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comp.lang.ruby

capture output in real time

Paul Winward

5/10/2008 12:09:00 AM

I'm wondering how to capture data written to stdout (or stderr) in real
time using popen3. The problem occurs with the following sample
scripts:

sleep.rb
--------
puts "before sleep"
sleep 3
puts "after sleep"


capture.rb
---------
require 'open3'

Open3.popen3("ruby sleep.rb") do |stdin, stdout, stderr|

Thread.new do
loop do
out = stdout.gets
puts out if out
end
end

sleep 5
end


capture.rb outputs both "before sleep" and "after sleep" after 3 seconds
have passed. When using stderr, however, output is seen in real time.

Note, I'm avoiding STDOUT.flush in sleep.rb since I won't be able to
change the actual scripts I'll be passing to popen3.

Thanks for any help.
Paul
--
Posted via http://www.ruby-....
7 Answers

ara.t.howard

5/10/2008 12:15:00 AM

0


On May 9, 2008, at 6:08 PM, Paul Winward wrote:
>
> Note, I'm avoiding STDOUT.flush in sleep.rb since I won't be able to
> change the actual scripts I'll be passing to popen3.

popen3 is going to be using fork/exec under the hood and should
inherit the sync setting of STDOUT, so try setting

STDOUT.sync = true

before the popen and see if that helps - untested, but i'm running out
the door....

cheers.

a @ http://codeforp...
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

Paul Winward

5/10/2008 6:09:00 AM

0

Thanks for the suggestion but I'm still having the problem.

Paul

ara.t.howard wrote:
> On May 9, 2008, at 6:08 PM, Paul Winward wrote:
>>
>> Note, I'm avoiding STDOUT.flush in sleep.rb since I won't be able to
>> change the actual scripts I'll be passing to popen3.
>
> popen3 is going to be using fork/exec under the hood and should
> inherit the sync setting of STDOUT, so try setting
>
> STDOUT.sync = true
>
> before the popen and see if that helps - untested, but i'm running out
> the door....
>
> cheers.
>
> a @ http://codeforp...

--
Posted via http://www.ruby-....

Paul Winward

5/10/2008 5:47:00 PM

0

Another approach I had tried was to use IO.select, as shown in
http://whynotwiki.com/Ruby_/_Process_... ( attributed to Jamis
Buck). This too, produced the same delay in output, but is shown below:

Thank you for any help

def read_until(pipe, stop_at, verbose = true)
lines = []
line = ""
while result = IO.select([pipe]) #, nil, nil, 10)
next if result.empty?

c = pipe.read(1)
break if c.nil?

line << c
break if line =~ stop_at

# Start a new line?
if line[-1] == ?\n
puts line if verbose
lines << line
line = ""
end
end
lines
end



Paul Winward wrote:
> Thanks for the suggestion but I'm still having the problem.
>
> Paul
>
> ara.t.howard wrote:
>> On May 9, 2008, at 6:08 PM, Paul Winward wrote:
>>>
>>> Note, I'm avoiding STDOUT.flush in sleep.rb since I won't be able to
>>> change the actual scripts I'll be passing to popen3.
>>
>> popen3 is going to be using fork/exec under the hood and should
>> inherit the sync setting of STDOUT, so try setting
>>
>> STDOUT.sync = true
>>
>> before the popen and see if that helps - untested, but i'm running out
>> the door....
>>
>> cheers.
>>
>> a @ http://codeforp...

--
Posted via http://www.ruby-....

Paul Winward

5/10/2008 7:28:00 PM

0

I found a perl solution at http://www.perlmonks.org/?node... that
makes use of pseudo tty to get line-buffered output. Unfortunately, I
can find very little documentation on how to use the ruby module PTY,
and it looks like it is no longer supported
(http://www.ruby-...topic/...). Also, I wonder if a
pseudo tty can capture stderr as well
(http://unix.derkeiler.com/Newsgroups/comp.unix.programmer/2003-07...)

The perl solution is system dependent, but it worked for me on Linux
2.6.22. I did see a debian package libpty-ruby but I don't see any
documentation and am not sure if this is still supported.

The notion of a pseudo tty is new to me and I'd appreciate any help. Is
there a ruby equivalent for the perl solution below?

