14 ms·

What is TTY?

240 points by arunc 4y ago

arpa 4y ago
recently i tasked myself into creating a bash TUI library/framework, thinking it would be as easy as taking good old pascal in late 1990s and just drawing on screen using ascii. Boy was I mistaken!
- rascul 4y ago
  Something like dialog or whiptail? Dialog uses ncurses and whiptail uses s-lang.
  - arpa 4y ago
    yes, but no underlying libs, just pure bash... "How hard can that be"...
    - kevin_thibedeau 4y ago
      PC TUIs have the advantage of direct access to the character buffer. Doing it indirectly with control codes is much more involved when it comes to tracking terminal state.
  - Aloha 4y ago
    Using dialog is.. not comfortable - its a great tool, but its hard.
davegauer 4y ago
This is an excellent and entertaining intro to the world of TTYs and I think the first question is a particularly clever way to introduce it: > The answer is Ctrl+W. Because Ctrl+W is provided by something called TTY, and the other three are provided by the shell. My only criticism is a pet peeve of mine: screenshots of terminals. I know it's more work to copy/paste and make sure formatting is correct, and you lose the ANSI colors. But it just feels so wasteful and inaccessible (<img alt="tty">).
- Ennea 4y ago
  You could preserve the colors with some CSS, even :) I have the sudden urge to write a small tool/some CSS that can automatically convert bits of terminal output into ready-to-use fancy HTML+CSS.
  - rascul 4y ago
    Something like aha? (It's probably in your distro's repo) https://github.com/theZiz/aha/ https://github.com/theZiz/aha/
    - Ennea 4y ago
      I knew that somebody would already have something ready that does this :) Thanks!
- rascul 4y ago
  You can use the script(1) utility (from util-linux) to record a session and it will save the ANSI colors. This only works if you do it ahead of time, though. And if you're putting it in a pastebin, the color codes will get in the way.
  - blueflow 4y ago
    There exists tooling to convert these typescripts into, for example, HTML output to include on your website. I use the script(1) util as part of my dayjob to create a bit of a trail when there are questions later. Some terminal emulators also support dumping the buffer as HTML.
    - rascul 4y ago
      > There exists tooling to convert these typescripts into, for example, HTML output to include on your website. Indeed, I mentioned aha in another comment. > Some terminal emulators also support dumping the buffer as HTML. That's a nifty feature that I haven't run into yet. Seems like it could be quite useful.
bluetomcat 4y ago
It was probably a good abstraction for its time. The driver for the particular terminal was abstracted away in the OS, meaning that programs only had to handle a pre-defined set of signals and IO. It is, however, a needlessly complex abstraction as of now. The lack of a real terminal device means that the OS maintains such pseudo devices for the sole purpose of being able to spawn multiple virtual terminals.
- JonathonW 4y ago
  But it's a convenient least-common-denominator and abstraction layer for text-mode applications. We might not be using physical teletypewriters now (or VT100-style video terminals), but it's really convenient to be able to write an application once and know that it's going to just work whether I'm running it in a text-mode console or in xterm or in gnome-terminal or over ssh or in tmux within any of those or... Sure, we could come up with some other standard, but the TTY interface is one we already have, with a huge amount of code already written to work with it; why fix what isn't broken? Contrast this with Windows; until recently, command-line applications there were pretty tightly coupled to the console host that they ran within [1]; this made remoting of command-line applications difficult and means you had to go through extraordinary hoops just to do things like replace the system's console UI with something like ConEmu or run a terminal within something like Visual Studio Code. More recent versions of Windows try to improve this situation, by adding support for Unix-style pseudoterminals [2] (what they call ConPTY) to the console subsystem. And since ConPTY terminals are based around the same standards as TTY terminals on *nix, things like OpenSSH remoting can now "just work", even for existing command line apps on Windows. [1] See here for some of the gory details of how this works and how it differs from the conventional *nix approach: https://devblogs.microsoft.com/commandline/windows-command-line-inside-the-windows-console/ https://devblogs.microsoft.com/commandline/windows-command-l... [2] Blog post describing the PTY infrastructure on Windows: https://devblogs.microsoft.com/commandline/windows-command-line-introducing-the-windows-pseudo-console-conpty/ https://devblogs.microsoft.com/commandline/windows-command-l...
  - zokier 4y ago
    > why fix what isn't broken? Its brokenness is debatable, see for example https://lwn.net/Articles/343828/ https://lwn.net/Articles/343828/
commonlisper 4y ago
> Ctrl+B to move a letter forward Did they mean Ctrl+B to move a letter backward? Ctrl+F moves a letter forward.
rascul 4y ago
Kind of related, Textmode Window Environment is interesting. https://github.com/cosmos72/twin https://github.com/cosmos72/twin A couple screenshots are on the old SourceForge site. https://sourceforge.net/projects/twin/ https://sourceforge.net/projects/twin/
np_tedious 4y ago
Further evidence that I am so clueless about the workings of many tools I use every day
- GrabbinD33ze69 4y ago
  I mean modern computing is built upon many layers of abstraction, where you only need to understand the technicalities of the thing you're specifically working on, and the tech you don't have time to focus on will seem like "magic" due to abstraction.
zokier 4y ago
