Is it ok to ask questions about command line, bash, environment variable, etc. here?
What is better to ask here and what is better to ask on Super User?
Well, the sites do have a significant overlap. Briefly:
Super User (SU) : Any question about using/administering computers. This includes simple scripts of the type a sysadmin is likely to write (file and user management scripts for example). SU is platform agnostic. You can ask about any operating system there. Most of the questions here would also be on topic on SU.
Unix & Linux (U&L) : Anything related to *nix. We deal with installing and configuring your system at any level and for any flavor of *nix (including Linux, Unix, OSX, BSD etc.). We also accept simple scripting questions about the shell or perl and python. Again, however, this will depend on the complexity of the issue. If you want a detailed answer about how Perl handles pointers internally, you go to SO. If you want help writing a script that parses text, that would be welcome here.
So, how do you chose? Often, it comes down to what kind of answer you want. Administering a Linux box is on topic here and on SU. Personally, I have had a much better experience here though. An exception might be questions about the GUI side of things. They are welcome here but the community is not very interested in them. Those might be better asked on SU instead.
My rule of thumb is that when I want a detailed answer about the nitty-gritty of how a system works under the hood, I will ask here. If I want something more general, and a user-level answer (say, how to change the color of my buttons) I might ask on SU instead. Shell scripts are on topic on SU but you are much more likely to get a good answer here. Configuring your desktop environment is on topic here but you might get a better answer, or at least get your question more noticed, on SU.
At the end of the day, it's up to you. As long as your question is on topic, it is welcome on either site. Just make sure you don't ask the same question on both sites. Finally, if you feel you asked on the wrong place, you can flag your question and ask a mod to migrate it.