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it_tips:screen [2014/03/27 11:08] (current)
florido created
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 +===== Screen =====
 +
 +Screen is the best sysadmin and cluster user companion. It does a magical thing: if you disconnect abruptly from a server, it will keep everything running as is you've never disconnected. Think of it as an open window to your work on a remote machine, to which only you can have access.
 +
 +To give an idea of what ''​screen''​ does, follow this simple tutorial in which we will be using the ''​top''​ utility that shows processes running on a machine. The tutorial shows how, if you start top in a screen session and you disconnect from a server, the top program is still running on the remote machine.
 +
 +Please read tips at the end on how to use //copy mode// and move in the open window.
 +
 +== 1) login via ssh into a server and run screen: ==
 +<​code>​[pflorido@n12 pp]$ screen</​code>​
 +An intro page will appear where you have to press enter or space. At this point you are in a virtual shell running on the server, independent from the ssh connection. If connection ​ drops by during this time you don't loose your session.
 +
 +== 2) start the top command ==
 +<​code>​[pflorido@n12 pp]$ top</​code>​
 +
 +A lot of info about running processes will fill the terminal.
 +
 +==  3) press the three keys ''​Ctrl a d''​ alltogether ==
 +The screen session will be //​detached//​. you'll see a message like:
 +<​code>​[detached]</​code>​
 +and you will be sent back to the shell where you wrote ''​screen''​ before.
 +
 +== 4) list the existing screen sessions ==
 +With the command
 +<​code>​[pflorido@n12 pp]$ screen -list</​code>​
 +
 +You'll see a list of running detached or attached screens, like this:
 +<​code>​
 +There is a screen on:
 + 20440.pts-1.n12 (Detached)
 +1 Socket in /​var/​run/​screen/​S-pflorido.
 +</​code>​
 +
 +== 5) re-attach the open screen by copy pasting its name: ==
 +<​code>​
 +[pflorido@n12 pp]$ screen -DR 20440.pts-1.n12
 +</​code>​
 +
 +== 6) gaze at top command still running! ==
 +You should now see the running processes as if top command was never stopped. In fact it had never stopped!
 +
 +Press ''​q''​ to exit it and keep working on the same screen shell.
 +
 +== 7) >:-| I don't believe it works! ==
 +You can try ''​screen''​ effectiveness this way:
 +  - log into the cluster
 +  - running ''​screen''​
 +  - close the terminal window wgere you ran it
 +  - relogin via ssh and run ''​screen -list'':​ the screen session is still there!!
 +
 +
 +=== The Copy Mode and other stuff ===
 +Inside a screen session, interaction with the shell changes. In particular, to scroll up and down you should use the combination:​
 +  * ''​Ctrl a ESC''​ to enter //copy mode//. Then you can move the cursor up and down in the screen session.
 +  * ''​Ctrl a ?''​ will give you a list of commands
 +  * ''​man screen''​ will give you  more info.
 +  * On the same ssh connection you can open multiple screen terminals so that you can have multiple windows open, with ''​Ctrl a c''​
 +    * you can move between existing screen terminals with
 +      * ''​Ctrl a p''​ previous
 +      * ''​Ctrl a n''​ next
 +  * You can name each session so that you remember what that was about with the ''​-S''​ flag:<​code>​[pflorido@n12 pp]$screen -S " Don't screen at me!"
 +[pflorido@n12 pp]$ screen -list
 +There are screens on:
 + 20550.Don'​t screen at me! (Detached)
 + 20440.pts-1.n12 (Detached)
 +2 Sockets in /​var/​run/​screen/​S-pflorido.
 +</​code>​
 +
 +  * You can hang the following quick reference card om the wall to remember the commands:
 +    *  http://​aperiodic.net/​screen/​quick_reference
 +
 +== References ==
 +
 +  * http://​www.gnu.org/​software/​screen/​
 +  * http://​www.gnu.org/​software/​screen/​manual/​screen.html
 +  * http://​www.rackaid.com/​resources/​linux-screen-tutorial-and-how-to/​
 +  * Quick reference card: http://​aperiodic.net/​screen/​quick_reference
 +
  
it_tips/screen.txt ยท Last modified: 2014/03/27 11:08 by florido