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Howto/Screen

Screen

Screen is a great tool which has many uses. You could think about it as a kind of a window manager for your console.

For example, if you wish to run two programs at the same time, without having to open multiple connections to the server, you can do this with screen. Maybe you want to run irssi, while at the same time checking your E-mail.

Another great thing with Screen is that it allows you to keep your programs running while you are disconnected from the server. This makes it possible to stay connected to IRC and log everything even while your computer is turned off! All you have to do is start your IRC client in a Screen and then "detach" when you go offline.

However, since screen is very powerful and flexible, and it's all text-only, it can be hard to understand how to use it properly. It's also easy to start too many screens and lock yourself out of the system (since you can only have so many logins/processes). If this happens, talk to an admin to get help exiting your processes. Also remember that you are only allowed to run two programs in the background, for example one instance of irssi and one instance of lynx. Also, it's not ok to run multiple instances of irssi.

Screen basics

If you want to run a program on the server even while you are not connected, you first have to start the program inside a Screen. You can then detach this screen, and attach it another time (from another computer maybe).

To start a new screen, use the command screen in an empty shell on the server: 

screen

This will spawn a new screen for you. You will get a new empty shell, and at the bottom you can see a status bar, called a hardstatus line. In this hardstatus line you can see the different "windows" you have running in this screen session.

Now you can now start whatever program you would like to run.

When you want to log out, you can detach your program by entering the key combination of Ctrl-a d (that is; press Ctrl-a and then press d). Now your program is running in the detached screen, waiting for you to return and attach the screen session again.

You can now log out from the shell, and the screen session will still be there when you log back in (unless the server reboots).

To attach your screen session again, use this command in an empty prompt: 

screen -r

Remember to not start mulitple screens, it will only make things harder for you.

If you want to list your running screens, you can use this command: 

screen -ls

If you have multiple screens, resume them one by one and exit.

To exit just type logout or exit (you can also do this by pressing Ctrl-d). If you only have one window, you will exit the screen session and you will get a message like this: 

[screen is terminating]

That wasn't too hard, was it? :) Whith the few things you have learned this far, you can really make so much more of your shell experience. It's a truly great tool!

Good to know

If your connection to the shell is lost, you might not be able to resume your screen right away because the server thinks it's still in use until a timeout is reached. If you want to resume right away, you can use this command: 

screen -rd

This command will detach any previously attached clients and then attach you.

If you somehow started two alltogether different screen sessions you might not be able to resume the right one just by tying screen -r. In this case, you have to specify screen -r <id>. You can get a list of current running screen sessions and their id with the command screen -ls. Just resume any extra screens you don't need and exit them by typing exit, logout or Ctrl-d.

Sometimes when the server reboots, your screens "dies". If this happens, you won't be able to reattach it. Instead, you have to clear the old one by doing screen -wipe and start a new one. FAQ/DeadScreen

An easier way

Blinkenshell has a custom command named scr which you can use to handle screens more easily. It checks for dead screens and removes them, checks if you have multiple screens running and warns you about them, creates a new screen if you don't already have one, or resumes an already running screen. All you have to do is typ scr and it will take the approperiate action and tell you what's happening. This is script is quite new and might not take every possible situation into account, please report any issues you find!

Multiple windows

Now, if you want to run multiple programs at the same time, and easily switch between them, you can create several "windows" in your screen session. To create a new window, use this key combination: Ctrl-a c (c for Create).

You should be presented with a brand new prompt, where you can start yet another program. You can create even more windows if you want, but be careful not to start any more than you need. If you create too many, you won't be able to log back in again. Try not to have more than three windows running at the same time. You can see your windows in the hardstatus line at the bottom of your terminal. To exit a window, just exit the shell in that window typing logout or exit. When you close the last window, the screen will terminate.

To switch between two windows inside a screen, press Ctrl-a a. This immediately switches back and forth between two windows (really, it switches to the last used window). If you have more that two windows, you can use Ctrl-a n for stepping forward (next window), and Ctrl-a p for stepping backwards (previous window).

You may also also use Ctrl-a <0-9> where the number is the window you want go to. Ctrl-a " gives a list of current windows, and you can select the one you want to go to.

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