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Let us briefly look into what happens when you turn on linux based system. Following diagram explains various steps involved 

Explanation:-

1:- Initially system is put on by system administrator. If you are running unix/linux , you yourself are the system administrator. For a large configuration with more than one terminal attached to main system, a person called system administrator does this for you.

2:- It may happen that all terminals attached to the system may not be on after the booting is done. The system starts a program "getty" for each such terminal. This program periodically checks whether you have put on the terminal. This period may be fraction of second. After the terminal is put on, a new program "login" takes over from "getty". This "login" program prompts the message, as described earlier. it reads the user name and password and matches the password with the one stored in the system

3:-After the user login, this "login" program is taken over by another program called "sh" (for "shell"). This "shell" is a command interpreter program that reads each command from the terminal and performs accordingly.

4:-After user finished his/her work, he/she log out. There might be another user who want to use the system. In this case the terminal is not put off by the first user, consequently, the "sh" is taken over by "login again to enable the next user to log in.

5:-On the other hand, after logging out, the user might put the terminal off. As soon as it put off, the "login" program is taken over by "getty".

6:-At the end, before the system is put off, the system administrator puts all the "getty" properly off, and then shut the system down.

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