The module system is a program, running on most supercomputers, that simplifies using different software in a precise and controlled manner. Oftentimes a supercomputer has more software installed than the average user will ever use. Frequently there are also multiple version of the same program available at the same time, to enable backward compatibility. A prominent example of this are Compilers, which sometimes have the tendency to break older programs with new versions.
To get a list of all currently loaded modules, one can employ
$ module list
and might receive something like:
Currently Loaded Modulefiles: 1) DEVELOP 3) intelmpi/2017.4 2) intel/16.0 4) likwid/system-default
$ module avail
lists all available (loadable) modules and module groups. With the information of these two commands, one can select module to load/unload/switch with
$ module load/unload x
to load or unload module x or
$ module switch x y
to switch out module x for module y. Switching is especially useful, to switch to a different version of the same program.
If you recurrently need lots of modules, this loading can be automated with an sh-file, so that you just have to execute the file once and it loads all modules, you need.
Using these commands, the module system takes care to set all the system variables, so the desired applications are referenced and called correctly.