screen []

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====== 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:

<file config .screenrc>

hardstatus alwayslastline
hardstatus string "%{WK}%-LW %{Y.}%n%f* %t%{-}%+LW"

</file>

(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:

<code>
        3340.pts-6.HOST        (Detached)
        4522.pts-8.HOST        (Attached)
2 Sockets in /tmp/screens/S-YOU
</code>

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:

<file config .screenrc>

startup_message off

</file>

=== 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

<file config .screenrc>

bind m screen -t mail 1 mutt

</file>

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]])