Why Screen?

You SSH into a server, start a backup, a large file copy, or a database import that will take an hour — and then your Wi-Fi drops, your laptop sleeps, or you simply need to go home. When you reconnect, the job is gone. This course is about stopping that from happening, and about understanding exactly how Linux manages the jobs you start.

Why Jobs Die When the Connection Drops

Every program you start from an SSH session is attached to a terminal — strictly, a pseudo-terminal that sshd creates for your login. Your shell, and every job it starts, reads from and writes to that terminal. When the connection breaks, the terminal "hangs up." The system then sends the SIGHUP ("hang-up") signal to the shell, and the shell passes it on to its jobs. The default reaction to SIGHUP is to exit, so your hour-long import stops wherever it was.

What a Terminal Multiplexer Does

GNU Screen is a terminal multiplexer. It runs a background server process that owns its own pseudo-terminals, and your shells and jobs run inside those instead of inside the SSH session's terminal. What you see is just a client connected to that server. Disconnect, and only the client disappears; the server, and everything running in it, carries on. This gives you three things:

• Persistence — detach from a session and reattach later, even from a different computer (Chapter 2).
• Multiplexing — many shells, called windows, inside one session, and split views of several at once (Chapter 4).
• Sharing and logging — two people can attach to the same session, and a window's output can be logged to a file (Chapter 9).

A Short History, and Screen vs. tmux

Screen was designed by Oliver Laumann and Carsten Bormann at the Technical University of Berlin and first published in 1987. It is now part of the GNU Project and still actively maintained, with a 5.x release line. Its best-known alternative, tmux, first appeared in 2007 and has a similar feature set.

The ideas are the same in both — sessions, detaching, windows, panes — so what you learn here carries over. This course uses screen because it is the one you are most likely to find already installed on an unfamiliar server.

Installing and Checking Screen

What About Job Control?

Screen is one answer to the "my job died" problem, but not the only one. The shell itself has job control: running commands in the background with &, pausing them with Ctrl-Z, listing them with jobs, and protecting them from SIGHUP with nohup and disown. Chapters 5–7 cover these, and Chapter 8 combines both — including the common situation of wishing you had started a job inside screen after it is already running.

Hands-On Exercises