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Intro to terminal multiplexers

Author:Gregg Thomas · Last updated: October 19, 2026

If you work on a remote server or cluster over SSH, you have probably had this happen: you start a long process, your laptop goes to sleep or your Wi-Fi drops, and when you reconnect the process is gone. Or you find yourself opening five separate SSH connections just to watch a log in one, edit a file in another, and run a command in a third.

A terminal multiplexer solves both problems. It lets you:

  • Keep sessions alive even after you disconnect. You can start something, close your laptop, go home, reconnect from a different computer, and pick up exactly where you left off.
  • Run many shells in one window, split into panes or organized as tabs, all inside a single SSH connection.

The two classic tools are screen and tmux. byobu is a friendly wrapper that sits on top of either one. This tutorial covers the core concepts and the handful of commands you need to get productive with each.

Where this matters on the cluster

On the FAS cluster, a multiplexer running on a login node is perfect for keeping an interactive workflow open — monitoring jobs, editing files, or holding a long salloc/srun interactive session so it survives a dropped connection. It is not a tool for running heavy computation directly: login nodes are shared, so real work still belongs in a Slurm job. Think of a multiplexer as a durable, organized workspace, not a way around the scheduler.

What is a terminal multiplexer?

A multiplexer wraps your shell in a layer that keeps running independently of your connection. There are three concepts to understand, from biggest to smallest:

  • Session — a whole workspace that keeps running on the server. You detach from it (leave it running in the background) and later reattach to it. This is the piece that survives a disconnect.
  • Window — like a tab inside a session. Each window is a full-screen shell. A session can hold many windows.
  • Pane — a split within a window, so you can see several shells side by side at once.
Session  "analysis"
├── Window 0: editing
├── Window 1: monitoring   ┌───────────┬───────────┐
│                          │  pane 0   │  pane 1   │   <- one window,
│                          │  (htop)   │  (tail)   │      split into panes
│                          └───────────┴───────────┘
└── Window 2: git

The prefix key

Almost every multiplexer command is triggered by a prefix key: you press the prefix, release it, then press a command key. This keeps the tools from intercepting normal keystrokes.

  • screen prefix: Ctrl-a (written C-a)
  • tmux prefix: Ctrl-b (written C-b)

So "press the prefix, then c" means press Ctrl-a (or Ctrl-b in tmux), let go, then press c. Throughout this tutorial, C-a c means exactly that.

Detach vs. exit

Detaching leaves the session running on the server — this is what you want when you log off. Exiting the shell (typing exit in every window) closes the session for good. If you want your work to survive, detach; don't exit.

screen basics

screen is the older of the two and is installed almost everywhere by default, which makes it a safe fallback on any server.

Starting and naming a session

screen                  # start an unnamed session
screen -S analysis      # start a session named "analysis" (recommended)

Naming your sessions makes them much easier to find later.

Detaching and reattaching

# From inside a session, detach with the keystroke:
#   C-a d          (Ctrl-a, then d)

screen -ls              # list running sessions
screen -r analysis      # reattach to the "analysis" session
screen -r               # reattach if there is only one

You can now disconnect entirely, SSH back in later, run screen -r, and your session is exactly as you left it.

Working with windows

Keys Action
C-a c create a new window
C-a n next window
C-a p previous window
C-a " list windows to pick from
C-a A rename the current window
C-a 0–9 jump to window by number

Splitting into panes

Keys Action
C-a S split horizontally (top/bottom)
C-a \| split vertically (left/right)
C-a <Tab> move focus to the next pane
C-a X close the current pane

A newly split pane is empty

In screen, a fresh pane has no shell in it yet — move into it with C-a <Tab> and press C-a c to start one.

Scrolling back

Your mouse wheel often won't scroll inside screen. Enter copy mode to scroll through history:

  • Press C-a <Esc>, then use the arrow keys / PageUp to scroll.
  • Press <Esc> again to leave copy mode.

tmux basics

tmux is the modern successor to screen. It has cleaner defaults, a scriptable config file, and mouse support, so it's the tool most people reach for today. The concepts are identical — only the keybindings differ (prefix is C-b).

