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comp.lang.ruby

IO.popen - continuous output

andy

7/20/2007 12:56:00 AM

5 Answers

Sebastian Hungerecker

7/20/2007 9:35:00 AM

0

Andy Koch wrote:
> Hi,
>
> is it possible to get a continuous output stream with IO.popen('xxx')
>
> example,
>
> IO.popen('tail -f some_log_file')...
>
> where the goal is to see the tail output as it's generating, rather than
> waiting for the process to complete to dump the generated output.

That *is* what you get with IO.popen.
For example,
IO.popen('tail -f testfile').each_line do |line|
puts line.upcase
end
prints out the upcased version of any line added to testfile as soon as it is
added to the file.


--
Ist so, weil ist so
Bleibt so, weil war so

Dan Zwell

7/22/2007 5:23:00 AM

0

Sebastian Hungerecker wrote:
> That *is* what you get with IO.popen.
> For example,
> IO.popen('tail -f testfile').each_line do |line|
> puts line.upcase
> end
> prints out the upcased version of any line added to testfile as soon as it is
> added to the file.
>
>

I was just wondering about this, and I see that you are correct,
*usually*--this is very strange.

file1:
#!/usr/bin/env ruby
5.times {puts "hi"; sleep 0.5}

file2:
#!/usr/bin/env ruby
IO.popen("./file1").each_line { |line| puts line }

When running file2 (which should stream the output from file1), the
output is not streamed, but instead saved up until "file1" finishes
running and then dumped. Note that this only occurs when file1 is a ruby
file--replacing it with a bash script that does the same thing causes
the output to stream properly. I am perplexed. Any insight? (Is this a bug?)

I'm using ruby 1.8.5 on linux.

Dan

Dan Zwell

7/22/2007 6:20:00 AM

0

Dan Zwell wrote:
> Sebastian Hungerecker wrote:
>> That *is* what you get with IO.popen.
>> For example,
>> IO.popen('tail -f testfile').each_line do |line|
>> puts line.upcase
>> end
>> prints out the upcased version of any line added to testfile as soon
>> as it is
>> added to the file.
>>
>>
>
> I was just wondering about this, and I see that you are correct,
> *usually*--this is very strange.
>
> file1:
> #!/usr/bin/env ruby
> 5.times {puts "hi"; sleep 0.5}
>
> file2:
> #!/usr/bin/env ruby
> IO.popen("./file1").each_line { |line| puts line }
>
> When running file2 (which should stream the output from file1), the
> output is not streamed, but instead saved up until "file1" finishes
> running and then dumped. Note that this only occurs when file1 is a ruby
> file--replacing it with a bash script that does the same thing causes
> the output to stream properly. I am perplexed. Any insight? (Is this a
> bug?)
>
> I'm using ruby 1.8.5 on linux.
>
> Dan
>
I've made this a bit more accessible for the non-linux folks:
In IRB, try running thes one-liner (with wide margins):
IO.popen("ruby -e '5.times {|n| puts n; sleep 1}'").each_line {|line|
puts line}

It should print a number every second, right? Well, it doesn't for me...

Dan

Sebastian Hungerecker

7/22/2007 9:19:00 AM

0

Dan Zwell wrote:

> file1:
> #!/usr/bin/env ruby
> 5.times {puts "hi"; sleep 0.5}
>
> file2:
> #!/usr/bin/env ruby
> IO.popen("./file1").each_line { |line| puts line }
>
> When running file2 (which should stream the output from file1), the
> output is not streamed, but instead saved up until "file1" finishes
> running and then dumped.

That seems to be a buffering issue. If you set STDOUT.sync to true in file1,
it will work as expected.


--
NP: Explosions in the Sky - Welcome, Ghosts
Ist so, weil ist so
Bleibt so, weil war so

Dan Zwell

7/22/2007 9:30:00 AM

0

Sebastian Hungerecker wrote:
> Dan Zwell wrote:
>
>> file1:
>> #!/usr/bin/env ruby
>> 5.times {puts "hi"; sleep 0.5}
>>
>> file2:
>> #!/usr/bin/env ruby
>> IO.popen("./file1").each_line { |line| puts line }
>>
>> When running file2 (which should stream the output from file1), the
>> output is not streamed, but instead saved up until "file1" finishes
>> running and then dumped.
>
> That seems to be a buffering issue. If you set STDOUT.sync to true in file1,
> it will work as expected.
>
>

Thanks, that's good to know. Having this behavior unexplained made me a
bit nervous about using IO.popen.