Expect makes use of a number of features that are present in all UNIX systems. The family of standards known as “POSIX” describes most but not all of these features. So while Expect can run on any UNIX system, it may have trouble providing all of its features on non-UNIX systems that nonetheless claim strict POSIX compliance. While Expect works just fine on some non-UNIX POSIX systems and can work in a limited way on all POSIX systems, I prefer to be conservative in my claims, so I use the phrase “UNIX systems” when referring to the systems on which Expect runs.
Fortunately, Expect can be used to control other operating systems indirectly. Since Expect is capable of making network connections (through telnet, rlogin, tip, etc.), it can remotely contact other computers even while running on a UNIX computer. In this way, it is very common to use Expect scripts to control non-UNIX computers such as Lisp machines, PostScript printers, modems, pagers, etc.
Testing and setting modems and other network devices (routers, bridges, etc.) is a particularly popular use of Expect. It is possible to write scripts that regularly test sets of modems to make sure that they are functional and that previous users have not left them in a bad state. Such scripts can even simulate a real user—placing a phone call, connecting to a remote host, and even logging in. An Expect script can remember the port numbers and other trivia that users do not bother to note until they have scrolled off the screen. So if a line turns out to be faulty, the script can record this and other information to a log.
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