Speaking UNIX: Stayin' alive with Screen

Create and manage multiple shells on your console

The command line is a powerful tool, but it has a fatal weakness: If the shell perishes, so does your work. To keep your shell and your work alive—even across multiple sessions and dropped connections—use GNU Screen, a windowing system for your console.

Martin Streicher (martin.streicher@gmail.com), Software Developer, 自由职业者

Photo of Martin StreicherMartin Streicher is a freelance Ruby on Rails developer and the former Editor-in-Chief of Linux Magazine. Martin holds a Masters of Science degree in computer science from Purdue University and has programmed UNIX-like systems since 1986. He collects art and toys. You can reach Martin at martin.streicher@gmail.com.



10 February 2009

Also available in Chinese

Harry Potter may have his wand, Thor may have Mjöllnir, and Buckethead may have his axe, but all those gizmos pale in power to the QWERTY. With a few taps at the command line, you can launch a Web site, recruit legions to your cause, or vanquish a marauding thunder lizard.

Alas, even the mighty QWERTY has a fatal weakness: It succumbs easily to a cut connection. A noisy phone line, a lost cellular or wireless connection, or a network time-out spells certain death for a remote shell. If you've spent hours on a task, such as debugging an application, it can be frustrating and maddening to lose your work in an instant.

But don't smash Mjöllnir into your head. Instead, adopt GNU Screen. Screen creates and manages multiple shell windows within a console (say, a dumb terminal physically connected to the host), an xterm window, or a Secure Shell (SSH) login. You can switch from one shell window to another in a flash, and you can leave shells running perpetually and reconnect at any time. Effectively, Screen provides many virtualized consoles.

Figures 1 through 5 picture the features and operation of Screen. Looking at Figure 1, assume that you have used SSH to log in to a remote host. Initially, you have your original shell on your local host (say, your laptop or desktop) and a remote shell. As usual, you can use the remote shell to run commands on the remote host; output is encrypted and sent over the SSH connection to your local shell. (In the figures, blue highlights the shell output that is currently visible.) As lamented above, if your local shell or the remote shell or the connection between the two is terminated, the remote shell is terminated, taking your work to the big bit bucket in the sky.

Figure 1. A typical SSH connection
A typical SSH connection

Figure 2 depicts state after you launch Screen on the remote host. The Screen utility launches and in turn spawns a new shell window, A, in which you can run commands. The output of A is visible, as connoted by blue; the output comes from the shell, traverses through Screen, goes through the remote login, and over the SSH connection, arriving at your local login.

Figure 2. Screen manages shell windows
Screen manages shell windows

Screen does not have a presence per se; it's a proxy to select among the available and running windows it manages. Screen can only show the output of one window at a time. Think of Screen as a virtual keyboard-video-mouse (KVM) switch.

But you can also detach from Screen, as shown in Figure 3. The Screen proxy persists, as do all windows it controls, but the connection to Screen is temporarily severed, returning you to the prompt of your remote login shell.

Figure 3. You can detach from Screen and its windows persist
You can detach from Screen and its windows persist

Figure 4 shows a possible, eventual scenario. The connection to Screen has been reestablished, and two additional windows—B and C—have been created. Windows A and B continue to run a shell and any subordinate jobs, yet only the output of C is visible. Of course, you can switch among the windows to monitor the state of your varied jobs.

Figure 4. Screen can manage multiple windows
Screen can manage multiple windows

Finally, Figure 5 captures conditions if you detach from Screen and terminate your remote login. Screen and its windows persist. You can log in anew, reconnect to Screen (and by proxy to its windows), and carry on unperturbed.

Figure 5. You can detach and reattach to Screen at will
You can detach and reattach to Screen at will

If you regularly access a remote server to perform maintenance or develop software, GNU Screen is indispensable.

