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.
Tagged . All content copyright James Fisher 2017. This post is not associated with my employer.
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