From: gwtmp01@...
Date: 2006-09-29T01:30:40+09:00
Subject: Re: how to determine if pipe is given


On Sep 28, 2006, at 12:09 PM, ara.t.howard@noaa.gov wrote:

> On Fri, 29 Sep 2006 gwtmp01@mac.com wrote:
>
>> In both cases if you leave off the '-' the program will report an  
>> error
>> rather than reading and processing the data.  The next programmer
>> who comes along is going to be very confused by this behavior.
>
> not if they work on unix!  ;-)

Huh?  That is like saying that grep at the end of a pipeline is expected
to be called as:

	source | grep 'pattern' -

instead of simply

	source | grep 'pattern'

>> Can anyone think of a standard Unix program that will *only* read  
>> from
>> stdin in if you give an explicit sentinel as a file argument?
>
> #
> # tarfile on stdout
> #
> harp:~ > tar -cf - directory  > tarfile_on_stdout.tgz

Hmm. Tar is a bit of a special case, isn't it?  It's default behavior
is to access devices in /dev.  You don't need -f, if you are using the
default devices.

> # echo foobar into a gzip of stdin, sending compressed output to  
> stdout, pipe
> # that into another gzip which is decompressing stdin, dump that  
> output back
> # out to stdout
> #
> harp:~ > echo foobar | gzip - -c | gzip -d - -c
> foobar

But gzip works just fine without all those extra arguments:

	echo foobar | gzip > foobar.gz
	echo foobar | gzip | gzip -d -c

> # use image magick's convert command to convert stdin -> stdout
> #
> harp :~ > convert - - < map.png > map2.png
> harp :~ > file map2.png
> map2.png: PNG image data, 713 x 569, 8-bit/color RGB, non-interlaced

Are you saying that

	convert < map.png > map2.png

doesn't work?

If you look back at my posting I said that a program that *requires*  
the use of '-'
would be unusual.  I understand why some programs support the '-'  
notation, which is
still a hack if you've got /dev/stdin or /dev/stdout available.

Maybe I totally missed the point but I thought the OP was making the  
requirement that
his program would not bother to read from stdin *unless* it saw  
something special on
the command line, which led to your suggestion of the '-'.  My point  
is that the OP's
requirement would be very unexpected for the usual Unix programmer.