Building and installing Screen

The original version of Screen was released more than 10 years ago, so chances are your system has the utility—typically named /usr/bin/screen. However, if your system lacks Screen, it is easily installed from your distribution's package manager. For example, if you use a variant of Debian Linux®, such as Ubuntu, you can install Screen in one step with apt-get:

  $ sudo apt-get install screen

Alternatively, if you prefer to build from source, visit the GNU Screen project page (see Resources for a link), and download the latest code bundle. As of this writing, the most recent release of Screen is version 4.0.3, posted just months ago, in August 2008. To build and install from scratch, download and extract the code, change to the resulting source directory, and run the typical ./configure, make, and sudo make install sequence:

$ wget http://ftp.gnu.org/gnu/screen/screen-4.0.3.tar.gz
$ tar xzf tar xzf screen-4.0.3.tar.gz 
$ cd screen-4.0.3
$ ./configure
this is screen version 4.0.3
...
$ make
CPP="gcc -E " srcdir=. sh ./osdef.sh
...
$ sudo make install
...
You may also want to install ./etc/etcscreenrc in 
	/usr/etc/screenrc.
$ sudo cp ./etc/etcscreenrc /usr/etc/screenrc

Screen is now installed and ready to use. Type man screen to view the utility's man page.


Getting started with Screen

To use Screen, just launch it. When the license message appears, click Return; you should now see a new login shell prompt. (In the transcripts below, a nickname has been artificially added before each shell prompt to differentiate between the numerous shell instances and to draw parallels with Figures 1 through 5.)

Local $ ssh remote.example.com 
Last login: Sun Dec 21 17:23:16 2008 from local.example.com
Remote $ hostname
remote.example.com
Remote $ screen
A $ top

Your state now mirrors that of Figure 2. A Screen session is running (albeit unseen), and window A is active, running the top system monitor, which refreshes system performance information every few seconds.

To temporarily detach from the Screen session and all its windows, click Ctrl-a, then d (lowercase D for "detach"). Ctrl-a is the Screen command prefix, and d is the specific command to detach. You are now in the scenario shown in Figure 3. The Screen session and window A remain alive. To verify, run screen -list:

$ screen -list
There is a screen on:
21020.pts-2.remote  (Detached)
1 Socket in /tmp/screens/S-strike.

The screen -list command shows all available screen sessions. (As an aside, you can have multiple screen sessions running simultaneously, each with its own set of concurrent windows. This is shown in Figure 6. Each screen instance tracks its own current window. Some tips on the advanced use of Screen such as this are presented later.) The session numbered 21020 is detached and marked accordingly. Because only one screen session exists, you can reattach to it directly with screen -r. Oblivious to Screen's machinations, top continues uninterrupted.

Figure 6. Screen provides multiple sessions, each with multiple windows
Screen provides multiple sessions, each with multiple windows

To recreate Figure 4, reattach to the current session, then click Ctrl-a, then c (lowercase C for "create") to create a new window. Next, click Ctrl-a, then c again. You now have one Screen session managing three windows.

To view the available windows in a session in a menu-like interface, click Ctrl-a, then the double quotation mark (") key in the current window:

Num  Name           Flags

 0   bash           $
 1   bash           $
 2   bash           $

By default, the name of a window is taken from the first command it launches—commonly, a shell. Hence, there are three bash windows in the menu above. To navigate the window menu, use the arrow keys to move up and down the list; to choose a window, simply press Return.

To dispose of the current window, type exit at the window's shell prompt, or click the keyboard shortcut Ctrl-a, then k (lowercase K for "kill"). If you use the latter method, an alert appears at the bottom of the window to confirm that you want to kill the window. Click y (lowercase y for "yes") to acknowledge or n (lowercase n for "no") to reject the command. If you kill all the windows running under a Screen session, the originating screen command prints a message and is terminated:

Remote $ screen
... Create and manipulate windows ...
... Exit from all windows...
[screen is terminating]
Remote $ screen -list
No Sockets found in /tmp/uscreens/S-supergiantrobot.

If you're following along and have killed all the open windows, the screen -list command yields No sockets..., indicating that no screen sessions are available.


Smarter screens

So far, you've seen how to create multiple windows within a Screen session. Alone, this is enough to persist your command-line work and never lose work again.

Admittedly, however, managing seemingly identical, concurrent windows can become confusing; it would be superior if you could easily differentiate between one window and another without opening each one, especially if any amount of time passes between a detach and a reattach.

Indeed, Screen provides a number of options and tools to customize your work environment. You can name each window, and you can place a status bar at the bottom of each window to differentiate one from another.

