In win7, when I want to restart computer via keyboard until my operating system hasn't been loaded yet, I press Ctrl+Alt+Del.
What is equivalent to this command in Ubuntu?
(Note that I'm not asking for Task manager).
I wish to restart it if, for example, I pressed F10 but BIOS didn't load successfully so I have to reboot it. I really don't like brute way (using physical shutdown button).
Responding to your edited original question: I suggest the 'SysRq REISUB' method. This is a way to reboot linux systems gracefully, that often works to keep the file system healthy (in contrast to hard poweroff).
SysRq is often on the PrintScreen key:
Press Alt + PrintScreen continuously, sometimes the Fn key is involved too (in laptops),
and then slowly (one key after another) the keys R E I S U B to reboot.
When you press the 'letter keys' you need not specify caps lock or shift.
The corresponding sequence to shut down the computer gracefully is:
Press Alt + PrintScreen continuously, sometimes the Fn key is involved too (in laptops),
and then slowly (one key after another) the keys R E I S U O to shut down.
B 'Boot' is replaced by O 'Off'.
You find more details at the following links,
en.wikipedia.org/wiki/Magic_SysRq_key
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Edit: The key combo is a bit tricky on some computers. On several Dell laptops, for example Dell Precision M4800, the following works
