[R] capturing stderr/stdout

[Prof Brian Ripley]

Sundar Dorai-Raj wrote:
> Hi, Prof. Ripley,
> 
> Thanks for the reply. Mostly I want to capture output as it is written 
> to the stream. For example, I quite often do the following to view the 
> progress of a log file from a computationally intensive script.
> 
> 1. Open a console and type:
> 
> Rscript script.R >& script.log
> 
> which directs both stdout and stderr to "script.log"
> 
> 2. Open another console and type:
> 
> tail -f script.log
> 
> This way I get both the script's log file and its current progress.

The usual way to so this is via 'tee' .

> I guess my question is: Is there a way to accomplish the "tail -f" 
> command in R?

Yes, but why would you want to?  Note that sink() has a 'split' option, 
so you could in principle do all this in your script.R.

Otherwise you can write a simple R loop to do readLines(n=1); 
writeLines() to emulate tail -F.


> Thanks,
> 
> --sundar
> 
> Prof Brian Ripley said the following on 11/20/2008 11:43 PM:
>> I am not sure what the issue is here. Do you want to capture both 
>> stderr and stdout (use 2>1 in the command with an sh-like shell), or 
>> is the
>> problem that you don't get immediate output?
>>
>> The latter is a Perl issue: you need to circumvent output buffering.
>> See e.g
>>
>> http://perl.plover.com/FAQs/Buffering.html
>>
>> Sundar Dorai-Raj wrote:
>>> Hi,
>>>
>>> I have an application in perl that prints some output to either 
>>> stderr or stdout.
>>>
>>> Here's an example:
>>>
>>> # tmp.pl
>>> print STDERR "starting iterator\n";
>>> for(my $i = 0; $i < 1000000; $i++) {
>>>   print $i . "\n";
>>> }
>>>
>>> # tmp.R
>>> con <- pipe("perl tmp.pl")
>>> r <- readLines(con, n = -1)
>>> close(con)
>>>
>>> However, the second line stalls until the perl for-loop finishes. 
>>> What I would like is to process each line as it comes. E.g. something 
>>> like:
>>>
>>> while(TRUE) {
>>>   r <- readLines(con, n = 1) # read one line
>>>   if(r == "10000") print(r)
>>>   if(length(r) == 0) break
>>> }
>>>
>>> Of course, this won't work since I'm not calling readLines 
>>> appropriately. Answers must work on Windows but may include cygwin 
>>> utilities if necessary. Any advice would be appreciated. Version info 
>>> at the end if it matters.
>>>
>>> Thanks, --sundar
>>>
>>>
>>>  > version
>>>                _
>>> platform       i386-pc-mingw32
>>> arch           i386
>>> os             mingw32
>>> system         i386, mingw32
>>> status
>>> major          2
>>> minor          8.0
>>> year           2008
>>> month          10
>>> day            20
>>> svn rev        46754
>>> language       R
>>> version.string R version 2.8.0 (2008-10-20)
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide 
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>>


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595