What is a TTY?

Open a terminal and run tty. It will give you the unique ID of your terminal:

$ tty
/dev/ttys008

UNIX has this saying, “everything is a file”. Your terminal is just another file in the filesystem! What is its file path? /dev/ttys008, the output of tty.

Your terminal acts quite like a normal file. You can read from it and write to it:

$ echo "Hello, TTY" > /dev/ttys008
Hello, TTY

$ cat /dev/ttys008 > output.txt
Hello, file
~/dev/tmp/tty
$ cat output.txt
Hello, file

Because terminals are in the global filesystem, they can interact with each other. Now open a second terminal, and run echo "Hello, TTY" > /dev/ttys008 again. What happens? The text Hello, TTY appears in the original terminal! You can now chat to other people using the system. This is how programs like wall work.

A process gets three standard streams: stdin, stdout, and stderr. The process can inspect these streams to see which TTY it’s attached to, like this:

#include <unistd.h>
#include <stdio.h>
void print_tty(char* name, FILE * f) {
  printf("%s (fileno %d): ", name, fileno(f));
  if (isatty(fileno(f))) printf("TTY %s\n", ttyname(fileno(f)));
  else                   printf("not a TTY\n");
}
int main(void) {
  print_tty("stdin ", stdin);
  print_tty("stdout", stdout);
  print_tty("stderr", stderr);
}

When you start this process from your shell, all three streams are attached to your current TTY:

$ clang whatismytty.c
$ ./a.out
stdin  (fileno 0): TTY /dev/ttys008
stdout (fileno 1): TTY /dev/ttys008
stderr (fileno 2): TTY /dev/ttys008

We can use shell piping/redirection commands to change which TTYs the streams are attached to.

$ ./a.out | cat
stdin  (fileno 0): TTY /dev/ttys008
stdout (fileno 1): not a TTY
stderr (fileno 2): TTY /dev/ttys008
$ echo | ./a.out
stdin  (fileno 0): not a TTY
stdout (fileno 1): TTY /dev/ttys008
stderr (fileno 2): TTY /dev/ttys008
$ ./a.out < /dev/ttys003
stdin  (fileno 0): TTY /dev/ttys003
stdout (fileno 1): TTY /dev/ttys008
stderr (fileno 2): TTY /dev/ttys008
$ ./a.out < /dev/ttys003 2> /dev/ttys004
stdin  (fileno 0): TTY /dev/ttys003
stdout (fileno 1): TTY /dev/ttys008
stderr (fileno 2): TTY /dev/ttys004

Each process can also have a “controlling terminal”. A process’s controlling terminal is not necessarily the same as the terminals its streams are attached to. You can see these with the ps command:

$ sudo ps -ax -o pid,tty
  PID TTY
    1 ??
   50 ??
   51 ??
...
...
84721 ttys007
84722 ttys007
71296 ttys008
71297 ttys008
71305 ttys008

Also, notice that some processes have ?? as their TTY. Most of these processes are “daemons”, UNIX background processes.

This page copyright James Fisher 2017. Content is not associated with my employer. Found an error? Edit this page.

Jim Fisher

CV

Speaking

Blogroll

RSS

TigYog

Kickabout