Praji’s Weblog

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. Screen can not only save you from disconnection disasters, but it also can increase your productivity by using multiple windows within one SSH session.

Use Screen for Session Management!

What is Screen?

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 but be sure to see the man page for full details.

Installing Screen

Chances are that you already have screen on your system. On most RedHat 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.

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.

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.

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.

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

This guide shows how you can create a Live-CD from your Ubuntu Gutsy Gibbon or Linux Mint 4.0 system with a tool called remastersys.

Remastersys is available in the Linux Mint romeo repository. You can customize your Ubuntu/Linux Mint system and then let remastersys create an iso image of it which you can then burn onto a CD/DVD.

1 Installing Remastersys

Open a terminal and become root:

sudo su

Then add the Linux Mint romeo repository to /etc/apt/sources.list and update the package database:

echo “deb http://www.linuxmint.com/repository romeo/” >>/etc/apt/sources.list
apt-get update

Afterwards you can install remastersys like this:

apt-get install remastersys

Then leave the root shell so that you are logged in as your normal user again:

exit

2 Remastersys Usage

In order to learn how you can use remastersys, run

sudo remastersys

It will then print all available options:

falko@falko-desktop:~$ sudo remastersys
[sudo] password for falko:

Usage of remastersys is as follows:

sudo remastersys backup|clean|dist [cdfs|iso] [filename.iso]

Examples:

sudo remastersys backup (to make a livecd/dvd backup of your system)

sudo remastersys backup custom.iso
(to make a livecd/dvd backup and call the iso custom.iso)

sudo remastersys clean (to clean up temporary files of remastersys)

sudo remastersys dist (to make a distributable livecd/dvd of your system)

sudo remastersys dist cdfs
(to make a distributable livecd/dvd filesystem only)

sudo remastersys dist iso custom.iso
(to make a distributable iso named custom.iso but only
if the cdfs is already present)

cdfs and iso options should only be used if you wish to modify something on the
cd before the iso is created. An example of this would be to modify the isolinux
portion of the livecd/dvd

falko@falko-desktop:~$

3 Creating An ISO Image Of Your Installation

To create an iso image of your installation, simply run

sudo remastersys dist

This will create an iso image called customdist.iso in the /home/remastersys directory. The dist option makes that your personal folder (e.g. /home/falko) will not be included in the iso image. You might have to insert your Ubuntu/Linux Mint installation CD during the process.

This is how the end of the process looks:

[...]
92.16% done, estimate finish Wed Nov 28 15:31:25 2007
93.39% done, estimate finish Wed Nov 28 15:31:25 2007
94.62% done, estimate finish Wed Nov 28 15:31:24 2007
95.85% done, estimate finish Wed Nov 28 15:31:24 2007
97.08% done, estimate finish Wed Nov 28 15:31:25 2007
98.31% done, estimate finish Wed Nov 28 15:31:25 2007
99.54% done, estimate finish Wed Nov 28 15:31:25 2007
Total translation table size: 2048
Total rockridge attributes bytes: 3950
Total directory bytes: 9094
Path table size(bytes): 54
Max brk space used 0
406890 extents written (794 MB)
/home/remastersys/customdist.iso is ready to be burned or tested in a virtual machine.

Check the size and if it is larger than 700MB you will need to burn it to a dvd

796M /home/remastersys/customdist.iso

It is recommended to run ’sudo remastersys clean’ once you have burned and tested the customdist.iso

falko@falko-desktop:~$

As I’ve just mentioned, the iso image has been created in /home/remastersys:

ls -l /home/remastersys/

falko@falko-desktop:~$ ls -l /home/remastersys/
total 814596
-rw-r–r– 1 root root 73 2007-11-28 15:08 control
-rw-r–r– 1 root root 833310720 2007-11-28 15:31 customdist.iso
drwxr-xr-x 9 root root 4096 2007-11-28 15:07 dummysys
dr-xr-xr-x 5 root root 4096 2007-10-19 02:08 ISOTMP
-rw-r–r– 1 root root 904 2007-11-28 15:06 varexc
falko@falko-desktop:~$

Now you can burn /home/remastersys/customdist.iso onto a CD or DVD (if the iso file is bigger than 700MB, you must use a DVD).

4 Cleaning Up

After you’ve burnt the iso image onto a CD/DVD, you can run

sudo remastersys clean

to remove all temporary file created during the iso generation as well as the /home/remastersys directory.