Automating processes through Expect

Archive - Originally posted on "The Horse's Mouth" - 2008-04-05 08:03:46 - Graham Ellis
What is Expect? Let me give you an example of how it's used to help answer that question.

I want to connect to a remote host that I can access through FTP, and have a look at all the files with "top", "sql" and "txt" in their names in my home directory. And that's something I need to do on a regular basis - perhaps checking logs on a web server.

I could type in the instructions longhand - but that would get monotonous on a regular basis - so I would prefer to run FTP controlling it like a puppet on a string - and that string in Expect.

Firstly, I do the job manually, once, and note down the exact prompts that I get, and the exact strings I type in. You noted, I hope, that I say "exact", didn't you - I must get spaces, carriage returns, line feeds right.

Then I write my code. The spawn command is used to trigger FTP. The expect command is used to await a prompt. And the send command is used to answer those prompts. Finally, I can use the variable called expect_out(buffer) which stores the complete response received prior to each "expect" to look at my results.

Here's a working example.

log_user 0

 

# Expect is a Tcl extension

# So all the Tcl commands are available too

puts "yes I am tickling"

 

spawn ftp 192.168.200.67

expect "): "

 

send "trainee\r"

expect "sword: "

 

send "abc123\r"

expect "ftp> "

 

send "ls\r"

expect "ftp> "

 

# The response we just got will have included

# the lines we want (and much more!) so that we

# can now use regular Tcl commands to extract it

set parts [split $expect_out(buffer) \n]

foreach line $parts {

  if {[regexp top|sql|txt $line]} {puts $line}

  }

 

# Close out cleanly

send "quit\r"

expect "eof"

That's very much a "spike" solution - a demonstration of principles; the expect command can wait for a whole series of alternatives (including error conditions) and the application above could be extended to handle them as appropriate, for example. And you can even run multiple spawned processes in parallel.

As well as Tcl commands, you can use Tk commands with Expect - that means that you can put a graphic front end onto it. One of the things I need to do from time to time is to check whether my servers are up and running, and an Expect and Tk tool is ideal for the purpose. You can see my source code here.

Expect, and Tcl are covered on our Tcl and Expect course. We cover Tk on our Tk course

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Automating processes through Expect