> Because Ctrl+W is provided by something called TTY, and the other three are provided by the shell This is more complicated (or simpler?) than what the article explains. Or I might be really confused. But if you strace bash while typing in stuff, you see clearly that it is reading in characters one by one, including ^W, and handling them itself. I don't see TTY doing much magic here at all: read(0, "h", 1) = 1 write(2, "h", 1) = 1 read(0, "e", 1) = 1 write(2, "e", 1) = 1 read(0, "l", 1) = 1 write(2, "l", 1) = 1 read(0, "l", 1) = 1 write(2, "l", 1) = 1 read(0, "o", 1) = 1 write(2, "o", 1) = 1 read(0, " ", 1) = 1 write(2, " ", 1) = 1 read(0, "w", 1) = 1 write(2, "w", 1) = 1 read(0, "o", 1) = 1 write(2, "o", 1) = 1 read(0, "r", 1) = 1 write(2, "r", 1) = 1 read(0, "l", 1) = 1 write(2, "l", 1) = 1 read(0, "d", 1) = 1 write(2, "d", 1) = 1 read(0, "\x17", 1) = 1 <-- bash reading in ^W! write(2, "\x08\x08\x08\x08\x08\x1b\x5b\x4b", 8) = 8
- zokier 4y ago
  stracing bash more reveals another thing; when executing command like stty then before forking bash calls TCSETSW ioctl to set tty settings, and after child exists its calls TCSETSW again to set its own values, so stty does not actually show what mode the tty is in while bash is handling the input: ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B38400 opost isig icanon echo ...}) = 0 clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fe653abef10) = 5175 wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 5175 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=5175, si_uid=1000, si_status=0, si_utime=0, si_stime=0} --- wait4(-1, 0x7fff156cbad0, WNOHANG|WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes) ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B38400 opost isig -icanon -echo ...}) = 0 https://man7.org/linux/man-pages/man4/tty_ioctl.4.html https://man7.org/linux/man-pages/man4/tty_ioctl.4.html
teddyh 4y ago
I prefer this: https://www.linusakesson.net/programming/tty/ https://www.linusakesson.net/programming/tty/ In fact, the image with the “hardware” and “software” diagrams seems to have been copied directly from that link. I did not see any attribution or acknowledgement.
- knorker 4y ago
  It can be good to write things down for yourself, to think them through. But I agree that to actually teach others Linus describes it much better, without the sprinkling of errors this post has. Like for example the author confuses the terminal for the shell. ctrl-w very much is passed right through to bash. But the author is correct if the user types "cat\n". Now it's the terminal handling ctrl-w. But note that cat doesn't handle ctrl-a. It's more complex than the author seems to think. tl;dr: read the other post, not this one.
- laixintao 4y ago
  Hi teddyh, like wey-gu mentioned, I am the author of this original Chinese version post, I think this English version was translated by machine instead of human. You can see from the original post[0], I add a link, a description and credits to every image that not created by me. However, I accept all other technical criticism under this threads, I didn't dig deep enough while writing this. I will learn more. 0: https://www.kawabangga.com/posts/4515 https://www.kawabangga.com/posts/4515
throwaway09223 4y ago
bash is a bad example for these types of interview questions because, as others have noted, bash disables many of these tty features and handles them in-process. Another example: The author touches on line discipline, but then makes the mistake of saying "The command is actually stored in the TTY after it is typed and before the Enter key is pressed" This isn't true in bash, or on any modern shells that support line editing features like tab completion. If you're going to use this for an interview question, I suggest using cat(1) instead -- it will behave as the author suggests.
cebert 4y ago
I would hope this wasn’t a software engineering interview question. I’ve been in industry a while, and would have no idea what to say if asked about this in an interview.
- DethNinja 4y ago
  It would be a good red flag to stay away from that company, unless they were producing something very specific to Unix terminals.
decasia 4y ago
I mean, if someone asked me about this in an interview, I would say something like "As a working software developer, what I can tell you is that sometimes the line discipline gets broken and borks the line endings, and I know how to reset it at that point, and that is the only reason I normally have to think about this particular layer of the whole shell software stack, but if I ever had a professional reason to look into it in more detail, at least I do know where to start digging."
McNutty 4y ago
No mention of PuTTY?
deleted 4y ago
[deleted]
wey-gu 4y ago
This is a translation of a Chinese blog[0] without mentioning the origin. See this tweet [1] Update: it even didn’t change its example of using original blog’s domain name. [2] 0: https://www.kawabangga.com/posts/4515 https://www.kawabangga.com/posts/4515 1: https://twitter.com/laixintao/status/1558667661420404737 https://twitter.com/laixintao/status/1558667661420404737 2: > Imagine a scenario where you break into someone’s machine, such as the server where kawabangga.com is located, and you find a way to execute python code inside it, but you can only inject the code into it and execute it without seeing the output, what do you do?
ksherlock 4y ago
> Among these 4 shortcuts, there is one that is implemented differently from the others, which one is it? Ctrl-A vs Ctrl-E vs Ctrl-W vs up arrow. The correct answer is the up arrow and any interviewer who disagrees will walk out of the room with a broken nose. Bash, readline, etc, operate in raw mode so the tty driver isn't handling anything. Three are control characters - a single character. Meanwhile, a vt100 can generate 2 different sequences for the up arrow -- ESC [ A or ESC O A -- depending on cursor key mode. Either way, that's 3 characters instead 1 character. (There's also a third sequence -- ESC A -- in vt52 mode but we'll ignore that).

