screen…it’s not just for nerds anymore.

So after hearing from people at work how great the “screen” command was (yeah welcome to gnuland boys and girls), I decided to do a short tutorial on screen. This way I can stop telling them to RTFM and instead tell them to RTFB (Blog). Anyways, What is “screen” first of all….From the pages of wikipedia:

GNU Screen is a free terminal multiplexer developed by the GNU Project. It allows a user to access multiple separate terminal sessions inside a single terminal window or remote terminal session. It is useful for dealing with multiple programs from the command line, and for separating programs from the shell that started the program. GNU Screen can be thought of as a text version of graphical window managers, or as a way of putting virtual terminals into any login session. It is a wrapper that allows multiple text programs to run at the same time, and provides features that allow the user to use the programs within a single interface productively.

Think of screen as a Virtual Machine (I know it’s not but bear with me). Once you run the command, the ‘virtual machine’ takes over and allows you to create multiple interactive command line sessions. In each of those sessions you can run commands that are either interactive (menu based) or serialized. Once you’re done you can disconnect the session — keeping in mind that the session is actually alive and running, including all the programs that were spawned inside that session — go to another computer and ‘restore’ the session with all the programs still running. By far one of the coolest things about screen is that it automatically allows you to nohup your commands, by just disconnecting the session and reconnecting to it later. So without any further due here is screen:

Obviously you need to run it, so first step is to type screen at the command line. When you do that you get a new shell window and the adventure starts. Remember that pretty much all screen commands start with Ctrl-a followed usually by a character (ie: you press Ctrl button and c together, let go, and follow it with the character).

So now you have a new shell, run a command (ie: pine, vi or something). Okay so now we can simulate you leaving your machine and detaching your session.

– To Detach : Ctrl-a d (this will detach the session but your command is still running inside that screens shell….you’ll see)
– To Reattach : screen -r (without the quotes. You should get the session back with whatever command you were running in it).

So now you’ve got the very basics of screen. Detaching allows you to run commands, leave them halfway, detach and go somewhere else and use Re-attach to restore the session.

Now, how about multiple sessions. Yeah you can do that too, one screen process with multiple sessions inside it.

– Use screen -r to reattach to your process (If you haven’t done so already). Note that your program is still running (say vi). If you now want to run lynx for example you can use the Ctrl-a c command to create another session (c for create). So now you have two sessions inside your “screen virtual machine”.
– Use Ctrl-a n and Ctrl-a p to flip between sessions (n for next and p for previous). You can also create more screens with Ctrl-a c. Lets create 2-3 more sessions.
– Use Ctrl-a followed by a number between 0-9 to switch between up to 10 recently created sessions.
– Now use Ctrl-a d to detach from the session, logoff (don’t reboot, that will kill the screen process) and log back in. Now reattach to the process using screen -r. Note that all your sessions are still there (you can check using Ctrl-a n and Ctrl-a p to cycle through the sessions).

One last thing before I take away the training wheels, to kill your screen process (and all sessions running inside it) use Ctrl-a Ctrl-\.

Okay, so here is a small list of the many screen options and commands:

– Ctrl-a “ : gives you a full screen list of all your sessions and you can scroll down to the one you want to switch to and press Enter (remember to get “ you have to use Shift-‘ and ESC gets you out of the list).
– Ctrl-a A : (that’s a shift-a) allows you to give a meaningful name to your session window.
– Ctrl-a k : allows you to kill your current session (not all sessions spawned inside a screen process, just the current session).
– Ctrl-a S : will split your current session screen in half. It is easy to confuse Ctrl-a S, which uses a capital ‘S’ with Ctrl-a s, which uses a lower case ‘s’. The upper case command causes screen to be vertically split (that is, with one region on top of the other), while the lower case command causes the parent terminal to freeze (Scroll Lock). To unfreeze the parent terminal, use the Ctrl-a q command.
– Ctrl-a : will jump between the regions in a split session. Keep in mind that the new region will have nothing in it until you designate another open session to pop in there using Ctrl-a p and/or Ctrl-a n which will cycle the next or previous session into the new split region.
– Ctrl-a X : (that’s a shift-x) will close a region (ie: split region goes back to full).
– Ctrl-a + : will enlarge the current region (and shrink the other).
– Ctrl-a – : will shrink the current region (and enlarge the other).
– Ctrl-a M : (that’s a shift-m) allows you to monitor the current window for output. I use the MSN command line client pebrot occasionally, and always set its window to notify me when something happens (ie: a join message).
– Ctrl-a _ : does the same thing as above, but in a opposite way. It switches into the monitoring mode for 15 seconds of silence, which triggers a notification in xterm’s status area. So when our compile finishes, we will be told so even in another session.
– Ctrl-a [ : will place you in copy mode. Use this when you need to copy some text from one session to another. Do Ctrl-a [ in the source session to enter copy mode (you can exit copy mode using ESC). Move around using cursor keys to the beginning of where you want to start copying and press Spacebar to mark the beginning. Now move to the end and press Spacebar again to mark the end of your copy block. You can now switch to another session, move to where you want to paste the block and press Ctrl-a ] to paste what was put in the buffer.

Here are a couple of more useful startup screen commands:

– screen -ls : will list all the screen processes running under your userid (yes you can run multiple screen processes with multiple sessions inside each).
– screen -r screenname : restores a specific screen process.
– screen -R : will try to reattach if there is a detached process, if not it will start a new process.
– screen -D -RR : this is the “I want control now” command. It will detach already attached clients and attach to the first session listed.

As usual screen is controlled via .screenrc file for configuration parameters (there is a system wide file in /etc/screenrc and the personal one in your home directory, under ~/.screenrc). You can add the following commands in your personal .screenrc to make life a bit simpler:

#kill startup message
startup_message off
# define a bigger scrollback, default is 100 lines
defscrollback 1024
# An alternative hardstatus to display a bar at the bottom listing the
# windownames and highlighting the current windowname in blue. (This is only
# enabled if there is no hardstatus setting for your terminal)
hardstatus on
hardstatus alwayslastline
#hardstatus string "%{.bW}%-w%{.rW}%n %t%{-}%+w %=%{..G} %H %{..Y} %m/%d %C%a "
#hardstatus string "%{= mK}%-Lw%{= KW}%50>%n%f* %t%{= mK}%+Lw%< %{= kG}%-=%D %d %M %Y %c:%s%{-}"
#hardstatus string "%{= kG}%-Lw%{= kW}%50> %n%f* %t%{= kG}%+Lw%< %{= kG}%-=%c:%s%{-}"
#hardstatus string "%{= kG}[ %{G}%H %{g}][%= %{= kw}%?%-Lw%?%{r}(%{W}%n*%f%t%?(%u)%?%{r})%{w}%?%+Lw%?%?%= %{g}][%{B} %d/%m %{W}%c %{g}]"
hardstatus string "%{gk}[ %{G}%H %{g}][%= %{wk}%?%-Lw%?%{=b kR}(%{W}%n*%f %t%?(%u)%?%{=b kR})%{= kw}%?%+Lw%?%?%= %{g}][%{Y}%l%{g}]%{=b C}[ %m/%d %c ]%{W}"


As usual there is a lot more to screen, so once you’ve got the basics nailed, take a peek at the man pages for more goodies and don’t forget…..Command line is your friend :-).