What command are you looking for in particular? The command for listing a folder is 'ls' or 'ls -l' for a list view, which is the same for other distributions.
If you want a list of available programs, type 'ls /usr/bin'. That will list files in the folder '/usr/bin', which is where most programs reside (or at least a link to the real location). It is a long list and not that useful.
You should be able to start with any Linux command line guide and just try it out.
Most programs accept a parameter '-h' or '--help' to get usage information. There is also the 'man' command for manuals.
The standard text editor on NI Linux RT is 'vi', which takes a bit of getting used to. (One of the most frequently asked questions on stackoverflow is 'How to exit vi/vim')
The package manager is opkg, you can use it to install programs that are missing.
On windows, you can install the Windows Subsystem for Linux or wsl. That way you can familiarize yourself with the Linux shell without setting up a VM or connection to a cRIO.
