Linux Mint comes with imagemagick 6 ( Version: ImageMagick 6.9.7-4 Q16 x86_64 20170114 ) and I'm using mogrify to edit some pictures.
I have a bunch of pictures that are 700*1000 and I need to add whitespace to either end to make them 768x1000 so I use this command
mogrify *.jpg -gravity center -extent 768x1000
But the pic stays 700x1000, try the same with convert on a single picture and this works fine. Does extent not work with mogrify?
Searching the web I found the Imagemagick mogrify page and it says extent is supported in version 7.
Looking at the examples on the page I would try
mogrify gravity center -extent 768x1000 *.jpg
You should also add more details to the version number as version 6 covered a lot of years and had a lot of releases. Also extent may not have always been supported in V6 mogrify.