A terminal multiplexer for *nix operating systems.
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A multiplexer is, according to Merriam-Webster, "a system of transmitting several messages or signals simultaneously on the same circuit or channel". Tmux runs in a terminal shell and presents a number of panes of prompts, effectively splitting the one terminal session into several, enabling many programs to output to a segment of the original window. One can, for example, run an ssh session to another computer, edit files in another 'window pane', watch output from a command in a third. One can connect to a remote computer, run tmux to use several programs at the terminal, close the connection and reconnect to it later:
"…it enables a number of terminals to be created, accessed, and controlled from a single screen. Tmux may be detached from a screen and continue running in the background, then later reattached."
—tmux man page.
This is a great tool. Currently I am editing a couple of files in vi (this one and a config file), I have my mp3 player open in a third pane. For systems administrators, tmux is very valuable as one can open a full-sreen terminal and split it into separate panes, each running a separate task. The program is largely driven by keyboard shortcuts, all of which are configurable. It's easy to create panes as required by recursively splitting the screen horizontally or vertically.
Switching between tasks is easy, but tmux can be a rabbit-hole of customisation; thankfully many people have made example configuration files available for download, and the documentation is excellent.
For anyone who works a lot with the command line, this program can makes one's life easier, or at the least, clearer, reducing the need to switch between terminal windows to see what's going on.
$ xclip -o | wc -w
BQ'25 295