Exploring Expect

Running Expect Without A Controlling Terminal

When Expect has no controlling terminal, you must avoid using tty_spawn_id. And if the standard input, standard output, and standard error have not been redirected, expect_user, send_user, and send_error will not work. Lastly, a stty command without redirection will always fail.

If the process has been started from cron, there are yet more caveats. By default, cron does not use your environment, so you may need to force cron to use it or perhaps explicitly initialize parts of your environment. For example, the default path supplied by cron usually includes only /bin and /usr/bin. This is almost always insufficient.

Be prepared for all sorts of strange things to happen in the default cron environment. For example, many programs (e.g., rn, telnet) crash or hang if the TERM environment variable is not set. This is a problem under cron which does not define TERM. Thus, you must set it explicitly—to what type is usually irrelevant. It just has to be set to something!

Environment variables can be filled in by appropriate assignments to the global array env. Here is an assignment of TERM to a terminal type that should be understood at any site.

set env(TERM) vt100

Later, I will describe how to debug problems that arise due to the unusual environment in cron.