mjhjmtu

One thing with the mysql program caught my attention. It can be told to ask for password interactively with the -p flag, in which case, as you would expect, a prompt appears in your terminal and whatever is put in is accepted as the password. What surprised me though is that this happens even if you redirect all three of stdin,out,err:

$ mysql -p </dev/null >/dev/null 2>/dev/null
Enter password:

It reads the password properly too. If you redirect stderr to a file, you can check if the login failed.

How does the process know which terminal to ask for the password on? Does it check for attached terminals to parent processes?

A unix process can read from /dev/tty, and so circumvent redirection.

It uses getpass() from libc (where available) which is described in man pages as follows:

The getpass() function opens /dev/tty (the controlling terminal of the process), outputs the string prompt, turns off echoing, reads one line (the "password"), restores the terminal state and closes /dev/tty again.

It can call the isatty unistd function.

NAME
 isatty - test whether a file descriptor refers to a terminal

SYNOPSIS
 #include <unistd.h>

 int isatty(int fd);

DESCRIPTION
 The isatty() function tests whether fd is an open file descriptor
 referring to a terminal.

Probably, as @Gerard H. Pille noted, mysql doesn't check at all but simply uses /dev/tty when you call it with -p.

Some additional detail

I checked into glibc sources the isatty implementation.

It simply uses the tcgetattr function to get the terminal capabilities of the file descriptor. If the function returns true, then it is a terminal.