Adding Expect on top of Tcl - what is it and where can I get a training course to learn about it?
Expect adds extra commands on top of Tcl - allowing the Tcl programmer to control additional processes as is they were really being run from the command line. But because they're being run from within a program, a whole lot of commands can be run with little effort, responses analysed easily, with the normal command output suppressed and replaced by a short report ...
We don't run a separate Expect course - but we do include Expect in our Learning to program in Tcl course
and our Tcl programming course
. The extra modules that cover the Expect extension account for sme 15% of the course - if Expect is not relevant to any
delegates, we just 'overview' it - otherwise it gets thorough coverage. And that can extend into an extra session on the penultimate evening of the course. For those for whom Expect is important, it is very
Let's see an example - I want to check that various web servers that I look after are online from time to time. I could ping
each in turn, manually - for example:
munchkin:tcl12 grahamellis$ ping -c 3 www.wellho.net
PING www.wellho.net (126.96.36.199): 56 data bytes
64 bytes from 188.8.131.52: icmp_seq=0 ttl=53 time=36.290 ms
64 bytes from 184.108.40.206: icmp_seq=1 ttl=53 time=27.647 ms
64 bytes from 220.127.116.11: icmp_seq=2 ttl=53 time=142.831 ms
--- www.wellho.net ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 27.647/68.923/142.831/52.380 ms
but if I have five or six (or many more computers) to check, that's too much typing, and prone to errors if I forget one of the hosts one time. And I also have to read back some of the data figures and analyse the results. Far better to write an Expect script to report as follows:
munchkin:tcl12 grahamellis$ expect qp
www.wellho.net came back with a trip time of 26.545
wellho.net came back with a trip time of 166.697
SERVER SLOW OR DISTANT
www.wellho.co.uk came back with a trip time of 33.698
www.firstgreatwestern.info came back with a trip time of 34.132
www.sheepbingo.co.uk came back with a trip time of 39.882
whm.wellho.net came back with a trip time of 18.160
In that example, I have checked six hosts and had my Expect script analyse the results returned. Looking at one particular piece of data within the returned report, I've done some arithmetic so see if the server answer time is good, and if it isn't, I've flagged that there's a potential issue.
Taking this script a stage further, I could run it every hour under cron
so that it becomes a monitoring tool, and have it send an alert / text / email to the system admin of servers which fail to respond (we actually have a scheme like this in place - not only crosschecking servers, but also checking that our hotel and office networks are online even when we're not physically present).
The two main extra lines that I've added to what is otherwise just a Tcl script are:
spawn ping -c 3 $mysite
t instigate each ping and
to wait for the returned information (up to the text "round-trip") which can then be analysed.
command returns a whole load of information in the expect_out
global array, and in particular my program looked at $expect_out(buffer)
member, which contains the text received prior to the words "round-trip".
A final extra note on this program - there's a line
in it. This sets a flag telling expect NOT to echo all spawned command output on the screen. When you're testing an Expect program, you need to see what's happening (in case the exchange hangs up because expect never receives what it's waiting for, for example) and the default setting which echos everything is a huge help. Then you'll add the log_user 0
line when you're happy that the code works.
Complete source of the program described above - [here]
. Note that it relies on the data format returned by ping and if your operating system has a different format, my program may need amendment to suit. I am using OSX, by the way.
Expect programming sounds simple, and indeed it is if you're well trained / know what you're doing. However, there are a number of non-obvious mistakes that newcomers can make which can result in flaky performance - there's an example [here]
from our course to show such an issue. There are further issues other than the ones we can show in that sample demo - come on a course to learn ;-).
There is a further example of expect_out
You need also to consider timeout
conditions when you're writing an Expect script - these are beyond the simple examples already quoted.
If you are running a series of processes (such as ping) each of which takes a noticeable elapsed time to run, you can
set Expect to spawn multiple processes and then ask the expect
command to listen for a string on any of them - requires careful through in programming, but very effective. There's an example of this pulling in /robots.txt from multiple servers [here]
. And indeed there are a lot more resources on our site relating to expect processes [here]
(written 2012-01-08, updated 2012-01-14)
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Expect in Perl - a short explanation and a practical example - (2010-10-22) 
Parallel Pinging, using Python Threads or Expect spawn lists - (2009-11-02) 
Using Tcl and Expect to automate repetitive jobs - (2009-10-24) 
Automating processes through Expect - (2008-04-05) 
Expecting a item from a list of possibles - (2008-02-04) 
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What is Expect? - (2007-10-26) 
Installing Tcl and Expect on Solaris 10 - a checklist - (2007-05-02) 
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Expect for Windows - (2005-09-04) 
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