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What Is tmux?

tmux is a terminal multiplexer. That sounds fancy; the idea is simple: it runs your terminal sessions as a background server on the machine, and your visible terminal is just a client looking at it.

Normally, your shell lives and dies with your terminal window. Close the window (or drop the SSH connection) and every running program gets killed. With tmux, the programs run inside the tmux server. Close the window, and the server keeps going. Reconnect later — from anywhere — and everything is exactly where you left it.

The three layers

tmux organizes things in three layers, biggest to smallest:

  • Session — a whole workspace. One session might be "the duel batch," another "fleet maintenance." Sessions are independent and can be detached and reattached.
  • Window — like a tab in a browser. One session holds many windows; you look at one at a time.
  • Pane — a split within a window. One window can show two (or more) terminals side by side or stacked.
session "duels"
├── window 0 "batch"      ← you see one window at a time
│   ├── pane 0            ← ...split into panes
│   └── pane 1
└── window 1 "logs"
    └── pane 0

tmux vs screen

If you've used GNU screen, tmux is the same idea done newer: both keep sessions alive across disconnects. Differences that matter:

  • tmux splits panes natively and cleanly; screen's splits are clumsy.
  • tmux has a real scripting interface (new-session, send-keys, split-window as commands, not just keys).
  • tmux is actively maintained; screen is essentially frozen.

If screen already does your job, keep it. If you're learning today, learn tmux.

What tmux is not

  • Not a remote desktop. It's text terminals only — no GUI apps.
  • Not a process manager. It keeps sessions alive, but if the machine reboots, sessions are gone. (For surviving reboots, you want systemd services — a different tool.)
  • Not SSH. tmux doesn't connect anywhere; it just holds the session on whatever machine it's running on. Pair it with SSH (see tmux Over SSH) and the combination is the whole trick.

Check: is tmux installed?

tmux -V

tmux 3.4

Any 3.x is fine for this guide. Install it with your package manager if it's missing (apt install tmux, dnf install tmux, brew install tmux).