Capturing stdout during unit tests

I’ve written a script for use with Nagios (a monitoring tool) that
I’d like to test. The requirements for the script is that it output a
single line of text to STDOUT and then exit with a status code (0 -
3). My script is designed to monitor a few different services… each
invocation includes a required command line argument specifying the
service to monitor in the current invocation. To do that, I created a
class for each type of service monitor.

I’m now writing unit tests for my script, and I’d like to check that
the output written to STDOUT by each service monitor class is
correct. But I’m new to Ruby and I’m not sure how to do that. In
other languages, I’d simply redefine STDOUT as a stream that goes to
a large in-memory buffer, but I haven’t seen anything that suggests
that is possible in Ruby. The best I’ve come up with so far is
creating a temporary file and calling $stdout.reopen() with the path
to that temporary file. However, I’d much rather do this in memory
because then I never have to worry about what file system and
permissions the user executing the unit test has.

Thanks,

Mark

One option is to write your test to execute your script on the command
line. I’m pretty sure this is the piece you’re missing:

asign the shell output of the ls command to a variable

ls_output = ls

Do something semi-useful with it.

items = ls_output.split
puts “There are #{items.size} objects in the directory.”

-Simon

Mark S. wrote:

I’ve written a script for use with Nagios (a monitoring tool) that
I’d like to test. The requirements for the script is that it output a
single line of text to STDOUT and then exit with a status code (0 -
3). My script is designed to monitor a few different services… each
invocation includes a required command line argument specifying the
service to monitor in the current invocation. To do that, I created a
class for each type of service monitor.

I’m now writing unit tests for my script, and I’d like to check that
the output written to STDOUT by each service monitor class is
correct. But I’m new to Ruby and I’m not sure how to do that. In
other languages, I’d simply redefine STDOUT as a stream that goes to
a large in-memory buffer, but I haven’t seen anything that suggests
that is possible in Ruby. The best I’ve come up with so far is
creating a temporary file and calling $stdout.reopen() with the path
to that temporary file. However, I’d much rather do this in memory
because then I never have to worry about what file system and
permissions the user executing the unit test has.

Thanks,

Mark

On Aug 7, 8:59 pm, Mark S. [email protected] wrote:

I’ve written a script for use with Nagios (a monitoring tool) that
I’d like to test.

I’m now writing unit tests for my script, and I’d like to check that
the output written to STDOUT by each service monitor class is
correct. But I’m new to Ruby and I’m not sure how to do that. In
other languages, I’d simply redefine STDOUT as a stream that goes to
a large in-memory buffer,

What language is this in?

but I haven’t seen anything that suggests that is possible in Ruby.
At any rate, Ruby does have similar functionality. An example:

require ‘stringio’
class Bs
attr :output_stream,true

def do_some_output(data)
output_stream.puts(data)
end
end

bs=Bs.new
bs.output_stream=$stdout
bs.do_some_output(“this is to stdout”)

expect=“grrrrrrrr”
result=String.new

bs.output_stream=StringIO.new(result,“w+”)
bs.do_some_output(“Waka waka waka!”)
bs.output_stream.close

puts "result= " + result
puts "expect= " + expect

[[email protected] sshaw]# ruby bs.rb
this is to stdout
result= Waka waka waka!
expect= grrrrrrrr

Awesome! Thank you. That’s exactly the functionality I was looking for.

Mark

On Aug 7, 2007, at 20:59, Mark S. wrote:

correct. But I’m new to Ruby and I’m not sure how to do that. In
other languages, I’d simply redefine STDOUT as a stream that goes
to a large in-memory buffer, but I haven’t seen anything that
suggests that is possible in Ruby. The best I’ve come up with so
far is creating a temporary file and calling $stdout.reopen() with
the path to that temporary file. However, I’d much rather do this
in memory because then I never have to worry about what file system
and permissions the user executing the unit test has.

Install the ZenTest gem, then:

require ‘test/zentest_assertions’

class TestBlah < Test::Unit::TestCase

def test_my_stuff
out, err = util_capture do the_thing end
assert_equal “…”, out.string
assert_equal “…”, err.string
end
end

On 8/08/2007, at 3:59 PM, Mark S. wrote:

correct. But I’m new to Ruby and I’m not sure how to do that. In
other languages, I’d simply redefine STDOUT as a stream that goes
to a large in-memory buffer, but I haven’t seen anything that
suggests that is possible in Ruby. The best I’ve come up with so
far is creating a temporary file and calling $stdout.reopen() with
the path to that temporary file. However, I’d much rather do this
in memory because then I never have to worry about what file system
and permissions the user executing the unit test has.

Thanks,

Try this:

require ‘stringio’

$stdout = StringIO.new

(This will break in IRB w/ Readline)

StringIO has all the IO methods, but acts on a string, not a file. So
you should be able to do what you want.

Cheers

Phil

Install the ZenTest gem, then:

require ‘test/zentest_assertions’

class TestBlah < Test::Unit::TestCase

def test_my_stuff
out, err = util_capture do the_thing end
assert_equal “…”, out.string
assert_equal “…”, err.string
end
end

if you’re not using test/unit:

require ‘test/zentest_assertions’
include Test::Unit::Assertions

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