Replacing sed

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Replacing sed

February 28, 2011 by Jared

I’ve spoken about processing logfiles with perl previously. Occasionally though, I still reach for sed.

Say I have a logfile that looks like this:

[ <timestamp> ] : somefunc()
[ <timestamp> ] : interesting line 1
[ <timestamp> ] : interesting line 2
... 1000s of lines
[ <timestamp> ] : somefunc()
[ <timestamp> ] : interesting line 1
[ <timestamp> ] : interesting line 2
... 1000s of lines

Picking out lines following a pattern is easy with sed – p prints the current match and n takes the next line.

$ < log.txt sed -n '/somefunc()/ {p;n;p;n;p}'
[ <timestamp> ] : somefunc()
[ <timestamp> ] : interesting line 1
[ <timestamp> ] : interesting line 2
[ <timestamp> ] : somefunc()
[ <timestamp> ] : interesting line 1
[ <timestamp> ] : interesting line 2

My first attempt to replace that with perl looks a bit ugly

< log.txt \
perl -ne 'if (/somefunc\(\)/) {print; print scalar(<>) for (1..2)}'

I’m not that happy with the module I came up with to hide the messiness either.

package Logfiles;

require Exporter;
our @ISA = qw(Exporter);
our @EXPORT_OK = qw(process);

use Carp;

sub process (&$;$)
{
    my ($sub, $regex, $lines) = @_;
    $lines ||= 0;
    return unless /$regex/;
    if (! $lines) {
        $sub->($_);
    } else {
        croak "process() arg 3 not ref ARRAY" unless ref($lines) eq 'ARRAY';
        my $line = 0;
        my @lines = @$lines;
        while (1) {
            if ($lines[0] == $line) {
                $sub->($_);
                shift @lines;
            }
            ++$line;
            last if ($line > $lines[0] or (not @lines));
            $_ = <>;
        }
    }
}

1;

But at least my typical spelunking looks a little cleaner now.

< log.txt perl -MLogfiles=process -ne \
    'process { print } qr/somefunc\(\)/, [0..2]'

Any suggestions on how to improve this (without reverting to sed)?

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Posted in Perl | Tagged logfiles, sed | 6 Comments

6 Responses

on February 28, 2011 at 08:38 Mark Fowler

perl -ne ‘print if $a– || ($a=/fsomefunc/*2)’
on February 28, 2011 at 08:44 Mark Fowler

What I was trying to say in my previous comment (before wordpress “smartened” my code) was:
on February 28, 2011 at 14:28 Sue D. Nymme

$lines ||= 0; serves no point, since the very next thing you do with $lines is check it for falsehood.

How about:
package S; # for "Sed" use base 'Exporter'; our @EXPORT = ('p');
sub p # print ARG lines from stdin, starting with current { my $nlines = shift || return; while (--$nlines) { print; $_ = ; } print; }
Then at the command line:
perl -MS -n '/somefunc()/ && p(3)'
on February 28, 2011 at 14:29 Sue D. Nymme

Hmmm, blank lines seem to screw up <code> formatting. Here’s another try.

$lines ||= 0; serves no point, since the very next thing you do with $lines is check it for falsehood.

How about:
package S; # for "Sed" use base 'Exporter'; our @EXPORT = ('p'); sub p # print ARG lines from stdin, starting with current { my $nlines = shift || return; while (--$nlines) { print; $_ = <> ; } print; }
Then at the command line:
perl -MS -n '/somefunc()/ && p(3)'
on February 28, 2011 at 14:31 Sue D. Nymme

Crud. And the second line of the while block in the above function was supposed to be $_ = <>.

Ah, HTML….
on February 28, 2011 at 22:27 Jared

Hi folks,

Sorry about the WordPress code mangling feature. If it makes it any better, it annoys the heck out of me too.

@Mark – neat golfing, and definitely better than my initial attempt :) I’m aiming for clearer rather than terser code (although terse is still important to compete with sed). Not sure if I got it.

@Sue –

I’ve tried to add back the missing angle-brackets. Let me know if I’ve missed any.

Ah yes, $var ||= 0 is a reflex to avoid undefined warnings when I have warnings turned on.

And I definitely like the way you’re going with S. My thinking was that sometimes the very next lines aren’t the interesting ones. Maybe you want the 2nd and 3rd lines following a particular match. It could be that doesn’t happen much in practice mind you.