· screen

Getting started with screen

Last week I had a ~10GB file I wanted to download to my machine but Chrome's initial estimate was that it would take 10+ hours to do so which meant I'd have probably shutdown my machine before it had completed.

It seemed to make more sense to spin up an EC2 instance and download it onto there instead but I didn't want to have to keep an SSH session open to that machine either.

I've previously come across screen and tmux which allow you to create a session in which you can do some work even if you aren't currently connected to it which was perfect for my use case.

screen was a bit more familiar so I decided to use that and I thought I should make a quick note of some of its basic flags for future me.

Starting a new session

Starting a new screen session is as simple as typing the following command:

$ screen

which leads to the following output:

Screen version 4.00.03jw4 (FAU) 2-May-06

Copyright (c) 1993-2002 Juergen Weigert, Michael Schroeder
Copyright (c) 1987 Oliver Laumann

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later

We can now start downloading our file using cURL, wget or a download accelerator like axel which is my personal favourite.

Detaching/Exiting from a session without it dying

Once I'd got the download running I wanted to close my SSH session to the AWS instance but first I wanted to detach from my screen session without killing it.

My first attempt was to use Ctrl + D but that actually results in the session being terminated and our download is therefore stopped as well which isn't quite what we wanted.

Instead what we want to do is detach from the session which allows us to leave it but keep it running. To do that we type Ctrl + A followed by Ctrl + D.

Reattaching to a session

After about an hour I wanted to checkup on my download and I assumed just typing screen would take me back to my session but instead it created a new one.

Alistair pointed out that I could get a listing of all the open screen sessions by typing the following command:

$ screen -ls
There are screens on:
	23397.pts-0.ip-10-243-5-102	(07/31/2013 05:25:30 AM)	(Detached)
	3981.pts-0.ip-10-243-5-102	(07/26/2013 07:59:28 AM)	(Detached)
	3910.pts-0.ip-10-243-5-102	(07/26/2013 07:58:42 AM)	(Detached)
	1094.pts-0.ip-10-243-5-102	(07/26/2013 07:49:31 AM)	(Detached)
4 Sockets in /var/run/screen/S-ubuntu.

As you can see, I'd created a bunch of extra sessions by mistake.

The one I had the download running on was '1094.pts-0.ip-10-243-5-102' and we can reattach to that one like this:

$ screen -x 1094.pts-0.ip-10-243-5-102

We can also attach using the '-r' flag:

$ screen -r 1094.pts-0.ip-10-243-5-102

I'm not quite sure what the difference is between '-r' and '-x', they both seem to behave in the same way in this scenario.

The manual suggests that '-x' is for attaching to a 'not detached screen session' which suggests to me that it shouldn't have worked since I wanted to connect to a detached session.

Hopefully someone with more knowledge of how these things work can explain what's going on!

Attach to an existing session or start a new one if none exists

I later learnt that had I not accidentally created all those extra sessions the following command would have been quite useful for finding the first screen session available and connecting to it:

$ screen -x -R

If there aren't any existing screen sessions available then it will create a new one which means that in my particular situation this would have been a more appropriate command to start with.

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