Exploring Expect

Example—chess Versus chess

Very useful results can be produced by communicating with multiple processes. A simple but amusing example is the problem of having one chess process play a second chess process. In order to accomplish this, the standard output of one process must be connected to the standard input of another, and vice versa.

Figure 10-1. 

As an Expect script, the basic idea might be implemented this way:

set timeout −1

spawn chess                  ;# start player one
set chess1 $spawn_id

spawn chess                  ;# start player two
set chess2 $spawn_id

while 1 {
    expect "(.*)\n"            ;# read move
    set spawn_id $chess1
    send $expect_out(1,string) ;# send move to other
                               ;# player

    expect "(.*)\n"            ;# read response
    set spawn_id $chess2
    send $expect_out(1,string) ;# send back
}

The first four lines start two chess processes and save the respective spawn ids. Then the script loops. The loop starts by reading a move from the first process. spawn_id is changed to the second process, and the move is sent there. The response is collected, spawn_id is set back to the original chess process, and the response is sent back to the first process. The loop repeats, allowing moves to go back and forth.

Alas, the UNIX chess program was not intended to read its own output, so the output has to be massaged a little before being used as input.^[40] Oddly, the program prints out moves differently depending on if it goes first or second. ...