The open command can ask questions
If you pass an argument that can’t be easily identified as a file or a URL, open will ask you what to do next. This may be a surprise if you were trying to use it in a script.
This is something I discovered by accident – I had a shell script that called open *.mp3, and it offered me a selection prompt rather than just opening the file.
After a bit of investigating, it turned out to be caused by a file with a colon in the filename. Here’s a minimal example that shows this behaviour:
$ touch 'A:B.mp3'
$ open A:B.mp3
A:B.mp3?
[0] cancel
[1] Open the file A:B.mp3
[2] Open the URL (null)
Which did you mean?
I suspect this is because open can open URLs in a web browser as well as open files on disk, and the colon makes it unclear which it should do. If you put something that looks more like a URL in here, the ambiguity is more obvious:
$ open http://localhost:5858
http://localhost:5858?
[0] cancel
[1] Open the file http://localhost:5858
[2] Open the URL http://localhost:5858
Which did you mean?
This could mean “a file called localhost:5858 in the http folder”, or it could mean “the URL http://localhost:5858” – and both of those options can work!
Can you tell open what to pick?
Kinda.
From the man page on macOS Ventura:
-u Opens URL with whatever application claims the url scheme, even if
URL also matches a file path
So for example, this command will open the URL in my web browser:
$ open -u http://localhost:5858
There doesn’t seem to be a corresponding flag for “always open this as a file path”. Even if you specify an application name with the -a flag, it still prompts you for ambiguous filenames.
You can force it to open with TextEdit (open -e) or the default text editor (open -t), but that’s only useful if you’re opening a text file.
Note also that the -u flag seems to be relatively new: it doesn’t appear in either copy of the open(1) man page that I found online (unix.com, ss64.com).
Never trust arbitrary input!
The reason I had filenames with colons is because I was using youtube-dl to download some YouTube videos, and it puts the video title in the filename. When I downloaded a file with a colon in the title, I got the unexpected open behaviour.
For a simple script this is fine, but it may cause issues elsewhere – e.g. if you’re calling open in a context where you’re not expecting it to ask for input.