JUSTPREP_FILENAME_IN

"main.just" is the default value.

It can be any base filename you like EXCEPT do not make it the same as the value for JUSTPREP_FILENAME_OUT otherwise that would be a mess.

Unlike just the justprep does not have command-line parameters. Instead it takes its configurations from the [System Environment Variables] that have been defined.

If you want to have a different value for JUSTPREP_FILENAME_IN for each project, you can always use the direnv CLI utility. It loads a temporary set of system environment variables whenever you cd into a directory then unloads them when you go somewhere else that is not in the lower sub-directories. However, I think it is a better practice to have the same value everywhere. Its only software. Set it up the way that works best for you.