Sometimes, plank closes itself and when I open it by writing plank in terminal, it opens up but when I close the terminal, plank also closes immediately.

That's because the plank process ran from the terminal is a child process of the terminal. If you kill the parent process then all child processes are ended.

If you press ALT + F2 and run it that way then you'll find the system runs as you expect

The Alt+F2 and then r and enter is not solving the problem. It goes away only after a reboot mandatorily. I would not have posted this question if this problem arise one or two times. It happens more often than that. But thank you for answering and explaining about terminal.

Did you add plank to your startup items?

Yes. It is in the list of start up items. Right.

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