# Capturing stdout during unit tests **URL:** \ **Category:** Ruby\ **Created:** [August 8, 2007, 6:39am UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520 "2007-08-08T06:39:46Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [August 8, 2007, 6:39am UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/1 "2007-08-08T06:39:46Z")
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 ---
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [August 8, 2007, 7:17am UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/2 "2007-08-08T07:17:01Z")
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 $stdout.reopen() with the path > to that temporary file. However, I’d much rather do this in memory > because then I never have to worry about what file system and > permissions the user executing the unit test has. > > Thanks, > > Mark ---
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [August 8, 2007, 9:05am UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/3 "2007-08-08T09:05:57Z")
On Aug 7, 8:59 pm, Mark S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I’ve written a script for use with Nagios (a monitoring tool) that > I’d like to test. > I’m now writing unit tests for my script, and I’d like to check that > the output written to STDOUT by each service monitor class is > correct. But I’m new to Ruby and I’m not sure how to do that. In > other languages, I’d simply redefine STDOUT as a stream that goes to > a large in-memory buffer, What language is this in? > but I haven’t seen anything that suggests that is possible in Ruby. > At any rate, Ruby does have similar functionality. An example: require ‘stringio’ class Bs attr :output\_stream,true def do\_some\_output(data) output\_stream.puts(data) end end bs=Bs.new bs.output\_stream=$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 [sshaw@localhost sshaw]# ruby bs.rb this is to stdout result= Waka waka waka! expect= grrrrrrrr ---
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [August 8, 2007, 11:09pm UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/4 "2007-08-08T23:09:26Z")
Awesome! Thank you. That’s exactly the functionality I was looking for. Mark ---
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [August 8, 2007, 9:09am UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/5 "2007-08-08T09:09:58Z")
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 ---
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [September 25, 2007, 11:09pm UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/6 "2007-09-25T23:09:17Z")
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 ---
**Author:** ![mslagell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e274bd/32.png) [@mslagell](https://www.ruby-forum.com/u/mslagell)\ **Post date:** [August 31, 2008, 8:44am UTC](https://www.ruby-forum.com/t/capturing-stdout-during-unit-tests/111520/7 "2008-08-31T08:44:12Z")
> 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