Introduction to screen

screen is for many people an unknown program and tool, but those who do know it consider it to be an essential one. This is because of the functionality of the program, that can end up saving you a lot of work and time. You can run any number of console-based applications – editors, mail clients, browsers, in just a single terminal. Sometimes it’s useful to have a terminal for each console-based application you’re running, but sometimes not, obviously.

For my part, the main reason for me using screen is the ability to decouple the terminal emulator from the running programs. In other words, I can detach and reattach things as I please. To put it in simple terms; it’s a tool for session management. Let’s go ahead with an example.

You are logged into your remote server via SSH, working on some configuration file, when suddenly you lose connection to it. Almost done, having spent hours, you grab your laptop in anger and smack the closest unfortunate person next to you. Not only have you lost valuable work, but soon enough people with shields, guns and helmets will come after you. Imagine, with screen, it could have been avoided! Had you been editing the configuration file within a screen, and should you have lost connection, the editing would still be in an active screen session, allowing you to detach and reattach the screen to your terminal once you logged back in.

Let’s have a closer look at what we can do with screen.

Starting screen

Fire up a terminal and just type:

If installed, you probably won’t get a message about having entered a screen session, but I’m sure you have. Everything functions just like a normal shell, except for a few special characters. screen uses the command Ctrl-A as a signal to screen instead of the shell.

To see the help page for screen, press Ctrl-A, then press ?. In other words:

As you can see here, Ctrl is represented by the ^, A is A, and well, you get the rest.

To start a program directly into screen, just type screen . Let’s use top as an example.

Using screen

Fire it up:

top is started in the usual way, just inside a screen session. To detach the session and let it run in the background, use Ctrl-a d:

Once your session is detached you can list the available screen sessions by using:

The output you’ll see will be something along the lines of:

To reattach the screen session to your current terminal, you can use screen -dr . In this case it would be:

This brings up an interesting question though. How can you differentiate what multiple screen sessions are holding, if you can only read the process number? Easy – you give each screen session a name. For most people, a name is easier to remember than a number. So, we start a new screen, in which we’re going to open an IRC client called irssi. Let’s use -S:

Entering irssi, we detach the session with Ctrl-a d, and do a screen -ls to see what we’ve got now:

There you clearly see the name irc that we entered earlier, and the session holding top, which we started up even before being able to specify a name. Pretty handy, huh? This is particularly hand if you’re only going to have one application in the screen session. If more, it would probably be better to have a more descriptive session name of some sort.

Sharing a screen

If someone else, or you, has created a screen, and would like to share with someone else what you’re doing, then screen -x does the trick. Simply use:

..if irc is the name of the session, or the process ID. Once done, you should have enabled multi display mode. And if you find out that the screen you have taken part of doesn’t match up to the size of your own terminal, simply use: force it to match up to the dimensions.

Multiple applications in one session

Now I’ve introduced you to the basics of using screen. Let’s proceed with using screen with multiple applications. Detach and resume the screen session called irc with:

Once resumed, press Ctrl-a Ctrl-c (a ^c) to create a new screen session from within your current one. This will start up a new shell. Let’s start a new application – nano. Simply type:

And the application will start. Let’s give this internal screen session a name. Press Ctrl-a Shift-a (a A). Notice that a is different from A (capital A).

You will be asked:

Type editor and press enter.

Navigating within the screens

You can switch between the various applications by pressing Ctrl-a 0-9, so:

will go to the first application, irssi in this case. Other ways of navigating is by using

To list up all of the shells available on the screen, type:

You’ll get a list that looks something like:

Use your Up/Down arrows and press enter to select a shell to go to.

Clearing destroyed sessions

If you have a screen session up, and the machine it is on goes down, then the screen session is probably destroyed. You can see the available screens by typing screen -ls. To get rid of destroyed screens, type:

Splitting windows

Another interesting thing many people have asked about is how you can split a window in a screen session. A useful feature if you say would like to have two documents open, or you have to do some work while having a conversation on IRC. To split the window, type:

You will see that your screen is now split, and you have a new area on the bottom. To move to this area, type:

Now that you’re in the new area, there’s not much to do but to create another shell, so do it with:

From here you can start another application in the regular way, and give the internal shell a name of its own.

If you want to close the application in the bottom area, do it as normal, then to kill the area, press:

If you want to kill every area EXCEPT then one you’re currently in, you can use:

And that’s pretty much it when it comes to screen. For more information, check out the command:

Good luck!

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