I've already checked out a bunch of repositories in my ~/cvs folder, using CrossVC, a graphical CVS client.
How can I use CVS in cmd with these already checked out repositories? Which variables should I export?
(I have 10 directories, which one with different checked ou repositories)
Checked out working copies usually already contain all the meta data that's needed to have the command line client work on them (look for CVS subfolders containing files such as Root, Repository and Entries.*). The client will always look at that sandbox meta data first before even considering environment variables.
So, no need to export any environment variables if you've already got a checked out working copy - at least no CVS-specific ones - you might need some for enabling SSH tunneling if you're using the :ext: protocol prefix though on *ix-systems even that should typically not be necessary as the defaults are already geared towards that kind of OS.
Note that even checkout or other commands that work directly on the repository (such as rlog, rtag, etc.) do not require any environment variables. Everything that could be specified by variables can also be specified via commandline arguments. The only purpose of defining CVS-specific environment variables such as CVSROOT is to avoid typing the same values over and over again.