5 ms·

Anatomy of a terminal emulator (2021)

93 points by alex_hirner 5y ago

atarian 5y ago
Interesting I didn't know that terminal sessions were files. So if I was root on a shared host, I could theoretically hijack and listen to what other people are doing in their SSH sessions.
- deathanatos 5y ago
  Yes, but if you're root, you could also peer into the terminal emulator's memory space itself, and see that, too. Or into their sshd process, and pull the plaintext out of that. You're root. You might also find the man pages wall(1) and write(1) (That's the terminal utility write, not the syscall write(2).) interesting. (Edit: well, actually, if you're SSH'd into a remote host, your terminal emulator is probably local, so you might not be able to see into its memory unless you're root locally, ofc.)
- RL_Quine 5y ago
  By definition as root you can, irrespective of if they are files or not. You have complete access to the memory, networking, and code execution state of the machine.
eterps 5y ago
Wow, high quality blog post.
nothrowaways 5y ago
Cool domain.
- teddyh 5y ago
  The .dev TLD is owned by Google.
deathanatos 5y ago
> The new process [ls -l] gets the desired data, sends it to the shell and terminates. > The shell sends the data to the terminal through the STDOUT side of the pty Unless redirected, ls inherits the pty file descriptor from the shell¹ when the shell forks it. But after that, it's writing to the terminal emulator directly, not to the shell; that is, the shell doesn't forward `ls -l`'s output to the terminal. ¹(or really, the child inherits whatever file descriptor the shell has for stdin, stdout, stderr for its stdin, stdout, stderr, respectively, unless, for each of those, there is a redirection specified, in which case the shell replaces the file descriptor in the child as specified by the command give to the shell. stdin/stdout/stderr might be a pty in the shell, or they might not be.)
- imsnif 5y ago
  True, and thanks for pointing this out. But this example is there to illustrate the basic relationship of shell<->terminal-emulator without going in to too much details that might confuse those readers for whom it is intended.
georgia_peach 5y ago
I feel this is the only article distinguishing this place from CNN at the moment.
georgia_peach 5y ago
An excellent article in his "Further Reading" section: https://www.linusakesson.net/programming/tty/ https://www.linusakesson.net/programming/tty/
danellis 5y ago
I was surprised by the assumption of ANSI and no mention of $TERM.
- imsnif 5y ago
  Fair point, but this post was mostly to tell how things are today. And these days most (all?) terminals the reader will interact with would be ANSI.
westoncb 5y ago
It's kind of incredible that after ~20 years of programming I'd never come across anything illuminating this breakdown: > The terminal emulator is a graphical application whose role it is to interpret data coming from the shell and display it on screen. This display is often textual but not always. > The shell provides an interface to the operating system, allowing the user to interact with its file-system, run processes and often have access to basic scripting capabilities. > These two programs are connected together by the pty (pseudoterminal) which provides a bi-directional asynchronous communication channel between the two.
- kevstev 5y ago
  Did you have a formal cs education? This was covered in my operating systems class- we had to write a basic shell as an assignment.
  - westoncb 5y ago
    Yep, I did, though I didn't take an OS class. What's significant about the article (imo) is in pedagogy. I've never been particularly interested in OS implementation details or shell programming, but just because it's around I have some level of curiosity and have wondered something roughly like, "why doesn't anyone ever just say what a shell/terminal is?"—basically I was looking exactly for a concise architectural breakdown like the author gave, just as a brief answer to a quick question, and hadn't come across it until now.
    - kevstev 5y ago
      Interesting, it was a required class in my program. Writing a shell is definitely a useful exercise, you may want to give it a go. We focused more on signal handling and forking/exec'ing processes though and less on display features, though from what I recall there was some raw/canonical mode stuff we had to deal with.
      - westoncb 5y ago
        In my program we could do either compilers, OSes, or networks. That does sound like a fun project though—if I wanted to learn more in the area, building a toy shell is definitely the direction I'd go.
dang 5y ago
Anatomy of a Terminal Emulator - https://news.ycombinator.com/item?id=29080561 https://news.ycombinator.com/item?id=29080561 - Nov 2021 (70 comments)

