# Capturing incremental output from STDOUT

**URL:** <https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780>\
**Category:** Ruby\
**Created:** [March 8, 2010, 3:47pm UTC](https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780 "2010-03-08T15:47:44Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [March 8, 2010, 3:47pm UTC](https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780/1 "2010-03-08T15:47:44Z")

</div>

Hi all,

If I run an external process, e.g.

IO.popen(‘cucumber’)

Then Ruby blocks while waiting for the whole output of the subprocess.  
Is  
there a way to read the subprocess’ output incrementally as it is being  
written?

---

<div class="post-metadata">

**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [March 10, 2010, 11:47am UTC](https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780/2 "2010-03-10T11:47:16Z")

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James C. wrote:

> Hi all,
> 
> If I run an external process, e.g.
> 
> IO.popen(‘cucumber’)
> 
> Then Ruby blocks while waiting for the whole output of the subprocess.  
> Is  
> there a way to read the subprocess’ output incrementally as it is being  
> written?

What’s almost certainly happening is that the process which is sending  
the output is buffering it, and not flushing the buffer.

If the subprocess is written in ruby, try adding “$stdout.sync = true”  
at the beginning.

Some apps will switch into unbuffered mode if they think they are  
talking to a human on a terminal (a tty or pty). There is an  
almost-undocumented ‘pty’ module in the standard library which you can  
use to run a program under a pty: see

[http://www.ruby-forum.com/topic/133560](http://www.ruby-forum.com/topic/133560)

so give that a try if the process you’re spawning is not one that you  
can modify.

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<div class="post-metadata">

**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [March 9, 2010, 10:32pm UTC](https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780/3 "2010-03-09T22:32:12Z")

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James C. wrote:

> Hi all,
> 
> If I run an external process, e.g.
> 
> IO.popen(‘cucumber’)
> 
> Then Ruby blocks while waiting for the whole output of the subprocess.

really?

irb(main):001:0\> IO.popen(“yes yes”) do |fp|  
irb(main):002:1\* loop do  
irb(main):003:2\* puts Time.now  
irb(main):004:2\> fp.gets  
irb(main):005:2\> sleep 1  
irb(main):006:2\> end  
irb(main):007:1\> end  
Wed Mar 10 06:30:25 +0900 2010  
Wed Mar 10 06:30:26 +0900 2010  
Wed Mar 10 06:30:27 +0900 2010  
Wed Mar 10 06:30:28 +0900 2010  
Wed Mar 10 06:30:29 +0900 2010  
Wed Mar 10 06:30:30 +0900 2010  
…

Seems it do not wait the EOF.

---

<div class="post-metadata">

**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [March 10, 2010, 1:17pm UTC](https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780/4 "2010-03-10T13:17:41Z")

</div>

2010/3/8 James C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> If I run an external process, e.g.
> 
> IO.popen(‘cucumber’)
> 
> Then Ruby blocks while waiting for the whole output of the subprocess.

IO.popen does not block anything. You would at least have to read  
from or write to the stream in order to get a chance of blocking.

> Is  
> there a way to read the subprocess’ output incrementally as it is being  
> written?

You can use IO#read(int), i.e. with a size limit, or you can read line  
based. What code do you have and what do you want to accomplish?

Kind regards

robert

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<div class="post-metadata">

**Author:** ![james-calivar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/2bfe46/32.png) [@james-calivar](https://www.ruby-forum.com/u/james-calivar)\
**Post date:** [March 10, 2010, 1:28pm UTC](https://www.ruby-forum.com/t/capturing-incremental-output-from-stdout/184780/5 "2010-03-10T13:28:35Z")

</div>

Robert K. wrote:

> 2010/3/8 James C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):
> 
> > If I run an external process, e.g.
> > 
> > IO.popen(‘cucumber’)
> > 
> > Then Ruby blocks while waiting for the whole output of the subprocess.
> 
> IO.popen does not block anything. You would at least have to read  
> from or write to the stream in order to get a chance of blocking.

Although the OP didn’t mention the platform, as Windows can be broken  
with regards to forking.

But I just tried it with the one-click installer under XP, and it seems  
to be fine:

ruby 1.8.6 (2009-08-04 patchlevel 383) [i386-mingw32]

> > c = IO.popen(“pause”,“w+”)  
> > =\> #IO:0x2bea818  
> > c.readpartial(1024)  
> > =\> "Press any key to continue . . . "  
> > c.puts  
> > =\> nil  
> > c.readpartial(1024)  
> > =\> “\n”  
> > c.readpartial(1024)  
> > EOFError: end of file reached

(readpartial reads between 1 and the given number of bytes, depending on  
how many are available at the time)