Starting a new tmux session and detaching it, all inside a shell script

New, detached, named and with command executed in default shell:

tmux new-session -d -s apiserver 'java -cp /root/apiserver.jar com.package.EntryPoint'

To attach use:

tmux attach-session -t apiserver

You are using the -d switch when creating the session. This means that the session will start detached, so you don't need to use the detach command. Besides, if your session is not running when you try to detach, it means that it no longer exists, so your command probably exited.

Start a shell, and send vagrant up to it, so you can see the errors.

tmux new-session -d -s rbt123
tmux send-keys 'vagrant up' C-m
tmux detach -s rtb123

The C-m means hit return.