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Expect with Ruby - a training example to get you started

The Expect library, originally written for use wth Tcl, is also available in other languages and today I wrote an example to introduce how it's used in Ruby. Please note this is a spike solution - not going to fail (if it does) in a pretty way.

The principle of expect is that you spawn a process - passing back file handles - and you can then read and write exactly as if you were at the keyboard. Unlike open3 which works on a line by line basis and can give buffering issues, you can read a prompt up to a specific character or pattern in expect, giving you much more flexibility. Beyond this example, you have timeouts, EOF handling, the ability to wait for alternatives, and much more.

When using expect, I recommend you to run the job by hand so that you have an example of the terminal session you're emulating:

  # WomanWithCat:4c grahamellis$ ssh -l wellho www.wellho.net uptime
  # wellho@www.wellho.net's password:
  # 17:23:14 up 5 days, 2:08, 0 users, load average: 0.30, 0.49, 0.52
  # WomanWithCat:4c grahamellis$


You'll require to load the pty and expect modules:

  require 'pty'
  require 'expect'


And here's the automation

  uptimedata = []
  PTY.spawn("ssh -l wellho www.wellho.net uptime") do |ssh_read,ssh_write,pid|
    ssh_read.expect(/sword: /) { |msg| ssh_write.printf("#{ARGV[0]}\n") }
    loop do
      ssh_read.expect(/\n/) { |line| uptimedata.push line}
      break if uptimedata[-1].nil?
      p uptimedata[-1]
    end
  end


And the results:

  WomanWithCat:4c grahamellis$ ruby exprub password-here
  ["\r\n"]
  [" 19:22:11 up 5 days, 4:07, 0 users, load average: 0.15, 0.37, 0.45\r\n"]
  WomanWithCat:4c grahamellis$


Complete example (which does a ping as well) [here]. Although we may not cover this on our regular public ruby course, I would be very happy to cover it in an extra session on the penultimate evening for interested delegates, and also to cover it on private courses.

You'll find more about Expect and its principles under Tcl [here] and [here].
(written 2016-05-18)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
R113 - Ruby - Further Input and Output
  [4676] Running shell (operating system) commands from within Ruby - (2016-05-18)
  [4553] RUby - loading, using, changing, storing JSON format data - (2015-10-23)
  [4502] Reading and parsing a JSON object in Ruby - (2015-06-01)
  [4008] Reading and checking user inputs - first lessons - Ruby - (2013-02-17)

R106 - Input and Output in Ruby
  [4499] Significant work - beyond helloworld in Ruby - (2015-05-27)
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [2974] Formatting your output - options available in Ruby - (2010-09-29)
  [2893] Exclamation marks and question marks on ruby method names - (2010-07-28)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2614] Neatly formatting results into a table - (2010-02-01)
  [2290] Opening and reading files - the ruby fundamentals - (2009-07-16)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1587] Some Ruby programming examples from our course - (2008-03-21)

T211 - Tcl/Tk - What is Expect? Why use it?
  [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)
  [1531] Expecting a item from a list of possibles - (2008-02-04)
  [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)

T212 - Tcl/Tk - Expect Processes
  [3448] Checking all the systems on a subnet, using Expect and Tk - (2011-09-18)
  [1785] What is running on your network? (tcl and expect) - (2008-09-04)
  [675] Adding PHP tags to an old cgi program - (2006-04-08)
  [287] Checking that all our servers are up and accessible - (2005-04-22)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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