I use tmux throughout the day and booting up Vagrant machines is a constant part of my daily workflow. Oftentimes I want to create two additional splits: one for SSHing into the machine/containers, and the other one to display some real-time logs. I used to wait until the Vagrant machine is all booted up before I can manually create the splits and type in the commands – until I learned about send-keys:

vagrant up \
&& tmux split-window -h -c "$PWD"; tmux send-keys -t 2 "vagrant ssh" C-j \
&& tmux split-window -h -c "$PWD"; tmux send-keys -t 3 "vagrant ssh -c 'tail -f /var/logs/app.log'" C-j \
&& tmux select-layout even-horizontal \
&& tmux split-window -v -c "$PWD"; tmux send-keys -t 4 "vagrant rsync-auto" C-j

Since these commands are ran sequentially, the commands executed within the new tmux splits are assured to work because the VM should already be up and running.

This is very useful when you are using something like tmuxinator to manage and automate the creation of tmux sessions on a project-to-project basis.

tmux select-layout even-horizontal will even out the width of the horizontal splits before the vertical split is created. This assures that you get nicely spaced-out windows.

See man tmux and tmuxinator.