Problem :
When I am under one of my user-names, I can fire off, say, screen -S -p 0 -X eval 'stuff "say command" 15'. However, I cannot su username screen -S -p 0 -X eval 'stuff "say command" 15'; it complains that permission is denied. Why is this?
Solution :
The username that you’r using with su might not be a super user.
Just type like this,
su screen -S -p 0 -X eval 'stuff "say command" 15'
then type your password