Expecting a item from a list of possibles

Archive - Originally posted on "The Horse's Mouth" - 2008-02-04 23:01:08 - Graham Ellis
There are times (and they're quite frequent!) that I'm asked a good question that's so good it's worth sharing the answer. Actually, that makes up a fair proportion of my inspirations for this spot, such as today's.

In Expect - the command line automation tool of the Tcl language - you can tell the program to wait until it receives a particular string, or you can code a series of strings (and globs and regular expressions) that you want it to wait for. But what if you don't want to [b]code[/b] the things it's to wait for - you want to read them from a file or otherwise vary them at run time?

I have two solutions - both standard computing techniques, but both techniques which may be outside the experience of the newer user to Tcl and Expect.

Option 1 Build the list of options into a regular expression, and expect from that - using expect_out(0,string) to identify which of the options was actually matched.

See Complete example

Option 2 Build a complete expect parameter into a string, and then pass that into expect. You'll need to use an eval to do this so that your variable is interpreted.

See Complete example

Here's an example of one of those scripts in action:

dolphin:~ graham$ expect wuff

dog cow

0: I got a dog

1: I got a cow

and also a giraffe and a hen and

3: I got a hen

so on we go

unil dolphin:~ graham$

where I was looking for any one of a series of 3 letter animals built into a list:

set possible {cat dog cow pig hen}

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Expecting a item from a list of possibles