Emacs: Tell Org which linked files to open externally

2024-07-12

When Org resolves the file system path to a link that uses the file: hyperlink type, it has a mechanism to decide whether to open that file in Emacs or an external program. Here is some sample Org markup:

A link to [[file:/path/to/test.html][an HTML file]] will open outside Emacs.

The idea is that some files are best handled by a specialised program, such as a video with a media player. Generally useful, yes, though there are cases where we may want to use Emacs. Hence the user option org-file-apps.

This option lets us specify a list of cons cells, each of which consists of a file matcher and method as (MATCHER . METHOD). The doc string describes the technicalities. For my case, the MATCHER is a regular expression that targets the file type extension, while the METHOD is the emacs symbol. Thus:

;; Tell Org to use Emacs when opening files that end in .html
(add-to-list 'org-file-apps '("\\.html\\'" . emacs))

The add-to-list function is especially useful here, because it appends the element to the existing value of the list. This is what we probably want in this case as the default value of org-file-apps has some reasonable associations already.

I learnt about this after troubleshooting a case with denote: link types in Org files (denote: links are designed to behave like the file: type). Org would open HTML files in the browser, making it awkward to get to an Emacs buffer from where to edit the code. With Denote in particular, it is common to rename files of any file type (images, videos, PDFs, …) to have the Denote file-naming scheme and then link to them from an Org buffer. It is also possible to generate new files in any writable file type, subject to some configurations. This was my case, with the use of HTML files.

[ I also wrote a relevant entry about this in the Denote manual. ]

Thankfully, org-file-apps is there to (i) let us enjoy the power of opening some file: (or denote:) links externally while (ii) deciding when we want those to be in an Emacs buffer instead.