240 points by arunc 4y ago

arpa 4y ago

recently i tasked myself into creating a bash TUI library/framework, thinking it would be as easy as taking good old pascal in late 1990s and just drawing on screen using ascii. Boy was I mistaken!

rascul 4y ago
Something like dialog or whiptail? Dialog uses ncurses and whiptail uses s-lang.
- arpa 4y ago
  yes, but no underlying libs, just pure bash... "How hard can that be"...
  - kevin_thibedeau 4y ago
    PC TUIs have the advantage of direct access to the character buffer. Doing it indirectly with control codes is much more involved when it comes to tracking terminal state.
- Aloha 4y ago
  Using dialog is.. not comfortable - its a great tool, but its hard.

davegauer 4y ago

This is an excellent and entertaining intro to the world of TTYs and I think the first question is a particularly clever way to introduce it: > The answer is Ctrl+W. Because Ctrl+W is provided by something called TTY, and the other three are provided by the shell. My only criticism is a pet peeve of mine: screenshots of terminals. I know it's more work to copy/paste and make sure formatting is correct, and you lose the ANSI colors. But it just feels so wasteful and inaccessible (<img alt="tty">).

Ennea 4y ago
You could preserve the colors with some CSS, even :) I have the sudden urge to write a small tool/some CSS that can automatically convert bits of terminal output into ready-to-use fancy HTML+CSS.
- rascul 4y ago
  Something like aha? (It's probably in your distro's repo) https://github.com/theZiz/aha/ https://github.com/theZiz/aha/
  - Ennea 4y ago
    I knew that somebody would already have something ready that does this :) Thanks!
rascul 4y ago
You can use the script(1) utility (from util-linux) to record a session and it will save the ANSI colors. This only works if you do it ahead of time, though. And if you're putting it in a pastebin, the color codes will get in the way.
- blueflow 4y ago
  There exists tooling to convert these typescripts into, for example, HTML output to include on your website. I use the script(1) util as part of my dayjob to create a bit of a trail when there are questions later. Some terminal emulators also support dumping the buffer as HTML.
  - rascul 4y ago
    > There exists tooling to convert these typescripts into, for example, HTML output to include on your website. Indeed, I mentioned aha in another comment. > Some terminal emulators also support dumping the buffer as HTML. That's a nifty feature that I haven't run into yet. Seems like it could be quite useful.

bluetomcat 4y ago

It was probably a good abstraction for its time. The driver for the particular terminal was abstracted away in the OS, meaning that programs only had to handle a pre-defined set of signals and IO. It is, however, a needlessly complex abstraction as of now. The lack of a real terminal device means that the OS maintains such pseudo devices for the sole purpose of being able to spawn multiple virtual terminals.

