Lost your shell
connection? Need multiple shell sessions?
You are logged into
your remote server via SSH and
happily plucking along at your keyboard and then it happens. Suddenly, the
characters stop moving and then you get the dreaded “Connection Closed”
message. You have just lost your session. You were halfway through some task
and now you have to start over. Ugh. Well you can prevent this from happening
by using screen. The Linux screen tool can not only save you from disconnection
disasters, but it also can increase your productivity by using multiple windows
within one SSHsession. I use this
tool all of the time in our server
management work.
Linux Screen for
Session Management!
Linux Screen Can Save
you from that Lost Connection
What is Screen for
Linux?
As the man page
states, “Screen is a full-screen window manager that multiplexes a physical
terminal between several processes (typically interactive shells).” This can be
a life saver when working on your dedicated server. Screen has a several great
features for helping you administer your server more productively and safely. I
am going to discuss the three features (multiple windows, logging, sessions)
that I use the most. Once you mastered screen, you may want to check out our 12 Point Server Maintenance Checklistand put your new found
skills to use.
Installing Screen on
Linux
Chances are that you
already have screen on your system. On most Red Hat distributions you can find
it in /usr/bin/screen. To see if screen is in your path, you can use the which
command:
[admin@ensim admin]$
which screen
If you do not have
screen, then you can install it easily from an RPM or the package file for your system. On Cobalt
Raq servers, you can safely use the RedHat RPMS appropriate for your system.
Screen RPMs: rpmfind
Screen Web site: GNU Screen
As you probably
already have screen or can use an RPM, I am not going to cover the building of screen from source.
Lets get on to how to use screen.
Using Screen
Screen is started from
the command line just like any other command:
[admin@gigan admin]$
screen
You may or may not get
a text message about screen. If you do not, then you probably think nothing has
happened, but it has. You are now inside of a window within screen. This
functions just like a normal shell except for a few special characters. Screen
uses the command “Ctrl-A” as a signal to send commands to screen instead of the
shell. To get help, just use “Ctrl-A” then “?”. You should now have the screen
help page.
Screen
key bindings, page 1 of 2.
Command key: ^A
Literal ^A: a
break ^B b
lockscreen ^X x reset Z
clear C
log H screen ^C c
colon :
login L select " '
copy ^[ [ meta a silence _
detach ^D d
monitor M split S
digraph ^V
next ^@ ^N sp n suspend ^Z z
displays *
number N time ^T t
fit F only Q title A
flow ^F f other ^A vbell ^G
focus ^I
pow_break B version v
help ? pow_detach D width W
history prev ^P p ^? windows
^W w
info i readbuf <
wrap ^R r
kill K redisplay ^L l
writebuf >
lastmsg ^M m
remove X xoff ^S s
license ,
removebuf = xon ^Q q
[Press Space for next page;
Return to end.]
Key bindings are the
commands the screen accepts after you hit “Ctrl-A”. You can reconfigure these
keys to your liking using a .screenrc file, but I just use the defaults. The
power of screen will become obvious, especially if you need to bounce around to
different file system locations and leave processes running. For example, when
I go in to clean out wasted
disk space, I can remove files in one screen while hunting for other
files in another.
Multiple Windows
Screen, like many
windows managers, can support multiple windows. This is very useful for doing
many tasks at the same time without opening new sessions. As a systems manager,
I often have four or five SSH sessions
going at the same time. In each of the shell, I may be running two or three
tasks. Without screen, that would require 15 SSH sessions, logins, windows, etc. With screen, each system gets
its own single session and I use screen to manage different tasks on that
system.
Another trick I use is
when I am editing configuration files. For example, if I want to harden SSH, I can cat out the configuration file in one
window and then edit it in another.
To open a new window,
you just use “Ctrl-A” “c”. This will create a new window for you with your
default prompt. For example, I can be running top and then open a new window to
do other things. Top stays running! It is still there. To try this for
yourself, start up screen and then run top. (Note: I have truncated some
screens to save space.)
