[olug] Grep Sed Question

[Will Langford]

This thread reminds me of something amusing I saw a while back.

There was a userfriendly 5 or 6 years back..... started with a bunch of
cavemen around a fire grunting one syllable phrases, followed by a bunch of
the geek staff around a computer grunting 'grep', 'awk', 'sed'.

-Will


On 6/16/07, Christopher Cashell <topher-olug at zyp.org> wrote:
>
> At Fri, 15 Jun 07, Unidentified Flying Banana Stan Coleman, said:
>
> > I have a project where I want to remove a specific line from a text
> > file. Piece of cake I've done that many times. Here is the twist I
> > want to remove the specific line AND the line just prior to the
> > specific line. Any thoughts?
>
> I think that because of the line oriented nature of sed and grep, this
> will be difficult, if not impossible to accomplish with them.  By the
> time they know that you've reached the match line, they've already
> printed out the preceding line.
>
> My suggestion would be to use awk or perl, provided there isn't some
> special restriction or requirement for grep or sed.
>
> I threw together a quick example in perl, which I've included here.
> It's only been tested against one little sample file I threw together,
> but it seems to work.  Note that you will have to change /tag/ to
> whatever regular expression will match the line you want removed.
>
> It's currently setup to act as a filter, such as:
>
>   rem_line+pre.pl <input_file >output_file
>
> Also note that I haven't written a whole lot of perl lately, so bugs are
> entirely likely.  If anyone happens to see an error, please let me know.
>
> --
> | Christopher
> +------------------------------------------------+
> | Here I stand.  I can do no other.              |
> +------------------------------------------------+
>
>
> ------------------CUT HERE------------------------
>
> #!/usr/bin/perl
> # vim: ts=3 sw=3 et sm ai smd sc bg=dark
>
> use strict;
> use warnings;
>
> my $previous;
> my $skip;
> my $match = 'tag'; # Regex to match line to remove
> $_ = ""; # We need to pre-initialize this so our until loop works
>
> # We need to grab the first line here and set $previous to it since
> # we're buffering 1 line to give us the ability to skip the line
> # preceding our match.
> #$_ = <STDIN>;
> until (not /$match/) {
>    $_ = <STDIN>;
> }
> $previous = $_;
>
> while (<STDIN>) {
>    if (/$match/) {
>       # Don't print anything, and set $skip so we skip the next one.
>       $skip = "yes";
>    }
>    else {
>       unless ($skip) {
>          print $previous;
>       }
>       undef $skip;
>    }
>    $previous = $_;
> }
>
> # We're operating one line behind, so now we need to print the final
> # line, provided we're not skipping.
> unless ($skip) { print $previous }
>
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