To add a spacer block in Mac Dock …
defaults write com.apple.dock persistent-apps \
-array-add '{"tile-type"="spacer-tile";}' \
&& killall Dock
To add a spacer block in Mac Dock …
defaults write com.apple.dock persistent-apps \
-array-add '{"tile-type"="spacer-tile";}' \
&& killall Dock