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Expecting a item from a list of possibles

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}
(written 2008-02-04, updated 2008-02-05)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T211 - Tcl/Tk - What is Expect? Why use it?
  [4678] Expect with Ruby - a training example to get you started - (2016-05-18)
  [4405] Backup procedures - via backup server - (2015-01-24)
  [3572] Adding Expect on top of Tcl - what is it and where can I get a training course to learn about it? - (2012-01-08)
  [3286] Should we cover expect and/or Tk on our public Tcl courses? - (2011-05-11)
  [3009] Expect in Perl - a short explanation and a practical example - (2010-10-22)
  [2489] Parallel Pinging, using Python Threads or Expect spawn lists - (2009-11-02)
  [2474] Using Tcl and Expect to automate repetitive jobs - (2009-10-24)
  [1602] Automating processes through Expect - (2008-04-05)
  [1469] Curley brackets v double quotes - Tcl, Tk, Expect - (2007-12-12)
  [1411] Buffering of inputs to expect, and match order - (2007-10-27)
  [1409] What is Expect? - (2007-10-26)
  [1174] Installing Tcl and Expect on Solaris 10 - a checklist - (2007-05-02)
  [1173] Cheat Sheet / Check list for Expect maintainers - (2007-05-02)
  [435] Expect for Windows - (2005-09-04)
  [286] Automating regular manual procedures - (2005-04-21)

T242 - Tcl/Tk - More on Expect
  [3448] Checking all the systems on a subnet, using Expect and Tk - (2011-09-18)
  [2475] Quick easy and dangerous - automated logins via Tcl / Expect - (2009-10-24)
  [1475] Tcl/Tk - updating your display while tasks are running - (2007-12-16)


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