Ubuntu – kill a screen session

I don't know about typing Ctrl-A twice, but Ctrl-a A should give a prompt to enter a new title for the current window. You might want to try Ctrl-q then Ctrl-d a couple of times until you see [Screen is terminating]. If you get that, then nothing has frozen, it was just waiting for input somewhere. What does your ~/.screenrc file look like, if you have one?

I think both -c parameters (su and bash) will have to be quoted at a minimum.

su - username -c "/usr/bin/screen -dmS test bash -c '/var/www/path/to/script/script.sh; exec bash'"

Also - is expected to be last and may not be desirable here (see man su).

A few more remarks. sudo could be a better choice for a one-shot command like yours, but not necessarily:

sudo -iu username /usr/bin/screen -dmS test bash -c '/var/www/path/to/script/script.sh; exec bash'

In particular, you can use one less level of quoting with sudo.

Another thing you want to be careful with is executing commands without absolute path in a privileged context. This holds for su (or sudo) but also for the path to bash in your command. You are doing it right with screen.

Just tested the following and it works nicely. I think the - is the main issue in your original line:

/bin/su username -c "/usr/bin/screen -dmS test bash -c '/home/username/test.sh; exec bash'"

Evil remark: why don't you give tmux a try? I have recently switched and never looked back. The only thing I needed to change immediately was the prefix key combination which in tmux defaults to Ctrl + B - back to GNU screen's Ctrl + A.

It allows splitting your window into an almost arbitrary number of panes (vertically and horizontally) and its configuration file format (including the one for the status par) is actually intelligible to humans. Of course tmux is as good as screen when you simply want to run some program/script not originally written as daemon in background. If you intend to interact with the terminal multiplexer, however, I warmly recommend tmux.