Start top
Mem:
506028K av, 500596K used, 5432K free,
0K shrd,
11752K buff
Swap: 1020116K av, 53320K used,
966796K free
393660K cached
PID USER PRI
NI SIZE RSS SHARE STAT %CPU %ME
6538 root 25
0 1892 1892 596 R
49.1 0.3
6614 root 16
0 1544 1544 668
S 28.3
0.3
7198 admin 15
0 1108 1104 828 R
5.6 0.2
Now open a new window
with “Ctrl-A” “c”
[admin@ensim admin]$
To get back to top,
use “Ctrl-A “n”
Mem:
506028K av, 500588K used, 5440K free,
0K shrd,
11960K buff
Swap: 1020116K av, 53320K used,
966796K free
392220K cached
PID USER PRI
NI SIZE RSS SHARE STAT %CPU %ME
6538 root 25
0 1892 1892 596 R
48.3 0.3
6614 root 15
0 1544 1544 668 S
30.7 0.3
You can create several
windows and toggle through them with “Ctrl-A” “n” for the next window or
“Ctrl-A” “p” for the previous window. Each process will keep running while your
work elsewhere.
Leaving Screen
There are two ways to
get out of screen. The first is just like logging out of a shell. You kill the
window with “Ctrl-A” “K” or “exit” will work on some systems. This will kill
the current windows. If you have other windows, you will drop into one of
those. If this is the last window, then you will exit screen.
The second way to
leave screen is to detach from a windows. This method leaves the process
running and simple closes the window. If you have really long processes, you
need to close your SSH program,
you can detach from the window using “Ctrl-A” “d”. This will drop you into your
shell. All screen windows are still there and you can re-attach to them later.
This is great when you are using rsync for server migration.
Attaching to Sessions
So you are using
screen now and compiling that program. It is taking forever and suddenly your
connection drops. Don’t worry screen will keep the compilation going. Login to
your system and use the screen listing tool to see what sessions are running:
[root@gigan root]#
screen -ls
There are screens on:
31619.ttyp2.gigan (Detached)
4731.ttyp2.gigan (Detached)
2 Sockets in
/tmp/screens/S-root.
Here you see I have
two different screen sessions. To re-attach to a session, use the re-attach
command:
[root@gigan root]#screen
-r 31619.ttyp2.gigan
Just use screen with
the -r flag and the session name. You are now re-attached to the screen. A nice
thing about this, is you can re-attach from anywhere. If you are at work or a
clients office, you can use screen to start a job and then logout. When you get
back to your office or home, you can login and get back to work.
Screen Logging
As a consultant, I
find it important to keep track of what I do to someone’s server. Fortunately,
screen makes this easy. Using “Ctrl-A” “H”, creates a running log of the
session. Screen will keep appending data to the file through multiple sessions.
Using the log function is very useful for capturing what you have done,
especially if you are making a lot of changes. If something goes awry, you can
look back through your logs.
Linux Screen Tips
Just wanted to mention
to other cool tricks you can do with screen. Screen can monitor a window for
activity or lack thereof. This is great if you are downloading large files,
compiling, or watching for output. If you are downloading something or
compiling, you can watch for silence. To start the monitor, go to the screen
you want to monitor and use “Ctrl-A” “M” to look for activity or “Ctrl-A” “_”
to monitor for silence. Then open or switch to a new window. When the monitor
detects activity or silence, you will get an alert at the bottom with the
window number. To quickly go to that window, use “Ctrl-A” “ (thats a quote
mark, ctrl-a then a “). After you do this, just type in the number of the
window and enter. To stop monitoring, go to that window and undo the monitor
with the same command. For example, to stop monitoring for activity you would
use “Ctrl-A” “M” again.
Reference
Screen was covered
recently in Linux Magazine by Adam Lazur (Jan 2003, Issue 105). Much of his
information was adapted for this rackTIP. Other information was collected from
the man pages.
