How to comment multiple images at once

On Windows I had that nice bar on the bottom with the exif entries that I can't manage to make appear on Ubuntu (I'm on Lubuntu but I've tried Mate and Plasma 5.4 too).I've tried with exiftool: exiftool -Comment='my comment'<directory> but once I check with gThumb, under Comment > Description there's nothing written. How can I batch comment a bunch of pictures?