Open app in background from Terminal
If you call the command open with --hide it works (at least in Sierra MacOs). For instance
open -a /Applications/Skype.app --hide
Also, the j flag helps some
open -gj -a /Applications/Skype.app
As of writing (10.14.3 Mojave), the correct answer is:
open -a /Applications/Skype.app -gj
If you do
man open
you can see that
-g Do not bring the application to the foreground.
-j Launches the app hidden.