I built a tmux session manager for myself, It uses a ruby configuration file similar to the one below, You can find it at https://github.com/minhajuddin/tummy . I feel ruby is more flexible at configuration than yaml.

    session "statictrain"
    directory "/home/minhajuddin/r/st/web"

    window "src", [
      pane("vim TODO"),
    ]

    window "server-iex", [
      pane("iex -S mix phoenix.server"),
      # the last argument is passed to tmux as raw arguments
      #pane("iex -S mix", :horizontal, "-l 8"),
      pane("git status; tail -f log/debug.log", :vertical),
    ]


    # if you comment this out it will focus the first window when the session is started
    #focus_window "server-iex"
    # focus_window 0 # you can even focus a window by index starting at 0

    # vim: filetype=ruby