Figure One: Processes and ptys with one screen window open
How can you start a job from one system and finish it on another? For example:
You start an IRC (chat) session in a terminal window on one computer, but you want to continue it from another system — without having to /leave (and lose your place) and try to /join again later.
Or, you’re at home and you connect to your office system and start work. But you have to leave home for the office before you’ve finished, and you need to pick up where you left off once you’re at the office. Or, perhaps your modem hangs up, the network connection goes down, or the firewall times out your connection and you need to restart with no pain.
The handy screen utility lets you resume a terminal session without missing a keystroke. (screen doesn’t work with GUI applications.) Read on for some surprising ways to use screen. Its clever use of processes, ttys, the environment, and more, can also give users and programmers insight into how “the guts” of Linux and Unix work.
screen Then and Now
In the mid ’80s, when Oliver Laumann released screen version 1, most users had single terminals with no windows — and no Internet. These days, screen still works on dedicated terminals, but it’s also very useful in terminal windows (under the X Window System) and over network connections.
What’s made screen unique, then and now, is that it maintains the state of one or more terminal window sessions on a particular host. (These “terminal windows” are character terminal interfaces, like the old VT100 — not GUI windows.) Each session can have many terminal windows — or just one (and we’ll see why you’d want this). You can detach from a session and then, later, reattach to that session, from the same system or from another one. This ability makes screen especially useful when it’s used on a server with network connections.
screen, Inside and Out
How does screen work? Let’s start with a simple example. (We’ll use screen version 3.9.13, setuid root, but all of the examples shown should also work non-setuid and with other versions.) In a terminal — or a terminal window, like xterm — type the command name screen followed by another command you want to run under screen‘s control.
For example, top, the system monitoring program, is a good choice because it keeps running and updating its display. (If you haven’t used top before, start it now, watch it for a little while, then type q to quit.) Use the “delay” option — which is either -d 60 or -s60, depending on your version — to make top refresh its display every sixty seconds. If your version of top uses option -s to set “secure” mode (instead of setting the delay time), use it too. Here’s what you might type:
$ screen top -d 60 -s
At this point, you’ve got the processes and ptys shown in Figure One.
When you’re using a terminal window (like the xterm here — as opposed to an old-fashioned “real” terminal), xterm uses a pty to manage the display. screen also creates a pty for each “window” that it manages. Right now, you’re using two ptys: pts/5, which is the xterm display, and pts/6, which is the top display.
(A tty is the original Unix terminal interface device. A pty, or “pseudo-tty”, is a virtual terminal. A pty has a screen buffer that can be manipulated like a terminal, and a number like pts/5 that identifies it. You can see any terminal’s number by running the Linux command tty from a shell prompt. The pty is another important Linux feature that screen uses.)
You can see the processes and ptys and their associations with one another with the ps command. Its –forest option arranges processes in a tree, as Figure Two shows.
Notice that the backend SCREEN process doesn’t have a tty. (This is also true of processes like daemons, cron and at jobs, and most other Linux processes that don’t need a user interface.) This backend process is the parent of all the other “windows” (processes) that screen manages. If you detach your terminal (here, pts/5) and log out, then, once you reattach, SCREEN can use pts/6 to refresh your new terminal with exactly what was there when you left. (We’ll soon see how to detach and re-attach.)
screen also watches the keystrokes you type. It passes every keystroke through to the child process running in the current window — until it sees the special escape character, which is CTRL-A (which we’ll write, Emacs-style, as C-a, and which you make by holding down the CTRL key and pressing the A key). After C-a, screen looks for exactly one more character, which is a screen command.
For instance, C-a c creates a new window process. (If you’re following along at a terminal, please do that now.) The current window (the top load-average monitor) isn’t shown anymore, but screen continues to “remember” its state, even if it changes while it’s not being displayed. In the new window, you should see a shell prompt. There you can start another program — for instance, a text editor like vi.
Running ps again shows the new processes. Now, both top-level window processes are children of SCREEN, and vi is a child of the bash shell. See Figure Three.
C-a C-a switches to the previously-displayed window — in this case, your top session. As before, vi will keep running; you can switch to it anytime to do more editing with C-a C-a.
If you want to be notified whenever the text changes in a “background” window, use C-a M to set monitoring of that window. Try that while you display the window running top, then change to the other window. Every minute, you should see Activity in window 1 in the titlebar or message area of window 0. This is handy for monitoring a system log (with tail -f, say) while you’re doing something else.
Figure Two: psoutput while onescreen window is open
If you quit one of the processes under screen‘s control (for instance, by typing q to top or typing exit at a shell prompt), that window will close. When its last window closes, screen, and its backend process, SCREEN, terminate.
If some screen windows are open, though, you can also leave screen temporarily. Here are two ways:
You can suspend screen by typing C-a z or C-a C-z from any window. You can restart it (before you log out!) by typing the shell’s fg command. This uses Linux job control.
You can detach screen from your current terminal. The SCREEN backend process and the ptys it manages keep running. You can log out and log in again later — even days or weeks later (as long as the system isn’t rebooted) — and reattach the SCREEN backend process to your new terminal. To detach screen from your terminal, type C-a d. You should see the message [detached]. Now you’ll be back at a prompt from the shell where you first started (or attached) screen.
For example, if you’re monitoring a long-running industrial process from a tty on your office workstation, and you’re afraid that it won’t finish before you have to go home, start that processes under screen control. When you leave, use C-a d to detach. Now you can turn off your display (but not your CPU!), go home, connect to your workstation by (for instance) ssh, and reattach. It’s that easy!
To reattach, type the command screen -r at a shell prompt. Your window should appear just as it was when you left it — unless its contents have changed in the meantime, of course. You can see previous lines of the display by using screen‘s scrollback history.
A single Linux system can have thousands of processes — including lots of SCREEN backend processes. So how can your screen find the right one to attach? Depending on your system’s configuration, there’ll be a subdirectory somewhere — .screen under your home directory, for instance — with a FIFO (named pipe) connected to the backend process. (Your version of screen may use a socket instead of a FIFO.) You can list running backends with the command screen -list.
Figure Four: Processes and ptys after logging out, in, and reattaching
Figure Four shows the complex setup after you’ve detached, logged out, logged in, and reattached. The backend SCREEN process and its windows are still running, with their same ptys and processes. When you type screen from your new shell prompt (which happens to be in pts/10 this time, instead of pts/5), it finds the backend process and repaints your terminal from the active window’s pty (here, that’s pts/6 or pts/7).
More Ways to Use screen
screen is useful when you’re planning to detach and reattach. But it’s also useful for a host of other things, such as protecting against network failures and capturing screen output.
screen can keep a process alive if your network connection to it is broken. For instance, if you started Emacs on your server from under screen, and your network connection is broken, you can simply re-login to the server and reattach.
screen can also copy and paste, paste copied text to a file, and write the scrollback history to a file — all without using a mouse! screen also lets you select text more precisely than a mouse typically does.
If you’re running a screen session on one system (at home, for example) and you leave without detaching, you may still be able to attach from another system with screen -x, which sets the multi-display mode.
There’s much more to know about screen. Check its extensive manual page… and prepare to be impressed!
Thanks to Kimmo Suominen of Global Wire for some great screen tips. In July, we’ll dig into email transport and processing tools.
Jerry Peek is a freelance writer and instructor who has used Unix and Linux for over 20 years. He’s happy to hear from readers at firstname.lastname@example.org.
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