How I use screen for server administration.

Wim Peeters

http://www.lubby.org

Abstract:

Sceen is what kde is for X. You can call it a console screen window manager. It allows you to run shell processes in a single terminal. Whenever you want to administrate a server system without X, sceen gives you more flexibility. Just try it out. It is not easy but once you get it, you do not want to miss this functionality anymore.

1 Getting started with sceen

1.1 Installation on Debian/Ubuntu/Kubuntu

The installation on debian based systems - these are all systems using deb packages - is easy. The installation is done by the following command:

apt-get install screen

When you start screen by typing "screen" on the command line, it seems as if you just get your bash again. Before really working with it you should configure it a bit.

1.2 Configuration

My configuration, which is a bit more complex, looks like the following:

# $HOME/.screenrc
# Wim Peeters <www.lubby.org>

# Defines the time a message is displayed 
# if screen is not disturbed by other activity.
msgwait 2

# Print the version and the compile date in the status line.
version

# Same as the login command except that the 
# default setting for new windows is  changed.
deflogin off

# Same as the scrollback command except that 
# the default setting for new windows is changed.
defscrollback 500

# If set to "on", screen will append to the "hardcopy.n" 
# files created by the command "C-a h", otherwise these 
# files are overwritten each time.
hardcopy_append on

# This command configures the use and emulation of the 
# terminal's hardstatus line
hardstatus on

# When you  are  familiar  with  the  game "nethack",  
# you  may enjoy the nethack-style messages which will 
# often blur the facts a little, but are much funnier to read. 
nethack on

# Select whether you want to see the copyright notice during 
# startup
startup_message off

# Sets the visual bell setting for this window.
vbell off

# If you prepend the word "always" to the type (e.g., 
# "alwayslastline"), screen will use the type even if the 
# terminal supports a hardstatus.

hardstatus alwayslastline "%{= dR} %-Lw%50>%{= dG}%n%f* %{= dW}%t%{= dR}%+Lw%<"

#
# Change the way screen does highlighting for text marking 
# and  printing  messages.
#
# explanation about color codes:
#
# sorendition <attribute> <color>
# sorendition ab cd  //  a,c=bg  b,d=fg
#
#  ab             c&d       c&d
#  00 normal       0 black   4 blue
#  1- invert       1 red     5 magenta
#  25 bright       2 green   6 cyan
#  3- 1+2          3 brown   7 silver
#
# * attributes are cumulative, so a=3 is invert+bright
# * colors use the same codes for bg & fg colors,
#   so cd=22 all you get is green, text unreadable
#
# examples:
#  00 02 - black bg (0), green fg (2), no attributes (00)
#  20 30 - brown bg (3), black fg (0), bright bg (2) -> turns yellow
#  35 74 - silver bg (7), blue fg (4), invert, bright bg (3)
#  bright fg (5)
#
#
# Coloring:
#sorendition 10 49 # Blue on White
#sorendition 05 46 # Cyan on Blue
#sorendition 05 42 # Green on Blue
#sorendition  05 70
sorendition  25 70

# The message specified here is output 
# whenever a `Power detach' was performed.
pow_detach_msg	"BYE"

# Keybindings
bind ~ activity	"activity in %n (%t) [%w:%s]~"
bind * activity	"activity in %n (%t) [%w:%s]"
bind m screen -t 'mutt' mutt
bind v screen -t 'vim' vim
bind w screen -t 'w3m' env TERM=xterm w3m -B
bind y screen -t 'mc' env PAGER=less.sh mc -c
bind z screen -t 'ROOT' su
bind L screen -t 'LOGFILE' sudo /usr/bin/tail -f /var/log/messages
#
bindkey -k F1 prev
#
bindkey -k F2 next

You can copy and paste this in a ".screenrc" file and safe it in your home directory. The program will find it automatically. You are now ready for the next step.

wpeeters@kubuntu:~$ ls -la .screenrc
-rw-r--r-- 1 wpeeters wpeeters 2867 2007-11-10 15:26 .screenrc
wpeeters@kubuntu:~$

1.3 First steps with screen

If we start screen with this configuration file, it looks a bit nicer. After the start we see one session "*0 bash" on the bottom. Screen is made to arrange more than one sessions. (with just one session, screen would be useless!). Screen uses "Ctrl-A" as a escape key for entering commands. Every command starts with Ctrl-A followed by other key sequences. To make a new session we type "Ctrl-A" followed by "c". To do this, you type in "Ctrl-A", release all keys and then type the character "c".

