Wednesday, April 26, 2017

Linux -- using the screen for uninterrupted command operations (especially when you don't have Vnc)

This is a very old topic actually. Especially Linux folks will not pay attention to this one (because they probably already know it for a long time ago). However, in Oracle DBA world, it is a valuable topic.
We always use vnc for uninterrupted operations.(sometimes nohup as well)
That's what said to us. Especially for critical and long running OS operation (including copy, move , manual database upgrades and Rman operations) , we most of the time use VNC and get a X session , which is not interrupted, even if we lose our connections to our servers.

the "screen" command, on the other hand; can be thought as an alternative to the VNC connections in a way.
Ofcourse screen doesn't have the capability (it is not designed for this aim) to give us a X session (GUI), but it let us to have uninterrupted command operations (like the VNC does).
Besides screen program let us continue with our shell/terminal even after we disconnect from the server. That is even after disconnecting and reconnecting , we can still see all the shell outputs which have been produced since we have started working in our screen terminal.

Following filtered info is from the man page of the screen program;

Screen  is  a  full-screen window manager that multiplexes a physical terminal between several processes (typically interactive shells). 
When screen is called, it creates a single window with a shell in it (or the specified command) and then gets out of your way so that you can use the program as you normally would. 
Programs continue to run when their window is currently not visible and even when the whole screen session is detached from the userâs terminal.

Well... screen is an easy to install and use tool. (In case of Oracle Linux, screen can be installed using "yum install screen" command)

Here is a demo for you ->

We connect to our server and execute the screen command as follows;
When we execute the screen command, a screen session/terminal gets created as shown below;
(I showed the pids of the sessions using echo $$ to let you see the terminal's pid gets changed when we execute the screen command).

In the screen session, we first execute the ls -al command and see its output. Then, we execute our next command, which is "du -sh / ".. While our command is working, we close our terminal (by closing our terminal program to mimic an unexpected disconnection).

After we close our terminal program (which is an SSH client in my case), we relogin to our server and check our ongoing screen sessions using "screen -ls" command, as shown below;

Once we identify our screen session using its id (pid actually, and it is 3729 in this case), we use "screen -r" command to attach our ongoing screen session, as depicted below;

After we execute the screen -r command, we attach to our screen terminal and see our du -sh operation is still on-going. (we even see the output of ls -al command , that we executed earlier)

If we decide to kill our screen terminal, we can always use exit command while we are in the screen terminal prompt. Alternatively, we can use "screen -S <pid> -X kill" command  to kill our screen terminal, without even attaching to it.

The last thing that I want to mention in this blog post is, the difference between the nohup command and the screen command. 
That is, screen is not only used for daemonizing a process. It works more like a terminal window manager. For instance, we can disconnect from the screen terminal while our command is working, and then reconnect to that terminal in case our command requires an input from us. Also, we can reconnect to the terminal and check the command outputs , which were produced while we are not connected to the server at all and so on.. So, screen and nohup are not the same thing.

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