Exploring Expect

Comments In expect Commands

It is tempting to add comments after patterns and actions. However, it cannot be done arbitrarily. For example, the following example does not do what was intended.

expect {
    "orange" squeeze_proc   ;# happens every morning
    "banana" peel_proc
}

The problem in this code fragment is that the comment occurs in a place where the expect command is expecting patterns and actions. The expect command does not have any special way of handling comments. In this example, the expect command simply assumes that ";#" is a pattern and happens is the associated action. every and morning are also interpreted as a pattern and action.

This particular comment is rather lucky. The pattern banana is still used as a pattern. However, if the comment had one more word in it, banana would be used as an action and peel_proc as a pattern!

Remember that comments behave a lot like commands.^[31] They can only be used where commands can be used. If you want to associate a comment with an action, then use braces to create a list of commands and embed both the comment and the action within the list. In the following fragment, all of the comments are safe.

expect {
    "orange" {
        # comments can appear here safely, too
        squeeze_proc   ;# happens every morning
        # comments can appear here safely, too
    }

    "banana" peel_proc
}

^[31] The only significant difference between comments and commands is that arguments of a comment are not evaluated.