How to raise and take care of shell scripts: find commands

How do I find the command I need?

If one moves about the shell for some time, one is ready with a set of commands suitable for various situations - but nearly not all. And I think that nobody has learned all available options for all possible commands, isn't it? The good news: Your own systems (not only google) provides methods to get such information:

The three commands described above are actually the only ones you really have to know ;-) but the following could be useful too, if you don't exactly know what you are dealing with: