From: nobu.nokada@...
Date: 2002-09-19T12:53:08+09:00
Subject: Re: Capturing stdout from cmd while letting it print it

Hi,

At Wed, 18 Sep 2002 17:55:43 +0900,
coma_killen@fastmail.fm wrote:
> > "mycmd" may buffer and not flush the output.  How large is the
> > output?
> >
> mycmd is really a checksum verification program that goes through
> all the files in the dir. It prints and updates a percentage of
> how much of each file has been verified, as it verifies the file.
> 
> Output is as many lines as there are files in the dir.
> Thus, not very large.

Standard C runtime libraries buffers output to IO descriptor
othar than tty, in order to improve throughput.  Since pipes
are subjects of this behavior, data will not passed to
consuming process until producing process flushes the buffer or
data size reaches the limit (perhaps 4k or 8k, defined as
BUFSIZ in stdio.h).  Some commands have unbuffering option to
get rid of it.

This issue will occur with "tee" command David Stagner
mentioned.

> > Or, it may send output to stderr or console instead of stdout.
> >
> Could be but not likely. How can i check this?

Redirection.

 mycmd > log

directs only stdout from "mycmd" to file "log".  And if the
command takes minuets, you may be possible to observe that the
log size doesn't increase continuously.

-- 
Nobu Nakada