How does Linux Expect script work?

Question

I ever tried to input password by I/O redirection like echo <password> | ssh <user>@<host> echo <password> | ssh <user>@<host> , but it didn't work of course. Then I got that ssh actually reads password directly from /dev/tty instead of STDIN, so I/O redirection doesn't work for it.

As far as I know, Expect script is the standard way for this kind of job. I'm curious about how Expect works? I guess it runs the target program in a child process, and it changes the /dev/tty of the child process to refer to another place, but I don't know how.

Answer 1

It uses something called a pseudo-TTY which looks to the called program like a TTY, but allows for programmed control. See eg Don Libes' Exploring Expect p498f