Re: BASH Redirect (STDOUT & STDERR) Syncing
michael <linux@networkingnewsletter.org.uk> writes:
> On Tue, 2005-04-12 at 05:09 +0200, Bruno Hertz wrote:
>> Hal Vaughan <hal@thresholddigital.com> writes:
>>
>> > I'm running a Perl script and redirecting the output to a file, and
>> > redirecting STDERR to the same file, like this:
>> >
>> > myprog >capture.txt 2>&1
>> >
>> > And I've noticed that the error output is not in sync with STDOUT. Is there
>> > any way, when doing a redirect like this, to synchronize the output, so I can
>> > tell when (in relation to STDOUT) the error messages were output?
>>
>> Hey
>>
>> this is a wild guess coming now, but I'm posting it just to compare it
>> to what finally comes out as being right.
>>
>> They aren't 'in sync', as you call it, presumably because of the
>> output buffering done in Perl, where one channel, either stdout or
>> stdin, receives significant less output than the other and hence
>> flushes more seldom.
>
> Actually, I've noticed this for many things I've run from the cmd line
> in Debian (and have noticed it not happening for other Unices)... the
> only thing that springs to mind at the moment is output from 'make'
> having std err and std out well out of sync.... any ideas what might be
> going on here? (Debian GNU/Linux, 2.4.27-1-686-smp, Sarge apt updates)
Hmmm, not sure I can confirm this. Especially with command line tools
it's common practice to do line buffering when output is directed to a
terminal and block buffering otherwise, e.g. in pipes or when output
is redirected into a file.
Especially regarding block buffering, if a program doesn't take extra
measures it's buffering strategy will usually be governed by the
standard library, i.e. glibc. And in that case the only possible
reason for differing behavior on different platforms could only be
some config option given to glibc, presumably during compilation time,
targeting e.g. the buffer size. This may be possible, but at least I'm
not aware of Debian doing something special in that regard and can't
myself really say that Sarge behaves any different than, say, FC3.
Maybe somebody else knows more ?
Regards, Bruno.
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