FR: use filenames instead of pipes for the pager (where appropriate)

[jyn514]

Is your feature request related to a problem? Please describe. i am running jj cat to see the contents of a file from an old revision. i want to have syntax highlighting, so i set PAGER='bat --color=always'. but bat does not know the data coming from stdin is a rust file, so it doesn't generate syntax highlighting; i have to use PAGER='bat --file-name=cli_util.rs --color=always' explicitly (which means i can't set this persistently in jj.toml).

Describe the solution you'd like write the file to disk before piping it to the pager.

Describe alternatives you've considered

• special-case bat and pass it --file-name flags
• decide this use case isn't supported and people should just use flags in PAGER explicitly.

Additional context Add any other context or screenshots about the feature request here.

[joyously]

Or have an environment variable hold the current file name, to be used as needed?

I think many tools use a file written to /tmp as when an editor allows you to run your compiler on unsaved changes, or the zip file viewer lets you look in a file in the zip.

[jyn514]

an environment variable would work, yeah. i'd have to write a little wrapper script that passes it to bat, but that's doable.

[martinvonz]

Or have an environment variable hold the current file name, to be used as needed?

Yes, or substitute a $filename variable like we do for diff editors. Then the user can set their config like this:

ui.pager = ["bat", "--filename", "$filename"]

We already allow passing static environment variables (like we do to less by default). We could allow passing the path that way too, so you could do this:

ui.pager = { command = ["my-ager"], env = { MY_PAGER_FILENAME = "$filename" } }

But there might not be a need for that, so that's probably just a nice to have.