JonathonW 4y ago
But it's a convenient least-common-denominator and abstraction layer for text-mode applications. We might not be using physical teletypewriters now (or VT100-style video terminals), but it's really convenient to be able to write an application once and know that it's going to just work whether I'm running it in a text-mode console or in xterm or in gnome-terminal or over ssh or in tmux within any of those or... Sure, we could come up with some other standard, but the TTY interface is one we already have, with a huge amount of code already written to work with it; why fix what isn't broken? Contrast this with Windows; until recently, command-line applications there were pretty tightly coupled to the console host that they ran within [1]; this made remoting of command-line applications difficult and means you had to go through extraordinary hoops just to do things like replace the system's console UI with something like ConEmu or run a terminal within something like Visual Studio Code. More recent versions of Windows try to improve this situation, by adding support for Unix-style pseudoterminals [2] (what they call ConPTY) to the console subsystem. And since ConPTY terminals are based around the same standards as TTY terminals on *nix, things like OpenSSH remoting can now "just work", even for existing command line apps on Windows. [1] See here for some of the gory details of how this works and how it differs from the conventional *nix approach: https://devblogs.microsoft.com/commandline/windows-command-line-inside-the-windows-console/ https://devblogs.microsoft.com/commandline/windows-command-l... [2] Blog post describing the PTY infrastructure on Windows: https://devblogs.microsoft.com/commandline/windows-command-line-introducing-the-windows-pseudo-console-conpty/ https://devblogs.microsoft.com/commandline/windows-command-l...
- zokier 4y ago
  > why fix what isn't broken? Its brokenness is debatable, see for example https://lwn.net/Articles/343828/ https://lwn.net/Articles/343828/

commonlisper 4y ago

> Ctrl+B to move a letter forward Did they mean Ctrl+B to move a letter backward? Ctrl+F moves a letter forward.

rascul 4y ago

Kind of related, Textmode Window Environment is interesting. https://github.com/cosmos72/twin https://github.com/cosmos72/twin A couple screenshots are on the old SourceForge site. https://sourceforge.net/projects/twin/ https://sourceforge.net/projects/twin/

np_tedious 4y ago

Further evidence that I am so clueless about the workings of many tools I use every day

GrabbinD33ze69 4y ago
I mean modern computing is built upon many layers of abstraction, where you only need to understand the technicalities of the thing you're specifically working on, and the tech you don't have time to focus on will seem like "magic" due to abstraction.

zokier 4y ago

> Because Ctrl+W is provided by something called TTY, and the other three are provided by the shell This is more complicated (or simpler?) than what the article explains. Or I might be really confused. But if you strace bash while typing in stuff, you see clearly that it is reading in characters one by one, including ^W, and handling them itself. I don't see TTY doing much magic here at all: read(0, "h", 1) = 1 write(2, "h", 1) = 1 read(0, "e", 1) = 1 write(2, "e", 1) = 1 read(0, "l", 1) = 1 write(2, "l", 1) = 1 read(0, "l", 1) = 1 write(2, "l", 1) = 1 read(0, "o", 1) = 1 write(2, "o", 1) = 1 read(0, " ", 1) = 1 write(2, " ", 1) = 1 read(0, "w", 1) = 1 write(2, "w", 1) = 1 read(0, "o", 1) = 1 write(2, "o", 1) = 1 read(0, "r", 1) = 1 write(2, "r", 1) = 1 read(0, "l", 1) = 1 write(2, "l", 1) = 1 read(0, "d", 1) = 1 write(2, "d", 1) = 1 read(0, "\x17", 1) = 1 <-- bash reading in ^W! write(2, "\x08\x08\x08\x08\x08\x1b\x5b\x4b", 8) = 8

