init.el

      (if (getenv "TMUX")
          (progn
            (defun keycast-mode-line-update ()
              "Display current `thisl-command' and `this-command-keys' using tmux message"
              (if (not (eq this-command 'self-insert-command))
                  (shell-command
                   (format "tmux display-message '[%s] %s &'" (this-command-keys) this-command))))
            (add-hook 'pre-command-hook 'keycast-mode-line-update t)))