Can you give an example for what it's needed? Do you know you can call cygpath -m /?

I've been playing around with a fork of this action that installs the last i686 msys release, and a workflow to build the packages I've missed out on since i686 stopped getting updated (it's actually been going surprisingly well). I wanted to call autorebase.bat after updating the packages I'd already built, so needed to know the path from a cmd run step. It didn't occur to me to call out to cygpath, but I had seen the docs that steps could output variables and thought that made enough sense to offer that change upstream.

ok, thanks. That seems like a very uncommon use case, though I'm not against it per se, except maybe the naming.

@eine any thoughts?

For reference, using cygpath instead of an output variable:

    - name: test output
      shell: cmd
      run: |
        set MSYSTEM=MSYS
        FOR /F "tokens=* USEBACKQ" %%g IN (`CALL msys2 -c 'cygpath -w /'`) DO (SET "MSYSROOT=%%g")
        echo %MSYSROOT%
        call "%MSYSROOT%\autorebase.bat"

Untested, but something like this should also work in powershell:

Invoke-Expression (msys2 -c 'cygpath -m /autorebase.bat')

Honestly, I don't like the feature to set outputs from Actions. It feels like a not very well thought ad-hoc solution. Instead, I'd use some more natural solution, such as setting an environment variable: https://github.com/actions/toolkit/tree/main/packages/core#exporting-variables.

However, if the last comment by @lazka works, maybe it's better to just document that in the README. On the other hand, I believe that msys2.cmd was always located in MSYS2's /, prior to latest cache changes. Hence the root could be extracted from the location of the command file. An alternative to setting an envvar would be to ensure that this holds.

To better match what I was doing, I tested the following:

& (Join-Path (msys2 -c 'cygpath -w /') 'autorebase.bat')

and confirmed that that works. I'm sure giving autorebase.bat to cygpath instead of using Join-Path would also work.

And if you really want an output for some reason, you can set one from a run step:

    - name: test cygpath
      id: cygpath
      run: |
        $env:MSYSTEM = 'MSYS'
        Write-Output ('::set-output name=msysRootDir::{0}' -f (msys2 -c 'cygpath -w /'))
    - name: autorebase
      run: |
        $env:MSYSTEM = 'MSYS'
        & (Join-Path '${{ steps.cygpath.outputs.msysRootDir }}' 'autorebase.bat')

for anyone like me wanted it in environment variable:

      - name: Export MSYS2_ROOT
        run: echo "MSYS2_ROOT=$(msys2 -c 'cygpath -m /')" >> $env:GITHUB_ENV