What happens when you open a terminal and enter `ls’ (opens in new tab)

Thank you for your article and I would be grateful for a deep dive series on how a terminal works would be great.

On the old terminals, the wire sits at one voltage level until a character is transmitted. After that, the bits are spaced out at agreed-upon time intervals. So the receiver detects the first transition, and then reads and stores the bits by applying the same timing, then waits for the next character. Meanwhile, the value of that character, say an 8-bit number, is stored in a register, and typically the processor is interrupted. This gives the processor time to deal with each character before the next one is received.

Oddly enough I could have answered the OP's question in an interview, 40 years ago, but stuff has gotten so complex that I can't even tell you what all of the layers of abstraction are.

It still works the same way, just abstracted as individual characters and not specifically the implementations of electrical representations of keys. Every character is stateless, so whatever's before or after doesn't matter. So this means that some operations are a lot easier and a lot harder than they otherwise might be, because while you only have to make a decision for one character at a time, you need to keep an internal state if you want to display characters or do something with them as a whole.

I binged CuriousMarc’s YouTube series on teletype restoration a few months ago, so here’s a simplified (and possibly somewhat inaccurate) explanation of how one works.

The only electrical components in a Teletype are (a) a continuously-running electric motor, and (b) an electromagnet and a few switches. Everything else is completely mechanical.

The Teletypes in a circuit, and the electromagnet and switches within them, are all connected in series, forming a current loop. (Current, rather than voltage, is used because you can use a constant-current power supply to get the same power at each electromagnet regardless of how many are in the circuit and how many miles of wire are between them.)

When the circuit is idle, current is flowing. This allows any Teletype to begin transmitting; if it were the other way around, each one would need its own individual line power supply. (Incidentally, this is the origins of the “break” key still found on many modern keyboards: when the circuit was disconnected, nobody could transmit, so “breaking in” to the circuit was how you interrupted somebody, and a lot of early computers used this as a primitive version of ^C.)

When you press a key, a rod is lifted, activating a clutch that connects the motor to the rest of the machine, which starts running. The first thing this does is break the circuit, deactivating the electromagnets in the receiving mechanisms of all the other Teletypes. When this electromagnet deactivates, it trips the clutch in those machines, starting them running so they can receive the character. (This is why it’s called the “start bit.”)

One-tenth of the way through the rotation, a cam in the sending machine switches from the “start bit” to the switch for the first data bit. If the rod on the key you pressed has a bump on it in the right spot, it will press this switch and send a “mark”; if it’s missing the bump you’ll get a “space” instead. On the receiving end, a clever mechanism connects the electromagnet to a lever: if the magnet engages, it pushes the lever one way; and if it doesn’t, the lever stays pushed the other way.

At two-tenths of the rotation, the first data bit switch is disconnected and the second data bit switch engages. This of course reads the second bump on the rod, and at the receiving end the clever mechanism has moved so the magnet pushes (or doesn’t) the second lever.

(Notable exceptions: the ‘ctrl’ key forces the first two bits low—this is why e.g. Ctrl+D and “End of Transmission” are the same—and ‘shift’ forces the first bit high.)

At three-tenths of the rotation, of course, the third bit is read, transmitted and received; this process continues for the remaining data bits.

The final two-tenths of the rotation are known as the “stop bits”, and consist of some signal that doesn’t matter much. The receiving end uses this time to trip the print mechanism: the levers that were pushed-or-not during the data bits engage with some bumps in some other rods, blocking all but one of them that matches the specific pattern, and then a thingie shoves forward, slamming the appropriate type bar up into the ribbon and paper, and as it returns the carriage advances one character. (The reason for using two bits is simply to give some time for all of this to happen.)

Finally, the mechanisms on the sending and receiving ends complete their full rotation, disengaging the clutch and coming to an abrupt halt; the transmitting end reconnects the line so the circuit is available, and everything is ready for the next character. All of this has happened in less than a tenth of a second, a mechanical ballet choreographed too fast for the human eye to see, and transmitting information potentially hundreds of miles at the speed of light.

Haha! The post was actually motivated by a common (interview) question: https://github.com/alex/what-happens-when. We thought it would make for a fun adaptation for the terminal world

I had the same for SRE @ Google - really simple question that probes the entire free-form depth of your systems understanding.

terminfo would be a good topic for a couple blogposts for sure. Back when I was first learning all this stuff I never felt like I truly understood all the intricacies of this system. I tried to understand back when alacritty made it's own entry, but a lot of things still feel somewhat arcane.

Basically, it's a capability list though, IIUC.

Haha, yeah we probably should have added a caveat that this is only applicable to Unix

Thanks! It was fun to write this up. Glad you enjoyed :)

Just like our rat brains still in our heads.

Thanks! Were there any topics in the blog that you think would benefit from more detail in a follow-up post?

Warp actually made telemetry optional last month https://www.warp.dev/blog/telemetry-now-optional-in-warp

I just found out about warp, installed it and having a similar issue.

Terms are really weird for something I thought was a terminal app, and their commong questions are talking about "cloud-oriented features" which I really don't know and probably don't want.

I'd be happy to pay (even per month) for a version of this that asks you "do you want to use our cloud features or just the local version" and has different pricing and definetely no login.

Note: I didn't end up actually trying this as I really didn't like the sound of not knowing what my terminal is sending out. They do list what they send for telemetry, but not sure what is considered "cloud-features"

Another issue is that when an update is available I need to be able to tell Warp that I don’t care at the moment.

Due to the corporate software on my work Mac auto-updates fail. Ordinarily this isn’t a huge deal as I’ll just download the DMG and replace it manually. But Warp drops down an obtrusive overlay and you can’t dismiss it until you update.

I tried to like it, but little forced choices like that sent me back to iTerm 2.

Hi. I definitely took some liberties with terms here. For example, I said "I/O driver" instead of, say UART driver because I didn't want to get overly specific on the historical part. That wasn't the focus of this article. Thanks for linking this other one, as it fills in those parts well.

That's quite an unnecessarily toxic comment, besides all your criticism seems fixated with the introduction on teletypes.

I enjoyed the blogpost, I trust the authors of a terminal emulator to give me a good overview on the topic and I enjoyed it.

I’ve seen many times descriptions of how original teletypes work and how that influences modern terminals.

However, something that I have not seen yet and I’d love to check is an attempt to redesign the terminal from scratch separating historical baggage from the parts that still make sense nowadays.

How would a designer create this tool without the historical precedence? Is tradition holding us back? What new standards could we reach ?

> This model was introduced with UNIX

The UNIX IO system was adapted from Multics.

> That article starts with too many inaccuracies to recommend it to anybody

Why would you even read it after reading the title?