Thanks

#!/usr/bin/perl -w
use strict;
use IO::Pty;

my $pty = new IO::Pty;
my $slave = $pty->slave();

my $pid = fork();
die "Couldn't fork: $!" unless defined $pid;

if($pid){ # dup STDOUT to Pty and run external program:
$pty->close_slave();
open(STDOUT, ">&",$pty)||die $!;
system "perl blackbox.pl";
print "\cD"; # send ^d to end

} else { # this is your monitoring process
$pty->make_slave_controlling_terminal();
print "*$_" while <$slave>;
exit;
}
__END__




Paul Winward wrote:
> Another approach I had tried was to use IO.select, as shown in
> http://whynotwiki.com/Ruby_/_Process_... ( attributed to Jamis
> Buck). This too, produced the same delay in output, but is shown below:
>
> Thank you for any help
>
> def read_until(pipe, stop_at, verbose = true)
> lines = []
> line = ""
> while result = IO.select([pipe]) #, nil, nil, 10)
> next if result.empty?
>
> c = pipe.read(1)
> break if c.nil?
>
> line << c
> break if line =~ stop_at
>
> # Start a new line?
> if line[-1] == ?\n
> puts line if verbose
> lines << line
> line = ""
> end
> end
> lines
> end
>
>
>
> Paul Winward wrote:
>> Thanks for the suggestion but I'm still having the problem.
>>
>> Paul
>>
>> ara.t.howard wrote:
>>> On May 9, 2008, at 6:08 PM, Paul Winward wrote:
>>>>
>>>> Note, I'm avoiding STDOUT.flush in sleep.rb since I won't be able to
>>>> change the actual scripts I'll be passing to popen3.
>>>
>>> popen3 is going to be using fork/exec under the hood and should
>>> inherit the sync setting of STDOUT, so try setting
>>>
>>> STDOUT.sync = true
>>>
>>> before the popen and see if that helps - untested, but i'm running out
>>> the door....
>>>
>>> cheers.
>>>
>>> a @ http://codeforp...

--
Posted via http://www.ruby-....

ara.t.howard

5/10/2008 9:47:00 PM

0


On May 10, 2008, at 1:28 PM, Paul Winward wrote:
> I found a perl solution at http://www.perlmonks.org/?node...
> that
> makes use of pseudo tty to get line-buffered output. Unfortunately, I
> can find very little documentation on how to use the ruby module PTY,
> and it looks like it is no longer supported
> (http://www.ruby-forum.com/topic/...). Also, I wonder if a
> pseudo tty can capture stderr as well
> (http://unix.derkeiler.com/Newsgroups/comp.unix.programmer/2003-07...
> )
>
> The perl solution is system dependent, but it worked for me on Linux
> 2.6.22. I did see a debian package libpty-ruby but I don't see any
> documentation and am not sure if this is still supported.
>
> The notion of a pseudo tty is new to me and I'd appreciate any
> help. Is
> there a ruby equivalent for the perl solution below?

pty is supported and the dist has a bunch of demo code in it (ext/pty/
*) but you cannot capture stderr akaikt.

a @ http://codeforp...
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

ara.t.howard

5/10/2008 9:49:00 PM

0


On May 10, 2008, at 12:08 AM, Paul Winward wrote:
> Thanks for the suggestion but I'm still having the problem.

if the child program makes a call to setvbuf (as ruby does) there is
little you can do to alter this behaviour. otherwise the 'normal'
approach will work:

cfp:~ > cat a.rb
r, w = IO.pipe

child = fork{ STDOUT.reopen w; w.close; system 'echo 4 && sleep 1 &&
echo 2' }

w.close

while(( char = r.read(1) ))
printf "%f : %s\n", Time.now.to_f, char.inspect
end


cfp:~ > ruby a.rb
1210456103.163009 : "4"
1210456103.163153 : "\n"
1210456104.166967 : "2"
1210456104.167023 : "\n"


i might be wrong, but i really don't think there is anything you can
do with a child program that alters the buffering mode itself.

a @ http://codeforp...
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

Paul Winward

5/12/2008 5:01:00 PM

0

Thanks for the pointers. I wasn't aware of setvbuf. I'll look more into
pty.

ara.t.howard wrote:
> On May 10, 2008, at 12:08 AM, Paul Winward wrote:
>> Thanks for the suggestion but I'm still having the problem.
>
> if the child program makes a call to setvbuf (as ruby does) there is
> little you can do to alter this behaviour. otherwise the 'normal'
> approach will work:
>
--
Posted via http://www.ruby-....