Exploring Expect

The send_tty Command

send_tty is analogous to expect_tty. The send_tty command sends its output to the screen in such a way that it cannot be redirected by the shell. send_tty shares the very same advantages and disadvantages of expect_tty.

expect_tty and send_tty work by communicating through a special file called /dev/tty. Expect is one of a few programs that has the power to redirect communications with programs that communicate through /dev/tty. When programs are spawned by Expect, all of their inputs (/dev/tty and unredirected standard input) are merged into a single input to which the send command can write. Similarly, all of their outputs (/dev/tty and any unredirected standard output or standard error) are merged into a single output from which the expect command can read.

Thus, Expect reads and writes from processes with the same view that a real person has. In the same way a person can read all of the unredirected output or write all of the unredirected input, so can Expect. If a human can control a program, then the program can be controlled with Expect also. This is the power of Expect.