An old-school shell hack on a line printer

Published 2019-10-30 on Drew DeVault's blog

It’s been too long since I last did a good hack, for no practical reason other than great hack value. In my case, these often amount to a nostalgia for an age of computing I wasn’t present for. In a recent bid to capture more of this nostalgia, I recently picked up a dot matrix line printer, specifically the Epson LX-350 printer. This one is nice because it has a USB port, so I don’t have to break out my pile of serial cable hacks to get it talking to Linux 😁

This is the classic printer style, with infinite paper and a lovely noise during printing. They are also fairly simple to operate - you can just write text directly to /dev/lp (or /dev/usb/lp9 in my case) and it’ll print it out. Slightly more sophisticated instructions can be written to them with ANSI escape sequences, just like a terminal. They can also be rigged up to CUPS, then you can use something like man -t 5 scdoc to produce printouts like this:

Plugging the printer into Linux and writing out pages isn’t much for hack value, however. What I really wanted to make was something resembling an old-school TTY - teletypewriter. So I wrote some glue code in Golang, and soon enough I had a shell:

The glue code I wrote for this is fairly straightforward. In the simplest form, it spins up a pty (pseudo-terminal), runs /bin/sh in it, and writes the pty output into the line printer device. For those unaware, a pseudo-terminal is the key piece of software infrastructure for running interactive text applications. Applications which want to do things like print colored text, move the cursor around and draw a TUI, and so on, will open /dev/tty to open the current TTY device. For most applications used today, this is a “pseudo-terminal”, or pty, which is a terminal emulated in userspace - i.e. by your terminal emulator. However, your terminal emulator is emulating a terminal - the control sequences applications send to these are backwards-compatible with 50 years of computing history. Interfaces like these are the namesake of the TTY.

Visual terminals came onto the scene later on, and in the classic computing tradition, the old hands complained that it was less useful - you could no longer write notes on your backlog, tear off a page and hand it to a colleague, or white-out mistakes. Early visual terminals could also be plugged directly into a line printer, and you could configure them to echo to the printer or print out a screenfull of text at a time. A distinct advantage of visual terminals is not having to deal with so much bloody paper, a problem that I’ve become acutely familiar with in the past few days1.

Getting back to the glue code, I chose Golang because setting up a TTY is a bit of a hassle in C, but in Golang it’s pretty straightforward. There is a serial port and in theory I could have plugged it in and spawned a getty on the resulting serial device - but (1) it’d be write-only, so not especially interactive without hardware hacks, and (2) I didn’t feel like digging out my serial cables. So:

import "git.sr.ht/~sircmpwn/pty" // fork of github.com/kr/pty

// ...
winsize := pty.Winsize{
  Cols: 160,
  Rows: 24,
}
cmd := exec.Command("/bin/sh")
cmd.Env = append(os.Environ(),
  "TERM=lp",
  fmt.Sprintf("COLUMNS=%d", 180))
tty, err := pty.StartWithSize(cmd, &winsize)

P.S. We’re going to dive through the code in detail now. If you just want more cool videos of this in action, skip to the bottom.

I set the TERM environment variable to lp, for line printer, which doesn’t really exist but prevents most applications from trying anything too tricksy with their escape codes. The tty variable here is an io.ReadWriter whose output is sent to the printer and whose input is sourced from wherever, in my case from the stdin of this process2.

For a little more quality-of-life, I looked up Epson’s proprietary ANSI escape sequences and found out that you can tell the printer to feed back and forth in 216th” increments with the j and J escape sequences. The following code will feed 2.5” out, then back in:

f.Write([]byte("\x1BJ\xD8\x1BJ\xD8\x1BJ\x6C"))
f.Write([]byte("\x1Bj\xD8\x1Bj\xD8\x1Bj\x6C"))

Which happens to be the perfect amount to move the last-written line up out of the printer for the user to read, then back in to be written to some more. A little bit of timing logic in a goroutine manages the transition between “spool out so the user can read the output” and “spool in to write some more output”:

func lpmgr(in chan (interface{}), out chan ([]byte)) {
	// TODO: Runtime configurable option? Discover printers? dunno
	f, err := os.OpenFile("/dev/usb/lp9", os.O_RDWR, 0755)
	if err != nil {
		panic(err)
	}

	feed := false
	f.Write([]byte("\n\n\n\r"))

	timeout := 250 * time.Millisecond
	for {
		select {
		case <-in:
			// Increase the timeout after input
			timeout = 1 * time.Second
		case data := <-out:
			if feed {
				f.Write([]byte("\x1Bj\xD8\x1Bj\xD8\x1Bj\x6C"))
				feed = false
			}
			f.Write(lptl(data))
		case <-time.After(timeout):
			timeout = 200 * time.Millisecond
			if !feed {
				feed = true
				f.Write([]byte("\x1BJ\xD8\x1BJ\xD8\x1BJ\x6C"))
			}
		}
	}
}

lptl is a work-in-progress thing which tweaks the outgoing data for some quality-of-life changes, like changing backspace to ^H. Then, the main event loop looks something like this:

inch := make(chan (interface{}))
outch := make(chan ([]byte))
go lpmgr(inch, outch)

inbuf := make([]byte, 4096)
go func() {
  for {
    n, err := os.Stdin.Read(inbuf)
    if err != nil {
      panic(err)
    }
    tty.Write(inbuf[:n])
    inch <- nil
  }
}()

outbuf := make([]byte, 4096)
for {
  n, err := tty.Read(outbuf)
  if err != nil {
    panic(err)
  }
  b := make([]byte, n)
  copy(b, outbuf[:n])
  outch <- b
}

The tty will echo characters written to it, so we just write to it from stdin and increase the form feed timeout closer to the user’s input so that it’s not constantly feeding in and out as you write. The resulting system is pretty pleasant to use! I spent about hour working on improvements to it on a live stream. You can watch the system in action on the archive here:

If you were a fly on the wall when Unix was written, it would have looked a lot like this. And remember: ed is the standard text editor.

?

  1. Don’t worry, I recycled it all. 

  2. In the future I want to make this use libinput or something, or eventually make a kernel module which lets you pair a USB keyboard with a line printer to make a TTY directly. Or maybe a little microcontroller which translates a USB keyboard into serial TX and forwards RX to the printer. Possibilities! 

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