to several flavors of it, like:


One can access/login an Unix server via tools like Putty (most popular), mRemote etc. Googling will give you several options.
For accessing it, one need to have a login id created on it via Unix adminstrator, along-with a password.
For security purpose, you will be asked to change your password at first login.
Once logged in successfully, you will find yourself at a command prompt, in your home directory like below:
		userid@servername /home/userid
	
This "userid@servername" combination, i.e. your custom prompt is detemined by a global variable named PS1.
You can change this variable to change your prompt. This can be changed by editing a hidden file called as .profile via vi editor.
This file is executed when one logs into the system, so any variables set in will come into effect for the rest of the session, unless changed explicitly.
Hidden files can be listed using -a option of ls command, like via ls -latr:
		u807735@vnxhamapcogd01 /home/u807735
		=> ls -latr
		total 424
		drwx--x--x    2 u807735  staff           256 Mar 20 2012  Mail/
		-rw-r--r--    1 u807735  staff            11 Mar 20 2012  .mh_profile
		-rwx------    1 u807735  staff           396 Apr 24 2012  .profile*
		-rw-r--r--    1 u807735  staff           514 May 10 2012  datafile
		.....
	
Here, the files with a leading dot are actually hidden files.
One can open it using vi editor and then edit it and save it.
=> vi .profile
One can control the access to files and directories w.r.t. read, write and execute permissions at owner, group and others level using chmod command.

Here is short description of these 3 levels:

ownerActual owner of file
groupMembers of owner's group
othersEveryone else, i.e. other than above 2

One very dangerous command in Unix is "rm -f", which removes things forcefully, without confirmation. Use it cautiously.


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