Recompress (progressive) .jpg and .png images in a folder with guetzli.
In guetzlify.sh specify path to guetzli and convert program and set your desired quality.
Before using this script backup your images. guetzlify overwrites them without asking. Maybe you're unsatisfied with compression results and want to play around with quality setting, you need your original files again.
For images in a directory, edit dir in guetzlify_rec.sh and run it.
For single files try guetzlify.sh $FILENAME.