Tee and file redirections are very slow to write anything.

Brian Inglis Brian.Inglis@SystematicSw.ab.ca
Wed Feb 24 23:50:19 GMT 2021

On 2021-02-24 15:41, Duncan Roe wrote:
> On Wed, Feb 24, 2021 at 04:58:24PM -0500, Eliot Moss wrote:
>> On 2/24/2021 3:48 PM, ASSI wrote:
>>> Hamish McIntyre-Bhatty via Cygwin writes:
>>>> I found recently when trying to save output from a script for later
>>>> inspection that "tee" and file redirections seem to have massive
>>>> delays when run in Cygwin - usually nothing is written to file or
>>>> stdout until after the command has finished - not very helpful.
>>> You will want to switch from fully buffered to line-buffered or even
>>> unbuffered output.

>> And this does not have to do with Cygwin.  The same happens on Linux.
>> The default is that terminal I/O is unbuffered while other stream are
>> buffered.  Pipes come under "other streams".  One can make programmatic
>> changes to get around this, but most programs won't override the
>> default behavior on their own ...

> The (Linux) default is that terminal I/O is *line* buffered
> The man page for tee doesn't show an option to change buffering, while that for
> grep does.

I believe the default for both Cygwin and Linux is 64KB pipe buffer, so if you 
want to see smaller chunks as they are generated, you need to add some utility 
that may allow you to change that e.g.

	$ tail -f access.log | stdbuf -oL cut -d ' ' -f1 | uniq

but read the disclaimers on the stdbuf and grep man pages, which is why it is 
not done more, especially under Cygwin where Windows adds its own performance 
Some utilities may use read(2/3p), write(2/3p), or mmap(3) if they can and don't 
care about text or lines, for more efficient access to disk files, rather than 
buffered stream I/O functions.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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