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Unix Dweeb

How the Linux screen tool can save your tasks – and your sanity – if SSH is interrupted

Sep 30, 20193 mins

The Linux screen command can be a life-saver when you need to ensure long-running tasks don't get killed when an SSH session is interrupted. Here's how to use it.

working w screen shs
Credit: Sandra Henry-Stocker

If you’ve ever had to restart a time-consuming process because your SSH session was disconnected, you might be very happy to learn about an interesting tool that you can use to avoid this problem – the screen tool.

Screen, which is a terminal multiplexor, allows you to run many terminal sessions within a single ssh session, detaching from them and reattaching them as needed. The process for doing this is surprising simple and involves only a handful of commands.

To start a screen session, you simply type screen within your ssh session. You then start your long-running process, type Ctrl+A Ctrl+D to detach from the session and screen -r to reattach when the time is right.

If you’re going to run more than one screen session, a better option is to give each session a meaningful name that will help you remember what task is being handled in it. Using this approach, you would name each session when you start it by using a command like this:

$ screen -S slow-build

Once you have multiple sessions running, reattaching to one then requires that you pick it from the list. In the commands below, we list the currently running sessions before reattaching one of them. Notice that initially both sessions are marked as being detached.

$ screen -ls
There are screens on:
        6617.check-backups      (09/26/2019 04:35:30 PM)    (Detached)
        1946.slow-build         (09/26/2019 02:51:50 PM)    (Detached)
2 Sockets in /run/screen/S-shs

Reattaching to the session then requires that you supply the assigned name. For example:

$ screen -r slow-build

The process you left running should have continued processing while it was detached and you were doing some other work. If you ask about your screen sessions while using one of them, you should see that the session you’re currently reattached to is once again “attached.”

$ screen -ls
There are screens on:
        6617.check-backups      (09/26/2019 04:35:30 PM)    (Attached)
        1946.slow-build         (09/26/2019 02:51:50 PM)    (Detached)
2 Sockets in /run/screen/S-shs.

You can ask what version of screen you’re running with the -version option.

$ screen -version
Screen version 4.06.02 (GNU) 23-Oct-17

Installing screen

If “which screen” doesn’t provide information on screen, it probably isn’t installed on your system.

$ which screen

If you need to install it, one of the following commands is probably right for your system:

sudo apt install screen
sudo yum install screen

The screen tool comes in handy whenever you need to run time-consuming processes that could be interrupted if your SSH session for any reason disconnects. And, as you’ve just seen, it’s very easy to use and manage.

Here’s a recap of the commands used above:

screen -S 	start a session
Ctrl+A Ctrl+D				detach from a session
screen -ls				list sessions
screen -r 	        reattach a session

While there is more to know about screen, including additional ways that you can maneuver between screen sessions, this should get you started using this handy tool.

Unix Dweeb

Sandra Henry-Stocker has been administering Unix systems for more than 30 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She lives in the mountains in Virginia where, when not working with or writing about Unix, she's chasing the bears away from her bird feeders.

The opinions expressed in this blog are those of Sandra Henry-Stocker and do not necessarily represent those of IDG Communications, Inc., its parent, subsidiary or affiliated companies.

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