Integrating a VT220 into my life March 22, 2016 on Drew DeVault's blog

I bought a DEC VT220 terminal a while ago, and put it next to my desk at work. I use it to read emails on mutt now, and it’s actually quite pleasant. There was some setup involved in making it as comfortable as possible, though.

Here’s the terminal up close:

Hardware

First, I have several pieces of hardware involved in this:

• VT220 terminal
• LK201 keyboard (later made obsolete)
• USB to serial adapter
• DB9->DB29 null modem cable

It took a while to get all of these things, but I was able to get a nice refurbished terminal and a couple of crappy LK201 keyboards. Luckily I was able to eventually remove the need for the keyboard.

Basic Setup

Getting this working on Linux is actually pretty simple thanks to decades of backwards compatability. Plug all of the cords together, turn on the machine, and (on Arch, at least) run:

systemctl start serial-agetty@ttyUSB0.service

This will start up a getty for you to log into on your terminal. For a while I would use the LK201 to log in to this getty and spin up a mail cilent.

I did have to make a couple of changes to serial-agetty@.service, though:

ExecStart=-/sbin/agetty -h -L 19200 %I vt220

This specifies the TERM variable as “vt220” and sets the baud rate to 19200. I had to also set the baud rate in the terminal’s settings to 19200 baud as well, to get the fastest possible terminal.

I eventually got into the habit of logging into the terminal with the LK201, then running tmux and attaching to tmux from my desktop session. I would then hide this tmux terminal in the upper left corner of my display, and move my mouse over to it when I wanted to interact with the terminal. This let me use the same keyboard I used for the rest of my computer experience to interact with the VT220, instead of trying to use the LK201 as well. This was a bit annoying, so eventually I did some more customization.

Removing the keyboard

I wanted to be able to make everything automatic, so I could just boot my computer and log in normally and treat the VT220 almost like a fourth monitor. I started by automating the process of logging in and running tmux.

First, I created a user for the terminal:

useradd vt220

Then, I wrote a shell script that would serve as the user’s login shell and would start tmux:

#!/usr/bin/env bash
if [[ $TERM == "screen" ]]
then
	sudo /usr/local/bin/login-sircmpwn
else
	tmux -S /var/tmux/vt220.sock
fi

I made that directory, /var/tmux/, and made sure both the vt220 user and my normal user had access to it. I also edited my sudoers file so that vt220 could run that command as root:

vt220 ALL=(ALL) NOASSWD: /usr/local/bin/login-sircmpwn

I put the script into /usr/local/bin and added it to /etc/shells, then made it the login shell for the vt220 user with chsh. I then moved to my own systemd unit for starting the getty on ttyUSB0, this time with autologin:

#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Serial Getty on %I
Documentation=man:agetty(8) man:systemd-getty-generator(8)
Documentation=http://0pointer.de/blog/projects/serial-console.html
BindsTo=dev-%i.device
After=dev-%i.device systemd-user-sessions.service plymouth-quit-wait.service

# If additional gettys are spawned during boot then we should make
# sure that this is synchronized before getty.target, even though
# getty.target didn't actually pull it in.
Before=getty.target
IgnoreOnIsolate=yes

[Service]
ExecStart=-/sbin/agetty -a vt220 -h -L 19200 %I vt220
Type=idle
Restart=always
UtmpIdentifier=%I
TTYPath=/dev/%I
TTYReset=yes
TTYVHangup=yes
KillMode=process
IgnoreSIGPIPE=no
SendSIGHUP=yes

[Install]
WantedBy=getty.target

The only difference here is that it invokes agetty with -a vt220 to autologin as that user. systemctl enable vtgetty@ttyUSB0.service makes it so that on boot, the getty would run on ttyUSB0 and autologin as vt220. Then the script from earlier will run tmux, and within tmux will run sudo /usr/local/bin/login-sircmpwn, which is this shell script:

#!/usr/bin/env bash
until who | grep sircmpwn 2>&1 >/dev/null
do
	sleep 1
done
sudo -iu sircmpwn

What this does is pretty straightforward - it loops until I log in as sircmpwn, then enters an interactive session with sudo as sircmpwn.

The net of all of this is that now, I can boot up my machine, and when I log in, the VT220 starts up with tmux running and logged in as me. Then I went back to the old way of attaching to this tmux session with a terminal on my desktop session hidden in a corner of the screen. And now I could ditch the clunky old LK201 keyboard!

Treating the terminal as another output

I said earlier that my goal was to treat the terminal as a fake “output” that I could switch to from my desktop session just like I switch between my three graphical outputs. I run sway, of course, so I decided to add a fake output in sway and see where that went. I made a somewhat complicated branch for this purpose, but the important change is here:

diff --git a/sway/handlers.c b/sway/handlers.c
index cec6319..60f8406 100644
--- a/sway/handlers.c
+++ b/sway/handlers.c
@@ -704,6 +704,21 @@ static void handle_wlc_ready(void) {
 		free(line);
 		list_del(config->cmd_queue, 0);
 	}
+	// VT220 stuff
+	// Adds a made up output that we can use for a tmux window
+	// connected to my vt220
+	swayc_t *output = new_swayc(C_OUTPUT);
+	output->name = "VT220";
+	output->handle = UINTPTR_MAX;
+	output->width = 1000;
+	output->height = 1000;
+	output->unmanaged = create_list();
+	output->bg_pid = -1;
+	add_child(&root_container, output);
+	output->x = -1000;
+	output->y = 0;
+	new_workspace(output, "__VT220");
+	// End VT220 stuff
 }

This creates a fake output and puts it to the far left, then adds a workspace to it called __VT220. I assigned it the output handle of UINTPTR_MAX and everywhere in sway that it would try to use the output handle to manipulate a real output, I changed to to avoid doing so if the handle is UINTPTR_MAX. Then I added this to my sway config:

for_window [title="__VT220"] move window to workspace __VT220 

And run this command when sway starts:

urxvt -T "__VT220" -e tmux -S /var/tmux/vt220.sock a

Which spawns a terminal whose window title is __VT220 running tmux attached to the session running on the terminal. The for_window rule I added to my sway config automatically moves this to the VT220 fake output and tada! It works. Now I have a nice and comfortable way to use my terminal to read emails at work. Now if only I could convince people to stop sending me HTML emails! I just bought a second VT220 for use at home, too. Life’s good~

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