93 points by alex_hirner 5y ago

Interesting I didn't know that terminal sessions were files. So if I was root on a shared host, I could theoretically hijack and listen to what other people are doing in their SSH sessions.

deathanatos 5y ago
Yes, but if you're root, you could also peer into the terminal emulator's memory space itself, and see that, too. Or into their sshd process, and pull the plaintext out of that. You're root. You might also find the man pages wall(1) and write(1) (That's the terminal utility write, not the syscall write(2).) interesting. (Edit: well, actually, if you're SSH'd into a remote host, your terminal emulator is probably local, so you might not be able to see into its memory unless you're root locally, ofc.)
RL_Quine 5y ago
By definition as root you can, irrespective of if they are files or not. You have complete access to the memory, networking, and code execution state of the machine.

eterps 5y ago

Wow, high quality blog post.

nothrowaways 5y ago

Cool domain.

teddyh 5y ago
The .dev TLD is owned by Google.

deathanatos 5y ago

> The new process [ls -l] gets the desired data, sends it to the shell and terminates. > The shell sends the data to the terminal through the STDOUT side of the pty Unless redirected, ls inherits the pty file descriptor from the shell¹ when the shell forks it. But after that, it's writing to the terminal emulator directly, not to the shell; that is, the shell doesn't forward `ls -l`'s output to the terminal. ¹(or really, the child inherits whatever file descriptor the shell has for stdin, stdout, stderr for its stdin, stdout, stderr, respectively, unless, for each of those, there is a redirection specified, in which case the shell replaces the file descriptor in the child as specified by the command give to the shell. stdin/stdout/stderr might be a pty in the shell, or they might not be.)

imsnif 5y ago
True, and thanks for pointing this out. But this example is there to illustrate the basic relationship of shell<->terminal-emulator without going in to too much details that might confuse those readers for whom it is intended.

georgia_peach 5y ago

I feel this is the only article distinguishing this place from CNN at the moment.

georgia_peach 5y ago

An excellent article in his "Further Reading" section: https://www.linusakesson.net/programming/tty/ https://www.linusakesson.net/programming/tty/

danellis 5y ago

I was surprised by the assumption of ANSI and no mention of $TERM.

imsnif 5y ago
Fair point, but this post was mostly to tell how things are today. And these days most (all?) terminals the reader will interact with would be ANSI.

westoncb 5y ago

It's kind of incredible that after ~20 years of programming I'd never come across anything illuminating this breakdown: > The terminal emulator is a graphical application whose role it is to interpret data coming from the shell and display it on screen. This display is often textual but not always. > The shell provides an interface to the operating system, allowing the user to interact with its file-system, run processes and often have access to basic scripting capabilities. > These two programs are connected together by the pty (pseudoterminal) which provides a bi-directional asynchronous communication channel between the two.

kevstev 5y ago
Did you have a formal cs education? This was covered in my operating systems class- we had to write a basic shell as an assignment.
- westoncb 5y ago
  Yep, I did, though I didn't take an OS class. What's significant about the article (imo) is in pedagogy. I've never been particularly interested in OS implementation details or shell programming, but just because it's around I have some level of curiosity and have wondered something roughly like, "why doesn't anyone ever just say what a shell/terminal is?"—basically I was looking exactly for a concise architectural breakdown like the author gave, just as a brief answer to a quick question, and hadn't come across it until now.
  - kevstev 5y ago
    Interesting, it was a required class in my program. Writing a shell is definitely a useful exercise, you may want to give it a go. We focused more on signal handling and forking/exec'ing processes though and less on display features, though from what I recall there was some raw/canonical mode stuff we had to deal with.
    - westoncb 5y ago
      In my program we could do either compilers, OSes, or networks. That does sound like a fun project though—if I wanted to learn more in the area, building a toy shell is definitely the direction I'd go.

dang 5y ago

Anatomy of a Terminal Emulator - https://news.ycombinator.com/item?id=29080561 https://news.ycombinator.com/item?id=29080561 - Nov 2021 (70 comments)