====== Screen ======

Contents

  - [[#intro|Introduction]]
  - [[#start|Starting Screen, detaching and reattaching]]
  - [[#multiwin|Multiple windows]]
  - [[#multisession|Multiple sessions]]
  - [[#split|Splitting windows]]
  - [[#share|Sharing screen sessions]]
  - [[#screenrc|.screenrc]]
  - [[#cheatsheet|Cheat sheet]]
  - [[#resources|Resources]]

===== 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 rettach 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[1] 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 willl 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 =====

[1] http://fungi.yuggoth.org/vsp4s/

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$Id: screen.html,v 1.8 2012/05/08 14:45:50 dickey Exp $