This is more about the invocation of a program, than any language or parser (though I’m sure choice of parser library can depend on this). See, I’ve used a lot of Linux command-line utilities. And there are some obvious patterns; ‘-‘ precedes a single letter for short options, multiple options that don’t take arguments can be combined, ‘–‘ precedes long versions of options, and so on.
However, in some cases, capitalization is used to invert an option. So, ‘-d’ might mean to run as a daemon, but ‘-D’ would be to not run as a daemon. (Why not just omit the option if you don’t want it? That’s never been clear, but it’s actually rather common, so I figure there must be some reason.) But in some programs, a capital is a completely unrelated option; if ‘-d’ is run as daemon, ‘-D’ might be to enable debug mode. Is there some kind of overarching principal behind this, and which is the best to choose? Or are we just dealing with “whatever works”?
There are also some commands that, in addition to (or instead of) options with arguments, just take lone arguments. cp is a good example of this; aside from a couple rarely used toggles, the last argument it receives is presumed to be the destination, and any arguments between the option list and the destination are presumed to be sources. Is there a rule of thumb when it’s “okay” to rely on order like that, instead of using explicit option flags with arguments?
Generally, yes.