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.