I maintain a manifest file for a Trigger.io mobile app which includes a set of images indexed by size. I’ve taken to generating the JSON with awk and Imagemagick’s identify command:

identify returns a wealth of information about an image file:

$ identify Default~iphone.png
Default~iphone.png[6] PNG 320x480 320x480+0+0 8-bit sRGB 11.5KB 0.000u 0:00.000

I want to transform that into some JSON. awk is designed for exactly this kind of text transformation problem.

$ identify *.png | awk '{gsub("\.png.*$", ".png", $1); printf "\"%s\": \"%s\",\n", $3, $1}' | sort
"1024x748": "Default-Portrait@2x~ipad.png",
"1024x768": "Default-Landscape~ipad.png",
"2048x1496": "Default-Landscape@2x~ipad.png",
"320x480": "Default~iphone.png",
"640x1136": "Default-568h@2x~iphone.png",
"640x960": "Default@2x~iphone.png",
"768x1004": "Default-Portrait~ipad.png",

Copy and paste into manifest, remove trailing comma, make cup of tea.

EDIT: Achieve the same effect with the -format parameter for identify (-format docs here):

identify -format '"%wx%h": "%f",\n' *.png