I am trying to use GRASS on the command line, specifically the `r.thin` module. It seems to execute properly but I have no output raster dataset to work with afterwards, it is as if it just gets swallowed into the void. This is the exact command:

`grass79 -c /data/mapset/PERMANENT --exec r.thin input=dummy_img output=thinned`

And I get the following output:

    Starting GRASS GIS...
    WARNING: Mapset </data/mapset/PERMANENT> already exists. Ignoring the request to create it. Note that this warning may become an error in future versions.
    Cleaning up temporary files...
    Executing <r.thin input=dummy_img output=thinned> ...
    Raster map <dummy_img> - 658 rows X 658 columns
    Execution of <r.thin input=dummy_img output=thinned> finished.
    Cleaning up temporary files...

So it seems that it executed properly.

But when I look inside the /data/mapset/PERMANENT folder, I see only the `dummy_img` (which was already there) and no mention of `thinned`:

```
$ ls data/mapset/PERMANENT
dummy_img
```

Am I doing something wrong? [here](https://grass.osgeo.org/grass76/manuals/r.thin.html) is the docpage but I don't see any specific caveats to this.

I also downloaded a docker image with GRASS 7.6 instead of 7.9, as it is supposed to be stable, but it still did not work, just seemingly no output whatsoever.

Furthermore, this algorithm seems to work when I run it in QGIS 2.18.23 so there must be something going on...