# Re: open3 and background processes

**URL:** <https://www.ruby-forum.com/t/re-open3-and-background-processes/146986>\
**Category:** Ruby\
**Created:** [August 27, 2008, 10:28am UTC](https://www.ruby-forum.com/t/re-open3-and-background-processes/146986 "2008-08-27T10:28:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![stickstone](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@stickstone](https://www.ruby-forum.com/u/stickstone)\
**Post date:** [August 27, 2008, 10:28am UTC](https://www.ruby-forum.com/t/re-open3-and-background-processes/146986/1 "2008-08-27T10:28:12Z")

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> I  
> thought there was a way to peek ahead on an IO object to see if any  
> data is available on the handle before attempting to read it

You’re probably thinking of Kernel.select. To poll if data is available  
for  
read on IO object foo, and read as much data is available, up to 1024  
bytes  
max:

if select([foo], nil, nil, 0)  
res = foo.readpartial(1024)  
end

(select also returns true if the file is closed, in which case read will  
return nil)

Beware of race conditions in your code - i.e. the child might not  
generate  
output on stderr until some time later, so you may need to continue to  
poll  
periodically.

---

<div class="post-metadata">

**Author:** ![stickstone](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@stickstone](https://www.ruby-forum.com/u/stickstone)\
**Post date:** [August 27, 2008, 4:21pm UTC](https://www.ruby-forum.com/t/re-open3-and-background-processes/146986/2 "2008-08-27T16:21:26Z")

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On Aug 27, 2:22 am, Brian C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> end
> 
> (select also returns true if the file is closed, in which case read will  
> return nil)
> 
> Beware of race conditions in your code - i.e. the child might not generate  
> output on stderr until some time later, so you may need to continue to poll  
> periodically.

This works, though I had to add a sleep call to give it time to pick  
up an error:

require ‘open3’  
require ‘timeout’

program = File.join(Dir.pwd, “miniserver.rb”)

cmd = “ruby #{program} &”

Open3.popen3(cmd) do |stdin, stdout, stderr|  
sleep 0.1

if select([stderr], nil, nil, 0)  
p stderr.readlines  
end  
end

puts “Done”

This approach actually makes me less comfortable than my original  
approach, however, because now I’ll have to worry that I didn’t sleep  
long enough and an error will go unnoticed.

Regards,

Dan