#### Sunday, December 7, 2014 · 2 min read

tmux is a terminal multiplexor, which is nerdspeak for a program that runs multiple processes simultaneously within a single parent process. You might have heard of screen; it’s similar (and, in fact, a lot of tmux quickstarts assume that you’re transitioning from screen). This lets you have, for example, a text editor and a test server running in the same physical terminal window. Instead of opening multiple ssh connections to your server, tmux allows you to maintain a single connection and divide your screen up virtually into multiple panes.

Another nice thing about tmux is that the virtual panes are independent of the processes running, so you can “detach” a process and leave it running in the background without any terminal displaying the output. In face, a detached tmux session lives on even if you disconnect the ssh session. When you log back on, you can reattach to that process again.

Anyway, let’s get started. You make a tmux session by typing tmux in bash.

Your screen should get a pretty green ribbon under it, saying 0:bash. This means you’re currently in window 0, running bash. You can do normal bashey things here (ls, vim, irssi, whatever): tmux simply feeds your user input along to the bash process.

Well, almost. tmux listens in and intercepts any input that begins with a special keypress, ^B. You type this with the control character and ‘B’ the way you would type ^C to kill a bash process. We call ^B the “prefix”.

Let’s detach from tmux! For the dramatic effect, feel free to leave some process running—perhaps a Python session or even your IRC client. Type ^B D (that is, the prefix followed by the D character).

You should be back to the old bash. But the process you started is still running in the background: just not getting any input from you (or showing you any output). To reconnect to it, type tmux attach and you should get your process again. The easiest way to kill a session is to simply exit all the processes in it; if only bash is running, then type exit.

You can use and manipulate multiple different named sessions, by specifying different command line arguments to tmux, such as tmux new -s name_of_new_session to make a new session, tmux attach -t name to attach to a named session, and tmux kill-session -t name to kill a session. tmux ls lists sessions.

But the more interesting stuff is multiplexing. Open up a session and type ^B %. Your pane should split into two columns. You now essentially have two virtual terminals. Use ^B arrow-keys to switch between panes. To close a pane you exit the process that was running in that pane (exit in bash).

You can use ^B " to split the other way (horizontally, so the new pane is below the old one). And there are a bunch of commands to resize and swap panes.

Instead of saying them all over again, I’m going to point you to this gist, which has all the information you need.

In general, I use tmux as a way to keep my session as I left it when I logout (for example, this post was written across a couple of days, but I didn’t close vim at all). Also, it’s an easy way to leave a server or a bot running perpetually.

Enjoy tmuxing!

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