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it_tips [2013/07/02 09:59]
florido created
it_tips [2013/08/27 19:24]
florido
Line 1: Line 1:
-{{:underc.png?200|}}+====== IT Tips ====== 
 + 
 +Geeky notes on how to survive the endless battle against man and machine. 
 + 
 +===== Screen ===== 
 + 
 +Screen is the best sysadmin and cluster user companion. It does a magical thingif you disconnect abruptly from a server, it will keep everything running as is you've never disconnectedThink 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.txt · Last modified: 2016/05/04 09:39 by florido

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