Exploring Expect

Overriding The Original Return Value

If Expect is evaluating a Tcl or Expect procedure or command when a signal occurs, it is possible to change the return code that would otherwise be returned. Given a -code flag, the trap command substitutes the return code of the trap handler for the return code that would have been returned. For example, a break command in the handler causes the interrupted loop to break. A return command causes the interrupted procedure to return. And a normal return causes a command that is failing to succeed.

Clearly, this can be very confusing and disruptive to normal script flow, so you should avoid using it if possible. However, there are valid uses. For example, you can force an interpreter command to stop what it is doing and reprompt. This can be done on ^C using the following command:

trap -code {
    error unwound -nostack
} SIGINT

The error command generates an error and the -code flag forces the error to override whatever code would have been returned. The precise handling of error in this context is further described in Chapter 9 (p. 224).

If no command is in execution when the signal occurs, the return code is thrown away. In vanilla Expect (with no change from the way it is distributed), a command is always in execution, but when using Expect with Tk, there can be times when no command is in execution.