[OT] writing filters in sh

[Chip Camden]

Quoth Polytropon on Thursday, 28 October 2010:
> On Thu, 28 Oct 2010 09:17:12 -0700, Chip Camden <sterling at camdensoftware.com> wrote:
> > Perhaps someone with more sh fu can transform the
> > 'if' paragraph into a one-liner at least.  When I tried to do so, I got an
> > unexpected ; error.
> 
> Not tested, but this should do the trick:
> 
> 	#!/bin/sh
> 	if [ $# -ge 1 ] && ( exec cat $@ | $0; exit )
> 	while read data; do echo $data; done
> 
> The ; denotes a line break, means that you can use EITHER ; or
> a newline. In the original construct, 
> 
> 	if [ $# -ge 1 ]; then
> 		exec cat $@ | $0
> 		exit
> 	fi
> 
> you can change the ; into
> 
> 	if [ $# -ge 1 ]
> 	then
> 		exec cat $@ | $0
> 		exit
> 	fi
> 
> which is often seen in scripts. In this case, no ; is required (or
> even allowed). The same rule applies for the while/do/done iterator.
> 
> 
> -- 
> Polytropon
> Magdeburg, Germany
> Happy FreeBSD user since 4.0
> Andra moi ennepe, Mousa, ...
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Tested, and it had a bug ('if' is extraneous).  It also waits for input
even if it has file arguments.  But it's easily corrected:

#!/bin/sh
[ $# -ge 1 ] && exec cat $@ && exit | $0
while read data; do echo $data; done

The first line can be converted to an alias, like so:

alias inargs='[ $# -ge 1 ] && exec cat $@ && exit | $0'

So, if you add the alias to your profile, you can use it to enable the
behavior in any script:

#!/bin/sh
inargs
while read data; do echo $data; done

Thanks for the pointer in the right direction.

-- 
Sterling (Chip) Camden    | sterling at camdensoftware.com | 2048D/3A978E4F
http://camdensoftware.com | http://chipstips.com        | http://chipsquips.com
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