fswatch to watch only a certain file extension

You may watch for changes to files of a single extension like this:

fswatch -e ".*" -i ".*/[^.]*\\.xxx$" .

This will exclude all files and then include all paths ending with .xxx (and also exclude files starting with a dot).

If you want to run a command on the file change, you may add the following:

fswatch -e ".*" -i ".*/[^.]*\\.xxx$" -0 . | xargs -0 -n 1 -I {} echo "File {} changed"

I'm fswatch author. It may not be very intuitive, but fswatch includes everything unless an exclusion filter says otherwise. Coming to your problem: you want to include all files with a given extension. Rephrasing in term of exclusion and inclusion filters:

• You want to exclude everything.
• You want to include files with a given extension ext.

That is:

• To exclude everything you can add an exclusion filter matching any string: .*.

• To include files with a given extension ext, you add an inclusion filter matching any path ending with .ext: \\.ext$. In this case you need to escape the dot . to match the literal dot, then the extension ext and then matching the end of the path with $.

The final command is:

$ fswatch [options] -e ".*" -i "\\.ext$"

If you want case insensitive filters (e.g. to match eXt, Ext, etc.), just add the -I option.