Tmux to create a window if it doesn’t exist yet

The command to do this is join-pane in tmux 1.4.

join-pane [-dhv] [-l size | -p percentage] [-s src-pane] [-t dst-pane]  
    (alias: joinp)
    Like split-window, but instead of splitting dst-pane and creating
    a new pane, split it and move src-pane into the space.  This can
    be used to reverse break-pane.

To simplify this, I have these binds in my .tmux.conf for that:

# pane movement
bind-key j command-prompt -p "join pane from:"  "join-pane -s '%%'"
bind-key s command-prompt -p "send pane to:"  "join-pane -t '%%'"

The first grabs the pane from the target window and joins it to the current, the second does the reverse.

You can then reload your tmux session by running the following from within the session:

$ tmux source-file ~/.tmux.conf

Looks like you need this:

move-window [-rdk] [-s src-window] [-t dst-window]
              (alias: movew)
        This is similar to link-window, except the window at src-window
        is moved to dst-window.  With -r, all windows in the session are
        renumbered in sequential order, respecting the base-index option.

Calling movew without parameters moves current window to first free position. movew -r will renumber all the windows at once.