Re: grep: show matching line from pattern file

[Jim Popovitch <jim@k4vqc.com>]

On Sat, 2022-05-28 at 17:11 -0400, Greg Wooledge wrote:
> On Sat, May 28, 2022 at 04:02:39PM -0400, The Wanderer wrote:
> > On 2022-05-28 at 15:40, Jim Popovitch wrote:
> > > I have a file of regex patterns and I use grep like so:
> > > 
> > >    ~$ grep -f patterns.txt /var/log/syslog 
> > > 
> > > What I'd like to get is a listing of all lines, specifically the line
> > > numbers of the regexps in patterns.txt, that match entries in
> > > /var/log/syslog.   Is there a way to do this?
> > 
> > I don't know of a standardized way to do that (if anyone else wants to
> > suggest one, I'm open to learn), but of course it *can* be done, via
> > scripting. Off the top of my head, I came up with the following
> > 
> > for line in $(seq 1 $(wc -l patterns.txt | cut -d ' ' -f 1)) ; do
> >   if grep $(head -n $line patterns.txt | tail -n 1) /var/log/syslog >
> > /dev/null ; then
> >     echo $line ;
> >   fi
> > done
> 
> The quoting here is... completely absent (and that's extremely bad), but
> also importantly, one would ideally like to avoid running grep a thousand
> times, especially if the target logfile is large.
> 
> I believe this is the kind of job for which perl is well-suited.  I'm not
> great at perl, but I'll give it a shot.
> 
> Here's a version with some extra information as output, so I can verify
> that it's doing something reasonably close to correct:
> 
> 
> #!/usr/bin/perl
> use strict; use warnings;
> 
> my @patlist;
> open PATS, "<patterns.txt" || die "can't open patterns.txt";
> chomp(@patlist = <PATS>);
> close PATS;
> 
> while (<>) {
>     chomp;
>     for (my $i = 0; $i <= $#patlist; $i++) {
> 	print "$i|$patlist[$i]|$_\n" if /$patlist[$i]/;
>     }
> }
> 
> 
> Now, to test it, we need a patterns.txt file:
> 
> 
> unicorn:~$ cat patterns.txt 
> PATH
> HOME|~
> a...e
> 
> 
> And an input (log) file:
> 
> 
> unicorn:~$ cat file
> zebra
> Home, home on the range.
> Oops, I meant HOME on the range.
> 
> applesauce
> 
> 
> And here's what it does:
> 
> 
> unicorn:~$ ./foo file
> 1|HOME|~|Oops, I meant HOME on the range.
> 2|a...e|applesauce
> 
> 
> Pattern numbers 1 and 2 (the second and third, since it starts at 0) were
> matched, so we have a line for each of those.
> 
> If that's kinda what you wanted, then you can adjust this to do precisely
> what you wanted.  It shouldn't take a lot of work, I hope.  Well, I guess
> that depends on what you really want.
> 
> Bash is not well-suited to this task, and even if we were to take The
> Wanderer's script and fix all the issues in it, it would still be a
> vastly inferior solution.  Some tools are just not meant for some jobs.
> 

Thanks Greg, that is exactly what I needed, and double thanks for the
details in explaining it, etc. 

-Jim P.