Terminal Multiplexers: tmux

Immortal Sessions

When you SSH into a server and run a long-running process — a database migration, a build, a log tail — your process is a child of your SSH session. If your network drops, your laptop battery dies, or you simply close the terminal window, the SSH session ends. The OS sends a SIGHUP (hangup signal) to every process in that session, and they terminate.

tmux decouples your processes from your network connection.

tmux (Terminal Multiplexer) runs a server process on the remote machine, entirely separate from any single SSH connection. Your shell and everything it launches live inside a session managed by that server; the SSH connection you're typing into is just a view into that session, not the session itself. You start a session, run whatever long-running process you need inside it, and then detach — which leaves the session, and everything running inside it, alive on the server while your terminal returns to normal. Reattaching later, whether from the same machine or a brand-new SSH connection, drops you straight back into that session exactly as you left it, scrollback and all.

This is precisely the mechanism that lets a multi-hour migration or deployment survive a dropped VPN, a closed laptop lid, or a flaky network connection: the process was never a child of your SSH session to begin with, so nothing about that connection dying ever reaches it. tmux sessions also support multiple windows and split panes within a single session, so one persistent session can hold several parallel views of a job — a log tail in one pane, the running process in another — that all detach and reattach together as a unit.

Essential tmux keyboard shortcuts

Every tmux command is triggered by a two-key sequence: a configurable prefix key (Ctrl+B by default), followed by a command key. The prefix's only job is disambiguation — it tells tmux "the next keystroke is a command for me, not input for whatever program is running inside the session." Without it, tmux would have no way to distinguish a d you meant to detach with from a d you meant to type into your shell.

Why not just use nohup or &?

nohup command & runs a command immune to hangups in the background. It works for fire-and-forget tasks, but you lose interactivity — you cannot attach to it, see its live output, or send it input. tmux gives you a fully interactive session that persists. For operational work on servers, tmux is the right tool.