Run a screen session on boot from rc.local
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.
Here is what I used I found it to be the cleanest and simplest (tested working myself):
Replace "user" with the user to run it as. Replace "nameyouchoose" as the name of the screen session Replace "/script/start.bash" to the path of your script.
/usr/bin/sudo -u user /usr/bin/screen -dmS nameyouchoose /script/start.bash
Source: http://realtechtalk.com/How_to_start_screen_in_bash_script_or_from_etcrclocal_on_startup_as_a_specific_user-1980-articles