Starting and naming a session

tmux                    # start an unnamed session
tmux new -s analysis    # start a session named "analysis"

Detaching and reattaching

# From inside a session, detach with:
#   C-b d

tmux ls                     # list sessions
tmux attach -t analysis     # reattach to "analysis"
tmux attach                 # reattach to the most recent session

Working with windows

Keys Action
C-b c create a new window
C-b n next window
C-b p previous window
C-b w list windows to pick from
C-b , rename the current window
C-b 0–9 jump to window by number

Splitting into panes

Unlike screen, a new tmux pane immediately opens a shell.

Keys Action
C-b % split vertically (left/right)
C-b " split horizontally (top/bottom)
C-b <arrow> move focus between panes
C-b z zoom the current pane to full screen (toggle)
C-b x close the current pane
C-b <space> cycle through pane layouts

Scrolling back

Press C-b [ to enter copy mode, then scroll with the arrow keys / PageUp. Press q to exit.

A minimal config file

tmux reads ~/.tmux.conf on startup. A couple of quality-of-life settings that beginners tend to want:

# ~/.tmux.conf
set -g mouse on            # click panes/windows, drag to resize, wheel to scroll
set -g history-limit 50000 # keep a much longer scrollback buffer

With mouse on, you can click to select panes, drag borders to resize, and scroll with the wheel — a big usability jump.

Reloading the config

After editing ~/.tmux.conf, apply it without restarting: from inside tmux press C-b :, type source-file ~/.tmux.conf, and hit Enter.

screen vs. tmux at a glance

Task screen tmux
Prefix key C-a C-b
New named session screen -S x tmux new -s x
Detach C-a d C-b d
List sessions screen -ls tmux ls
Reattach screen -r x tmux attach -t x
New window C-a c C-b c
Next / prev window C-a n / p C-b n / p
Split vertical C-a \| C-b %
Split horizontal C-a S C-b "
Switch pane C-a <Tab> C-b <arrow>
Scrollback / copy C-a <Esc> C-b [

Which should you use? If you're starting fresh, learn tmux — better defaults, mouse support, and a config you can carry between machines. Keep screen in your back pocket for servers where tmux isn't installed.

byobu: a friendlier front-end

If the prefix-key gymnastics feel like a lot, byobu is a wrapper around tmux (or screen) that adds a helpful status bar and maps the most common actions to plain function keys — no prefix required.

byobu                   # start (or reattach to) a byobu session

Common byobu keys:

Key Action
F2 new window
F3 / F4 previous / next window
F6 detach (leave it running)
F8 rename the current window
Shift-F2 split horizontally
Ctrl-F2 split vertically
F9 open the byobu config menu

The status bar along the bottom shows things like the time, load, and memory, and the config menu (F9) lets you toggle features and change the backend without editing files by hand.

byobu is still tmux/screen underneath

Because byobu just drives tmux or screen, everything you learned above still applies — you can mix function keys with the normal prefix commands. Check which backend is active with byobu-status or in the F9 menu.

Making byobu (or tmux) auto-start on login

Some people like their multiplexer to launch automatically every time they log in. You can enable that with byobu-enable (and undo it with byobu-disable). This is optional and a matter of taste — many users prefer to start it manually so they control when a session is created.

A few gotchas

Clusters with multiple login nodes

Your session lives on the specific machine where you started it. On a cluster with several login nodes (e.g. holylogin01, holylogin02, …), if you reconnect to a different login node your session won't be there. Note which node you're on (it's in your prompt) and SSH back to that same one to reattach.

Don't do heavy compute on a login node

A multiplexer is great for editing, monitoring, and holding interactive sessions open, but it does not exempt you from the scheduler. Launch real computation through Slurm (sbatch, salloc, srun).

Nesting

Running tmux inside tmux (for example, a local tmux and another on the server) means the prefix goes to the outer one. Send a command to the inner session by pressing the prefix twice: C-b C-b c.

Further reading