The following table shows a few (not all of them) of the interesting key bindings which I use the most:

command	Function	Explication
C-a 0	select 0	Selects screen 0
C-a 1	select 1	Selects screen 1
C-a n	select n	Selects screen n
C-a 9	select 9	Selects screen 9
C-a A	title	Allow the user to enter a name
		for the current window.
C-a c	screen	Create a new window with a shell
		and switch to that window.
C-a C	clear	Clear the screen.
C-a d
C-a C-d	detach	Detach screen from this terminal.
C-a D D	pow detach	Detach and logout.
C-a space
C-a n
C-a C-n	next	Switch to the next window.
C-a N	number	Show the number (and title)
		of the current window.
C-a backspace
C-a h	prev
C-a p
C-a C-p		Switch to the previous window
		(opposite of C-a n).
C-a ?	help	Show key bindings.
C-a C-	quit	Kill all windows and terminate screen.
C-a :	colon	Enter command line mode.
C-a [
C-a C-[
C-a esc	copy	Enter copy/scrollback mode.
C-a ]	paste	Write the contents of the paste buffer to the
		stdin queue of the current window.
C-a R	attach	Re-attach to a running session and if none
		exists create one
C-a R	attach	Re-attach to a running session and if none
		tell me without creating a session.
C-a x	multi user	Attach to a running session for
		for looking at it in multi screen mode.
		You can use it for attaching to!

1.4 A little exercise

In this little exercise you start screen from the commandline. Here you see a startup session.

This might be boring. Therefore I took the following screenshot which is a bit more interesting.

Here you see 5 sessions. Session 1 is active at the moment. The Session are "0 bash 1* bash 2- mutt 3 ROOT 4 vim". These sessions can run on different systems. In the case of rootserver administration, the screen session normally runs on the rootserver.

Please try out the following. Start Screen, it should look like the startup session above and then type Ctrl-A c to create a new session. Do this until you have 5 sessions. on the last session you type ls -la followed by <enter>. You screen should look like the following.

Cycle through your windows with Ctrl-A n for next and Ctrl-A p for previous. Cycle till you get your session with the ls -la text on it. Jump immediately to session 2 with Ctrl-A 2. If you think you could do this with diffent session form the local system, I agree. Now you should just close the terminal window (yes, without closing the sessions first) and reopen a new terminal. You type the command "screen -list". You should see a detached session. If you compiled some sources in one of the sessions, the compilation goes on. Your monitoring session will not sleep either. You can detach the hard way (which you did) or properly with the detach command Ctrl-A d. You can Re-attach with the "screen -R" command.

This exercise already explained the basic usage of screen but we have still more to offer.

1.5 Attach and detach

One very interesting feature of screen is the possibility to detach from a session and to attach it again later. The process is not effected in that time and stays running. This makes is ideal for long running jobs. To detach a screen we use C-a - d. To see if you still have a session running you can use screen -ls. The reattachment is done with screen -R.

1.6 Split screen

I never use this feature but I know people who always split their screen sessions. You split your session with Ctrl-A S (capital S!) You have to use Ctrl-A <Tab> to enter the split window. You can even resize those regions. An example can be seen in the following screenshot.

1.7 Security

Now you are telling me that you use screen for administration of rootservers! What happens when somebody can find your session unattended? Well in the case I leave my system unattended, I use Ctrl-x to lock my screen sessions. Only my password can unlock it again.

1.8 Copy and paste

I just use my mouse to select some text and use both left and right buttons to insert into my commands. You can switch between sessions and paste and copy in other sessions.

1.9 Troubleshooting

When your screen seems to be frozen you probably touched the Ctrl-a s (little s here) To unfreeze the window you can use Ctrl-a q.
A remote session is sometimes not properly detached. (e.g. your commection died) You still see this session as attached when listing them. To attach to this session you can use its name or tell Screen to detach it first using "screen -DR". (detach and re-attach) This is easier then typing "screen -r 23383.pts-5.kubuntu".
If you cannot attach in multi user mode, it might be disabled by your OS. (Gentoo used to disable this by default)

1.10 More information

For more information you can look at the man page of screen. We did not cover the configuration file, the monitoring features, Keyboard shortcuts, and more features. I have to admit that with the subset of functionality described in this article, I am doing my job for years.