Change mac app icon programatically

go.sh

#!/bin/sh

# I believe you need XCode installed to get sips/Rez/SetFile

# Have an Icon.png in the current working directory

# Resize it to 512 by 512 for some reason
sips -z 512 512 Icon.png --out myIconResized.png
# make it an .icns file
sips -s format icns myIconResized.png --out myIcon.icns
# do something else…?
echo "read 'icns' (-16455) \"myIcon.icns\";" >> tmpicns.rsrc
# add it to the app
rm $'/Applications/Slack.app/Icon\r'
Rez -append tmpicns.rsrc -o $'/Applications/Slack.app/Icon\r'
# enable the icon
SetFile -a C /Applications/Slack.app/
# restart slack
killall Slack
# Now manually reopen the application, doing it via command line doesn't update the UI for some reason.