[OT] writing filters in sh
Devin Teske
dteske at vicor.com
Thu Oct 28 18:08:18 UTC 2010
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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