How to use Screen Command

While working with remote servers, sometimes we need to run long-running scripts. Imagine you are running such a script on your remote server and suddenly the internet connection drops, this will terminate the SSH session, and your work will be lost. For situations like this, we can use the Screen command.

GNU Screen is a terminal multiplexer program. Simply put, it gives you the ability to launch a number of windows (virtual terminals) from a single screen session. You can connect to a remote machine, start a screen session, and launch a process, even if the connection is lost, after reconnecting to the remote host, your process will still be running. Let's see how we can use the screen command.

• When we type screen in the shell, we are presented with license information of the program. Upon pressing enter the screen process will start.

• The shell looks the same as before, If we do a screen -ls command, it will show the current screen session.

• Let's mimic a long-running script in this screen session and ping google.com and detach this screen, please ctlr+A + d to detach from the screen. Another way to detach an attached screen is to run screen -d <screen_name> .  

ubuntu@ip-172-31-29-101:~$ screen -ls
There are screens on:
	92359.pts-2.ip-172-31-29-101	(06/30/2021 03:22:38 AM)	(Attached)    
1 Sockets in /run/screen/S-ubuntu.

ubuntu@ip-172-31-29-101:~$ screen -d 92359.pts-2.ip-172-31-29-101
[92359.pts-2.ip-172-31-29-101 detached.]

ubuntu@ip-172-31-29-101:~$ screen -ls
There are screens on:
	91297.logs	(06/30/2021 02:10:32 AM)	(Detached)
1 Sockets in /run/screen/S-ubuntu.

• After detachment, we are now in the parent shell, if we do screen -ls now, we can see the detached screen with its name. Since we have not passed any name, while creating the screen, we get a default name.  In order to create a named screen, use screen -S <screen_name>, note its a capital s.

$ screen -S logs

$ screen -ls
There are screens on:
	91297.logs	(06/30/2021 02:10:32 AM)	(Attached)
2 Sockets in /run/screen/S-ubuntu.

• In the parent shell, we can run any program and the ping command will keep on working in the background. We can re-attach to the screen session, we will see ping command still working. In order to re-attach we use screen -r <screen_name>.

ubuntu@ip-172-31-42-95:~$ screen -r 12108.pts-0.ip-172-31-42-95

• Let's detach this session again using ctrl+a + d and this time let's disconnect the ssh session as well. Upon reconnection, we will find the same ping command running inside the screen.
• detaching and reattaching allows devs to run long-running scripts without the fear of losing connection over ssh. To kill a screen session, fire exit command or press ctrl+a + k .
• In order to kill a screen session. screen -X -S <screen_name> quit