Why is this tool useful?
Chances are if you are reading this you probably have already used
the fine Expect for tcl, and possibly even read Exploring Expect. You are
interested in learning how to accomplish the same things you've done in tcl
using perl, or perhaps are just totally irritated at tcl.
Expect is a generic tool for talking to processes that normally require
user interaction. This might be running an ftp client to grab a file,
telnetting to a router to grab statistics or reset an interface. Or, as in the
case of a place I recently administered, to start up a secure webserver without
having to be physically at the machine to enter the super secret password.
Expect talks to processes through ptys. To it, a process is mostly just
a bidirectional file handle, much the same as a socket. In fact, it is possible
to take a filehandle you've already used and pass it off to expect to interact
with.
Now, something you might say at this point is "well, but there are
tools that I can use to do that with for more common protocols like telnet and
ftp already, such as Net::Ftp and Net::Telnet. Why would I want to use your
tool?". This is true. You might never want to use it. However, there are a few
advantages Expect has over similar modules:
1. A consistent interface. You don't have to remember the syntax for
the other tools.
2. It is more intuitive (my opinion, of course) because you already
know how to use the clients you are familiar with. Once you learn how to
talk to a process using Expect you will have an easy time automating your
other tasks.
3. It is more versatile. With Expect you can connect multiple
processes together, write to log files, talk to sockets, etc.
4. Consistent debugging. Debugging, IMHO, is much easier in Expect
than in other tools because you have the ability to watch the interaction
take place, and it's really pretty easy to use.
One serious disadvantage of Expect is that scripts generated using it
are generally non-portable. The way a client 'looks' is important to building
a script to talk to it. Interacting with a client on DG-UX may be very
different than the equivalent client on SunOS. Or, and ncftp would be a good
example of this, a client may be different between versions. Similarly, if
an administrator changes versions of a server it might send back different
prompts than what you are looking for. These are things you should be aware of.
