[OT] writing filters in sh

[Devin Teske]

On Wed, 2010-10-27 at 15:28 -0600, Chad Perrin wrote:
> I know that in sh you can get the contents out of files specified as
> command line arguments:
> 
>     while read data; do
>       echo $data
>     done <$@
> 
> I know you can also get the contents of files from pipes and redirects:
> 
>     while read data; do
>       echo $data
>     done
> 
> In Perl, you can use a single construct to do both and, unlike the first
> sh example, it can also take multiple filenames as arguments and
> effectively concatenate their contents:
> 
>     while (<>) {
>       print $_;
>     }
> 
> I'm not exactly an *expert* in sh, in part because when things start
> getting "interesting" while I'm writing shell scripts I tend to just use
> a more robust language like Perl.  Please let me know if there's some way
> to use a simple idiom like the Perl example to get the same results in
> sh.
> 


see here:
http://druidbsd.sf.net/download/sysrc.txt

For example (NOTE: this is just an example -- the function itself is
inefficient when one considers instead `tail -r file' and `tail -r <
file' and `cat file | tail -r', though the function serves as a good
example of how to achieve what you want):

# ... | lrev
# lrev $file ...
#
# Reverse lines of input. Unlike rev(1) which reverses the ordering of
# characters on a single line, this function instead reverses the line
# sequencing.
#
# For example, the following input:
#
# 	Line 1
# 	Line 2
# 	Line 3
#
# Becomes reversed in the following manner:
#
# 	Line 3
# 	Line 2
# 	Line 1
#
lrev()
{
	local stdin_rev=
	if [ $# -gt 0 ]; then
		#
		# Reverse lines from files passed as positional arguments.
		#
		while [ $# -gt 0 ]; do
			local file="$1"
			[ -f "$file" ] && lrev < "$file"
			shift 1
		done
	else
		#
		# Reverse lines from standard input
		#
		while read -r LINE; do
			stdin_rev="$LINE
$stdin_rev"
		done
	fi

	echo -n "$stdin_rev"
}




-- 
Cheers,
Devin Teske

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