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
If “which screen” doesn’t provide information on screen, it probably isn’t installed on your system.
$ which screen /usr/bin/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 <process description> start a session Ctrl+A Ctrl+D detach from a session screen -ls list sessions screen -r <process description> 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.