zokier 4y ago
stracing bash more reveals another thing; when executing command like stty then before forking bash calls TCSETSW ioctl to set tty settings, and after child exists its calls TCSETSW again to set its own values, so stty does not actually show what mode the tty is in while bash is handling the input: ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B38400 opost isig icanon echo ...}) = 0 clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fe653abef10) = 5175 wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 5175 --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=5175, si_uid=1000, si_status=0, si_utime=0, si_stime=0} --- wait4(-1, 0x7fff156cbad0, WNOHANG|WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes) ioctl(0, SNDCTL_TMR_STOP or TCSETSW, {B38400 opost isig -icanon -echo ...}) = 0 https://man7.org/linux/man-pages/man4/tty_ioctl.4.html https://man7.org/linux/man-pages/man4/tty_ioctl.4.html

teddyh 4y ago

I prefer this: https://www.linusakesson.net/programming/tty/ https://www.linusakesson.net/programming/tty/ In fact, the image with the “hardware” and “software” diagrams seems to have been copied directly from that link. I did not see any attribution or acknowledgement.

knorker 4y ago
It can be good to write things down for yourself, to think them through. But I agree that to actually teach others Linus describes it much better, without the sprinkling of errors this post has. Like for example the author confuses the terminal for the shell. ctrl-w very much is passed right through to bash. But the author is correct if the user types "cat\n". Now it's the terminal handling ctrl-w. But note that cat doesn't handle ctrl-a. It's more complex than the author seems to think. tl;dr: read the other post, not this one.
laixintao 4y ago
Hi teddyh, like wey-gu mentioned, I am the author of this original Chinese version post, I think this English version was translated by machine instead of human. You can see from the original post[0], I add a link, a description and credits to every image that not created by me. However, I accept all other technical criticism under this threads, I didn't dig deep enough while writing this. I will learn more. 0: https://www.kawabangga.com/posts/4515 https://www.kawabangga.com/posts/4515

throwaway09223 4y ago

bash is a bad example for these types of interview questions because, as others have noted, bash disables many of these tty features and handles them in-process. Another example: The author touches on line discipline, but then makes the mistake of saying "The command is actually stored in the TTY after it is typed and before the Enter key is pressed" This isn't true in bash, or on any modern shells that support line editing features like tab completion. If you're going to use this for an interview question, I suggest using cat(1) instead -- it will behave as the author suggests.

cebert 4y ago

I would hope this wasn’t a software engineering interview question. I’ve been in industry a while, and would have no idea what to say if asked about this in an interview.

DethNinja 4y ago
It would be a good red flag to stay away from that company, unless they were producing something very specific to Unix terminals.

decasia 4y ago

I mean, if someone asked me about this in an interview, I would say something like "As a working software developer, what I can tell you is that sometimes the line discipline gets broken and borks the line endings, and I know how to reset it at that point, and that is the only reason I normally have to think about this particular layer of the whole shell software stack, but if I ever had a professional reason to look into it in more detail, at least I do know where to start digging."

McNutty 4y ago

No mention of PuTTY?

deleted 4y ago

[deleted]

wey-gu 4y ago

This is a translation of a Chinese blog[0] without mentioning the origin. See this tweet [1] Update: it even didn’t change its example of using original blog’s domain name. [2] 0: https://www.kawabangga.com/posts/4515 https://www.kawabangga.com/posts/4515 1: https://twitter.com/laixintao/status/1558667661420404737 https://twitter.com/laixintao/status/1558667661420404737 2: > Imagine a scenario where you break into someone’s machine, such as the server where kawabangga.com is located, and you find a way to execute python code inside it, but you can only inject the code into it and execute it without seeing the output, what do you do?

ksherlock 4y ago

> Among these 4 shortcuts, there is one that is implemented differently from the others, which one is it? Ctrl-A vs Ctrl-E vs Ctrl-W vs up arrow. The correct answer is the up arrow and any interviewer who disagrees will walk out of the room with a broken nose. Bash, readline, etc, operate in raw mode so the tty driver isn't handling anything. Three are control characters - a single character. Meanwhile, a vt100 can generate 2 different sequences for the up arrow -- ESC [ A or ESC O A -- depending on cursor key mode. Either way, that's 3 characters instead 1 character. (There's also a third sequence -- ESC A -- in vt52 mode but we'll ignore that).