[OT] writing filters in sh

[Devin Teske]

On Thu, 2010-10-28 at 09:17 -0700, Chip Camden wrote:
> Quoth Chad Perrin on Thursday, 28 October 2010:
> > On Thu, Oct 28, 2010 at 08:11:48AM -0700, Chip Camden wrote:
> > > 
> > > Here's a way to do what you're wanting to do.  Unfortunately, it isn't a
> > > generalized, single construct:
> > > 
> > > #!/bin/sh
> > > if [ $# -ge 1 ];then
> > >   exec cat $@ | $0
> > >   exit
> > > fi
> > > 
> > > while read data; do
> > >   echo $data
> > > done
> > > 
> > > My lame attempts to generalize the first paragraph into an alias,
> > > function, or shell script have met with disappointment.

Here you go:

alias myfilter="/bin/sh -c '"'myfilter() { if [ $# -gt 0 ]; then while
[ $# -gt 0 ]; do [ -f "$1" ] && myfilter < "$1"; shift 1; done; else
while read LINE; do : some transform ; echo $LINE; done; fi; }; myfilter
"$@"'"' -- /bin/sh"

The above is to be entered as a single command-line.

NOTE: If you're using either csh or tcsh, you only need to make one
change... change "alias myfilter=..." to instead "alias
myfilter ..." (that's right, just change the equals sign to a space).

Here's a demo:

$ printf "Line 1\nLine 2\nLine 3\n" | myfilter
Line 1
Line 2
Line 3

$ head -2 /etc/rc.conf | myfilter
# This file now contains just the overrides from /etc/defaults/rc.conf
# please make all changes to this file.

$ myfilter < /etc/rc.conf | head -2
# This file now contains just the overrides from /etc/defaults/rc.conf
# please make all changes to this file.

$ cat /etc/rc.conf | myfilter | head -2
# This file now contains just the overrides from /etc/defaults/rc.conf
# please make all changes to this file.

$ myfilter > somefile
Line 1
Line 2
Line 3
^D
$ cat somefile
Line 1
Line 2
Line 3



> > 
> > I was hoping for a generalized, simple idiom for this, rather than
> > needing to implement it myself, for demonstration purposes (and for easy
> > reuse later, of course).  Your solution does not exactly fit my
> > preferences for simplicity, but I might include it in an article I'm
> > writing anyway.  It's simple and readable enough that it should not
> > clutter up the article much.
> > 
> > I tested it for some simple use cases, and it works well.  Thanks for
> > saving me a little trouble.
> > 
> > -- 
> > Chad Perrin [ original content licensed OWL: http://owl.apotheon.org ]
> 
> 
> You're most welcome.  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.
> 



-- 
Cheers,
Devin Teske

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