MacOS Dock Spacers
Add dock spacer (use again and again for mores spacers):
defaults write com.apple.dock persistent-apps -array-add '{tile-data={}; tile-type="spacer-tile";}' && killall Dock
Add half-width spacer (use again for more):
defaults write com.apple.dock persistent-apps -array-add '{"tile-type"="small-spacer-tile";}' && killall Dock