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UsingScreenInMacos.md

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Using screen as a terminal multiplexer in MacOS

screen is sometimes called a terminal multiplexer. It's incredibly useful, but arcane. However, as is usually the case, diligent research usually reveals enough secrets to (at least) construct a useful & rational configuration. That's what we'll try to do here!

We'll cover the native version of screenhere - the one that comes as part of macos. Before investing time & effort, you should know there are alternatives to the native Apple version of screen. As of this writing, Apple packages a 16 year-old version of GNU screen (ver 4.00.03) with macos 10.15.6 (Catalina). The latest version of screen (ver 4.9 as of today) can be installed through MacPorts. There are also alternatives to screen, one being tmux. Again, we're going to stick with the native version for this exercise.

Due to the age of Apple's chosen release of screen, some of the information available for screen online may not be relevant here. This shouldn't be a problem if we stick to "basics". We will have some example configuration files (screenrc) to guide us - that, according to man screen:

When screen is invoked, it executes initialization commands from the files "/usr/local/etc/screenrc" and ".screenrc" in the user's home directory.

This is out-of-date and/or incorrect. The folder /usr/local/etc/ does not exist in macos. The manual goes on to say:

Two configuration files are shipped as examples with your screen distribution: "etc/screenrc" and "etc/etcscreenrc". They contain a number of useful examples for various commands.

As I learned, these configuration files are not included in the macos distribution; i.e. they are not on your mac. But you can find them in Apple's open-source archives.

After perusing the massive man screen document, I elected to end the research, and move to the trial-and-error phase! With no screenrc file installled (yet), let's start screen, and connect to a couple of Raspberry Pi's:

$ screen -S Pi4 ssh -Y [email protected]

This starts a terminal session via SSH with for the designated user@host ([email protected]), and applies a session name of Pi4 using the -S option. This session namewill make moving between sessions easier. If this command executes successfully, you will now be looking at the command line of the host you connected. Once you've explored this new screen, let's detach it, and return to the terminal app. At the command line, press & hold the controlkey, then hit the a key, release both keys, then hit the d key:

ctrl-a d

The ctrl-a sequence signals that what follows is a screen command instead of a shell command. Once this sequence is entered, we will now be detached from this screen, and back at the terminal. But you are still connected to your host in the screen with session name Pi4. You may want to do some things in terminal, and then attach to session Pi4again. From the command line in terminal:

$ screen -r

Which will toggle you back to screen session Pi4. Now, detach from Pi4 again, and start a second screenin terminal to connect to host raspberrypi3b.local :

$ screen -S Pi3 ssh -Y [email protected]

Explore screen session Pi3, and detach using ctrl-a d

Now, from the terminal command line:

$ screen -r
There are several suitable screens on:
	3124.Pi4	(Detached)
	3143.Pi3	(Detached)
Type "screen [-d] -r [pid.]tty.host" to resume one of them.

This command now lists the two screen sessions, and explains how to resume either of them. From here we can switch between two screen sessionsand the macos shell in the same

You may want to "split" your terminal window, and display a screen session in each one. There are several options here; let's go over (some of) them:

  • Use tabs in the terminalmenu: View, Show Tab bar and then add tabs for each screen
  • Split the terminal window using screen
  • Install iTerm2which has the ability to split its window with different shell sessions

Quitting screen:

At some point, you will want to end your screen session completely. Perhaps you are finished, or perhaps you have made an error:

$ screen -ls        # retrieves list of all screen sessions; for example:
There is a screen on:
	1172.ttys001.MyMacbookPro	(Attached)
1 Socket in /var/folders/9t/_1d0fdt969x5s97bnfz40jdw0000gp/T/.screen.
$ screen -XS 1172.ttys001.MyMacbookPro quit
$

References:

  1. Apple maintains a central repository for the open-source software included in macos
  2. Michael Levin has written an overview on screen, and why it's useful.
  3. When using screen in macos, the problem described in this Q&A may be the first issue you encounter.
  4. Softpanorama covers most of the details in his .screenrc examples webpage.
  5. Salty Crane's blog has some tips for using GNU screen that might be useful.
  6. Alain Francois' post on LinOxide offers a well-organized overview of screen commands.
  7. This was a productive search term for information on ~/.screenrc.
  8. There are alternatives to screen for macos... tmux seems to be popular and well-maintained.
  9. How to scroll up and down in sliced “screen” terminal, a Q&A on StackExchange.
  10. How to split the terminal into more than one “view”?, a Q&A on StackExchange.