It opens the file. In which particular way it opens that file of course depends on the system in question – I don’t have Emacs on my phone, I don’t want xdg-open to attempt to open text files with Emacs on my phone.<p>Whether or not the apparent distribution default of opening text files with Firefox makes sense is an issue somewhat orthogonal to whether xdg-open works – in that particular case it may have been misconfigured to open files with Firefox, but that doesn’t mean it a) didn’t open the file (it did) nor b) in a completely unreasonable way (the file was still displayed and such).