To name a window, activate the window. Click Ctrl-a A (uppercase A, for "Annotate"), click Backspace as needed to elide any existing moniker, then type a meaningful name at the prompt:

Num  Name            Flags

 0   Window A       $
 1   Window B       $
 2   Window C       $

This is shown in Figure 7, where a window is tagged Window A. A window nickname need not be unique.

Figure 7. You can assign a descriptive name to each window
You can assign a descriptive name to each window

You can provide yourself additional visual cues to distinguish among windows with a status bar. Launch your favorite text editor, and create the file .screenrc in your home directory with the following lines:

hardstatus on
hardstatus alwayslastline
hardstatus string "%{.bW}%-w%{.rW}%n %t%{-}%+w %=%{..G} %H %{..Y} %m/%d %C%a "

With such a .screenrc configuration file in place, each new window displays a status line, including the window's name. Figure 8 shows a window with a status line.

Figure 8. Use the status line to help identify each window at a glance
Use the status line to help identify each window at a glance

Helpful Screen tips

Screen has too many features to list in a brief introduction, but fluent use requires just a modicum of know-how and a few helpful options. Here are some additional pointers:

  • Type screen without any arguments in any window to open a new window. Clicking Ctrl-a, then c and typing screen are synonymous, except that the latter provides command-line options to immediately configure the new session.
  • You can name a window when you create it with screen -t name. For instance, to create a new window and tag it debugger, go to a Screen-managed window, then type screen -t debugger. If you open the window menu, one of your options should be marked debugger.
  • If you've detached from a Screen session, you can reattach to a specific window with screen -p ID, where ID is a number or a name. Let's try it:
    Local $ ssh remote.example.com
    Remote $ screen -t ghost
    Ghost $ screen -t new
    New $ 
    ... Press Control-a d to detach...
    Remote $ screen -r -p ghost
    Ghost $
  • You can log the output of every window with screen -L. Each window has its own log file, typically named ~/screenlog.n, where n is the window number shown in the window menu. This is another fantastic feature to record complex steps—say, when a system is being reconfigured.
  • See the Screen documentation for a complete list of accelerator keys. Some of the most useful combinations are Ctrl-a, then 0 (the digit zero) through Ctrl-a, then 9 to switch immediately to a specific window; Ctrl-a, then C (uppercase C for "Clear") clears a window; Ctrl-a, then H toggles logging on and off; Ctrl-a, then Ctrl-a flip-flops between the current window and the previous window; and Ctrl-a, then Ctrl-\ (the backslash) kills all windows and terminates the current Screen session.

Advanced use of Screen

As mentioned earlier, you can create multiple, concurrent Screen sessions, where each session can manage a suite of windows. Each session has a unique identifier; use screen -list to catalog all available sessions. As with a window, you can name a session to refer to by its handle. Use screen -S label to assign a label to the new session.

Sharing is one of the best uses of a named Screen session. If permitted, you can connect to an existing session and collaborate with other users within any of the session's windows. You can even switch to another window within the session independently! If you're in the same window as another, anything typed or displayed is reflected to all partners. Let's try it:

  1. Choose a machine to work on, and log in.
  2. Type screen -S sharing -t one> to create a new Screen session named, aptly, sharing, and a new window named one.
  3. Create a second window with screen -t two.
  4. If you like, check your work with Ctrl-a, then d, screen -list, screen -r sharing, and Ctrl-a, then ".
  5. Emit some output in the window named one by typing echo in that window.
  6. On the same machine, open a second login window.
  7. Within that window, type screen -x -r sharing -p one.

    The -x option specifies multi-user mode; -p one attaches directly to the window named one. You should immediately see the same output as the other login session, as shown in Figure 9.

    Figure 9. A session can be shared
    A session can be shared

Using each login window, run UNIX® commands to produce output, click Screen keyboard shortcuts to switch among the windows in the shared session, and watch the results.


Magic Screen

To delve further into Screen, read about split-screen mode and learn how you can prevent access to individual windows with an old-fashioned lock.

Screen is a remarkable tool that you will quickly find invaluable in any work you perform on the command line. In fact, use it once, and you will wonder how you ever lived without it.

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