What should I write in Terminal to see all main commands?
I think the most direct answer to your question is:
which lists all built-in commands
lists all commands (maybe better to use
A nice list of core commands can be found here.
I don't know it's the correct way or not but in my opinion you can try the following trick as you want the list of commands through terminal
To see the available commands in Ubuntu you can go to the following directories and get a listing of them
DO NOT TRY TO EDIT OR ANYTHING IN THOSE DIRECTORIES
By using the above commands you can see the list of commands then to get to know more about those commands use
Or try online resources which lists the commands so that you don't screw up anything on your system
Also see the link below:
A compilation of all commands used in linux is given here
they even linked the man pages, so it is a great resource to check out!