Send a command to a running tmux session (like screen -X)

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http://unix.stackexchange.com – I'm looking to switch from screen to tmux for my terminal multiplexing needs, but there's a feature of screen that I need to replicate in tmux: the -X option, which allows one to run arbitrary screen commands inside an existing screen session. This allows me to modify a running screen session's environment to point to a new X server, since my screen sessions tend to live longer than my X11 logins. For example, I could put something screen -X setenv DISPLAY $DISPLAY in my X11 startup scripts. Anyway, is there a way in tmux to send arbitrary commands to a running tmux session? Alternatively, a (HowTos)