Keep programs running after you disconnect: screen and tmux

When your SSH connection drops, everything running in that session normally stops with it. That is a problem for long jobs such as big file copies, database imports and system upgrades. Screen and tmux solve it. They keep a terminal session running on the server, so you can disconnect, close your laptop, and later reconnect exactly where you left off.

This guide covers both tools: how to install them, the handful of keys you need, splitting the screen into panes, logging output, and when you should use a proper background service instead. If you are starting fresh, tmux is the more modern choice. Screen is older, simpler and installed on many servers already, so it is worth knowing both.

What is in this guide

• A few words you will see
• Screen or tmux?
• Install them
• tmux: the essentials
• tmux: windows and panes
• tmux: a small settings file
• screen: the essentials
• screen: windows and logging
• Good uses
• When to use a service instead
• Troubleshooting
• Limits
• When to step up

A few words you will see

Term	What it means
Session	A set of terminal windows that keeps running on the server.
Detach	Leave a session running in the background and return to your normal prompt.
Attach	Reconnect to a running session.
Window	A full-screen terminal inside a session, like a browser tab.
Pane	A part of a window, when you split it to see two terminals side by side. tmux only.
Prefix key	The key combination you press before a command key. Ctrl+B in tmux, Ctrl+A in screen.
Scrollback	Earlier output that has scrolled off the screen.

Screen or tmux?

	tmux	screen
Prefix key	Ctrl+B	Ctrl+A
Split into panes	Yes, easily	Limited
Settings file	~/.tmux.conf	~/.screenrc
Status bar	On by default	Off by default
Often already installed	Sometimes	Often
Best for	Day-to-day work and new users	Quick jobs on servers where it is already present

The most important job, keeping a session alive after you disconnect, works the same in both. Pick one and learn its five or six keys.

Install them

sudo apt update
sudo apt install tmux screen

Both use very little memory, so they suit any plan.

tmux: the essentials

In the key table, "Ctrl+B, then D" means press Ctrl and B together, let go, then press D.

What you want	Command or keys
Start a named session	tmux new -s work
Detach, leaving it running	Ctrl+B, then D
List sessions	tmux ls
Reattach	tmux attach -t work
Reattach to the last session	tmux attach
Scroll back through output	Ctrl+B, then [ and use the arrow or Page Up keys. Press q to stop
End the session	Type exit in every window, or run tmux kill-session -t work

A typical use: log in, run tmux new -s copy, start a long rsync, press Ctrl+B then D, and log out. An hour later, log in and run tmux attach -t copy to see how it went. If your connection drops by accident, the session carries on in exactly the same way.

tmux: windows and panes

What you want	Keys
New window	Ctrl+B, then C
Next or previous window	Ctrl+B, then N or P
Go to window 1, 2 and so on	Ctrl+B, then the number
Split side by side	Ctrl+B, then %
Split top and bottom	Ctrl+B, then "
Move between panes	Ctrl+B, then an arrow key
Close a pane	Type exit in it
Make one pane full size, and back	Ctrl+B, then Z

Panes are handy for watching a log in one half while you work in the other, for example sudo journalctl -f next to the commands you are running.

tmux: a small settings file

A few settings make tmux friendlier. Create ~/.tmux.conf:

set -g mouse on
set -g history-limit 10000
set -g base-index 1

This lets you click on panes and scroll with the mouse wheel, keeps more scrollback, and numbers windows from 1. Reload it inside tmux with Ctrl+B, then : and type source-file ~/.tmux.conf. With mouse mode on, hold Shift while selecting text to copy it in most terminal apps.

screen: the essentials

What you want	Command or keys
Start a named session	screen -S work
Detach, leaving it running	Ctrl+A, then D
List sessions	screen -ls
Reattach	screen -r work
Reattach even if still attached elsewhere	screen -d -r work
Scroll back through output	Ctrl+A, then Esc, and use the arrow keys. Press Esc to stop
End the session	Type exit, or press Ctrl+A then K and confirm

If your connection dropped, screen may still think you are attached. screen -d -r work detaches the old connection and attaches the new one.

screen: windows and logging

What you want	Keys or command
New window	Ctrl+A, then C
Next or previous window	Ctrl+A, then N or P
List windows	Ctrl+A, then "
Start logging the window to a file	Ctrl+A, then H
Start a session with logging on	screen -L -S work

Logging saves everything shown in the window to a file named screenlog.0 in the folder where you started screen. It is useful for long jobs whose output you want to read later. With tmux, a simple way to get the same is to pipe a command through tee, for example long-command | tee job.log.

Good uses

• System upgrades. An upgrade that stops halfway because a connection dropped is painful to fix. Always run release upgrades inside tmux or screen. See upgrading an Ubuntu server safely.
• Large copies and imports. rsync jobs, database imports and big downloads. See moving to a new VPS.
• Watching while you work. A log in one pane and your commands in another.
• Unreliable connections. Working from a train, a phone or patchy Wi-Fi.
• Quick tests. Trying a program by hand before turning it into a service.

When to use a service instead

Screen and tmux keep sessions alive when you disconnect, but not when the server reboots, and they do not restart a program that crashes. For anything that should always be running, such as a bot, an app or a game server, use a systemd service instead. Create a file such as /etc/systemd/system/mybot.service:

[Unit]
Description=My bot
After=network.target

[Service]
User=alex
WorkingDirectory=/home/alex/mybot
ExecStart=/usr/bin/python3 /home/alex/mybot/bot.py
Restart=on-failure

[Install]
WantedBy=multi-user.target

Change the user, folder and command to match your program. Then start it, and make it start at every boot:

sudo systemctl daemon-reload
sudo systemctl enable --now mybot
systemctl status mybot

Its output goes to the system journal, which you can read with sudo journalctl -u mybot -f. For apps packaged as containers, Docker's restart setting does the same job. See installing and using Docker on a VPS.

Troubleshooting

Symptom	Likely cause	Fix
"no sessions" or "There is no screen to be resumed"	The session ended, the server rebooted, or you are a different user	Run tmux ls or screen -ls as the same user who started it
screen says the session is attached	The old connection did not close cleanly	Use screen -d -r NAME
"sessions should be nested with care"	You are already inside tmux	Detach first, or use the session you are in
The prefix keys do nothing	Your terminal app or another tool uses those keys	Check your terminal's shortcuts, or change the prefix in the settings file
Cannot scroll with the mouse	Mouse mode is off	Use the scrollback keys, or turn on mouse mode in tmux
A job stopped after a reboot	Sessions do not survive reboots	Run it as a systemd service instead
Strange characters or broken lines	Terminal type mismatch	Try a different terminal app, or set TERM=xterm-256color before starting

Limits

• Not for always-on programs. Sessions end when the server reboots, and crashed programs stay crashed. Use systemd for those.
• Sessions belong to one user. A session started with sudo, or as another user, is only visible to that user.
• Forgotten sessions keep running. A session with a busy program left in it keeps using CPU and memory. Check tmux ls now and then.
• Fair use applies. Keeping heavy jobs running for hours is fine for occasional work, but constant full CPU use can hit fair-use limits. Unmetered does not mean unlimited.

When to step up

Screen and tmux run on any plan. If you find yourself running long, heavy jobs in them often, such as builds, encoding or big data processing, the plan's CPU and memory may be the real limit. How to read VPS specs explains which specs matter for which jobs.

To compare what is available now, start with the cheapest RackNerd VPS guide and check the sales page. For more everyday tools, see useful Linux server tools.