====== Screen ====== ===== Introduction ===== [[http://savannah.gnu.org/projects/screen/|Screen]] is a “terminal multiplexer”, i.e., it will let you have multiple virtual terminals in a single window. You will also be able to 1) “detach” a screen session, and then detach it in a later moment; 2) share screen sessions. In this tutorial ''$'' indicates a terminal prompt (just for visual convenience). A Screen command is usually of the form **CTRL-a KEY**, i.e., you'll have to press the **CTRL** key along with “a”, followed by a generic KEY. (Screen's default escape key is **CTRL-a**.) You'll need to be a [[http://sdf.org/?join#meta|MetaARPA]] member in order to use Screen on SDF If you want a “screencast” tutorial, type: ''$ ttyplay /ftp/pub/users/jecxjo/howto/screen'' on a terminal connected to SDF. Alternatively, you can watch the screencast online at [[http://playterm.org/r/brief-screen-tutorial-1307563176|PlayTerm.]] ===== Starting Screen, detaching and reattaching ===== You can run screen by typing: ''$ screen'' When you run it, a window with some copyright and some other info will show up, with “[Press Space or Return to end.]” at the bottom. OK, as it says, you can now press “Space” or “Enter”. You will then see your shell prompt and nothing else. That's fine: you can now run your programs as usual, the difference is that they will run under a Screen session. You can detach it by typing **CTRL-a d**. Screen will keep your session running. Now if you logout, then log back in, and type: ''$ screen -r'' you'll see your terminal as you left it when you detached Screen! If you lost connection, or simply forgot to detach Screen, and want to resume your session by typing: ''$ screen -d -r'' The “-d” flag will detach Screen, while “-r” will reattach. There are various types of “detach” and flags. For instance, if you type the previous command and there was no screen session to be resumed, you'll get a “There is no screen to be detached.” message. In this case you'll probably need: ''$ screen -d -R'' which will first create a new session (or reattach an existing one). ===== Multiple windows ===== You can create various windows under your Screen session, each one running their own program. When you are in a Screen session, typing: ''$ screen'' will create a new window, leaving the previous window untouched, though not visible. Instead of typing “screen”, you can use the shortcut **CTRL-a c**. You can create as many windows as you want. You can swith between windows with **CTRL-a n** (next window) and **CTRL-a p** (previous window). It's also possible to select windows by number. Typing **CTRL-a 1** will open window number 1, **CTRL-a 2**, to window 2, and so on. How do you know what's a window number? Well, you can type **CTRL-a “** (yeah, that's a double quote). In that case a menu with a windows list will open. You can select a number and press enter to go to that screen. An easier way is to have a “hard status line” that shows you each window number. For doing this in a permanent manner, you'll have to create a “.screenrc” (Screen's customization file) in your $HOME directory and write in it the following lines: hardstatus alwayslastline hardstatus string "%{WK}%-LW %{Y.}%n%f* %t%{-}%+LW" (We will see what that crazyness means later.) For your .screenrc file to be read by Screen, you'll have to start a new session. BTW, a Screen session ends when the last window is closed. ===== Multiple sessions ===== It's possible to have various Screen sessions running, each one with their own set of windows open. If you started Screen by just typing “screen”, a new session is created. If you detach and type “screen” again, a new session will start. If you now type: ''$ screen -ls'' it will give you a list of Screen sessions like this: 3340.pts-6.HOST (Detached) 4522.pts-8.HOST (Attached) 2 Sockets in /tmp/screens/S-YOU where HOST is the hostname and YOU is your username. You can reattach a screen session by name. Say for instance that you want to resume the “4522.pts-8.HOST” session. In this case you'll have to type: ''$ screen -r 4522.pts-8.HOST'' Obviously “4522.pts-8.HOST” is not a very userfriendly session name. You can give a meaningful name by starting screen as: ''$ screen -S mysession'' where “mysession” is the name you want to give to your session (e.g., “work”). ===== Splitting windows ===== It could be useful to split a window so that you can have two programs running on the viewport. In this case, type **CTRL-a S** and you'll see that the window will be split in two regions, with the one at the bottom blank. You can swith to it with **CTRL-a TAB** (TAB is the tab key) and then select a window by typing its number, or “p” (previous) or “n” (next), e.g., **CTRL-a 2**. You can always swith through regions with **CTRL-a TAB**. It's possible to split the window in more then two regions, each time by typing **CTRL-a S** If you want to split a window vertically, you'll have to either use a //patch(([[http://fungi.yuggoth.org/vsp4s/|Vertical Split for GNU Screen]] – provides a vertical split feature for current releases of GNU Screen))// or use a recent version of Screen. ===== Sharing screen sessions ===== The scenario is... HOSTUSER and SOMEUSER are both MetaARPA members logged into sverige. HOSTUSER wants to give SOMEUSER read only access to screen session of HOSTUSER. HOSTUSER starts a screen session as normal. Only three commands are needed for basic read only access to the current active window. HOSTUSER types the following commands beginning with **CTRL-a** : each time ( //control and 'a' followed by a colon //) **CTRL-a:multiuser on** **CTRL-a:password none** **CTRL-a:aclchg SOMEUSER -w “#”** SOMEUSER should exit screen first, then in the shell, type: ''$ screen -r HOSTUSER/'' SOMEUSER now has read only access to the active window in HOSTUSER's screen session at the time of attaching. HOSTUSER can verify SOMEUSER is attached by typing: **CTRL-a*** HOSTUSER can disconnect SOMEUSER by typing: **CTRL-a:acldel HOSTUSER** HOSTUSER can verify SOMEUSER is no longer attached by typing: **CTRL-a*** HOSTUSER can allow SOMEUSER access to additional commands. The following commands allow SOMEUSER to cycle backwards and forward through the windows and detach. HOSTUSER types: **CTRL-a:aclchg SOMEUSER +x next** **CTRL-a:aclchg SOMEUSER +x prev** **CTRL-a:aclchg SOMEUSER +x detach** //**Some notes:**// * Screen version that was used “Screen version 4.00.03 (FAU) 23-Oct-06” * At the time of writing ( Tue Apr 17 23:01:50 IST 2012 ) all attempts at achieving the above read only functionality by editing .screenrc failed, usually resulting in SOMEUSER having FULL access to HOSTUSER's screen and shell account. * HOSTUSER might notice that screen resizes to fit the smaller of the two terminals. * After SOMEUSER is detachs HOSTUSER might try **CTRL-aF** to resize the screen windows to fit the terminal. * For convenience and to reduce typing in screen HOSTUSER might want to have “multiuser on” and “password none” in .screenrc. HOSTUSER's screen is still private until a user is permitted to attach with aclchg. * HOSTUSER will hear/see lots of bells if SOMEUSER tries to type into the screen session. * HOSTUSER can check what window is being view by SOMEUSER by looking at the output of **CTRL-a*** * HOSTUSER might notice SOMEUSER appears to have rwx access in the output of **CTRL-a*** ...this is a bug or an undocumented feature. * SOMEUSER can remain attached even if HOSTUSER detachs. ===== .screenrc ===== As we previously saw, it is possible to customize Screen by writing settings in a file named “.screenrc” in your $HOME directory. We will now some useful Screen settings. === Avoiding the startup message === A full screen message with copyright and other stuff is usually displayed at startup. This can get annoying. In order to avoid it, you can put in your .screenrc the following: startup_message off === Keybindings === It's possible to bind keys so that when you type **CTRL-a KEY** Screen will open a new window launching some program. For instance, if you write in your .screenrc file bind m screen -t mail 1 mutt each time you'll type **CTRL-a m** a new window (named “mail”)with mutt will be created. Be careful: when you bind a key, you'll overwrite Screen's defaults (e.g., we just lost the “lastmsg” key). ===== Cheat sheet ===== TODO ===== Resources ===== * http://aperiodic.net/screen/start * http://www.bangmoney.org/presentations/screen.html * SDF has some [[http://sdf.org/?tutorials/dotfiles|annotated dotfiles]]. **Notes** ---- $Id: screen.html,v 1.8 2012/05/08 14:45:50 dickey Exp $ [[http://sdf.org/?tutorials/screen|Screen]] - traditional link (using [[wp>Revision_Control_System|RCS]])