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Re: grep: show matching line from pattern file

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On 2022-05-28 at 17:11, 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.

A brother of mine has schooled me on several things I did wrong here
already, and I am now going over my long-tail stockpile of scripts with
shellcheck, seeing what I can learn. (I normally do quote variables, but
for some reason it slipped my mind this time until after I'd already hit
Send. Some of these are known-safe values which won't need quoting, but
others aren't, so the principle remains valid to cite.)

This wasn't especially intended as a great solution, in any case; it
started out as me trying to write shell-like pseudocode, but then I
couldn't see any obvious reason why it wouldn't work, and when I tried
it out I only needed a few tweaks before it did. There's a *reason* I
had the "YMMV" on that message.

-- 
   The Wanderer

The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore all
progress depends on the unreasonable man.         -- George Bernard Shaw

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grep: show matching line from pattern file Jim Popovitch <jim@k4vqc.com> - 2022-05-28 22:00 +0200
  Re: grep: show matching line from pattern file The Wanderer <wanderer@fastmail.fm> - 2022-05-28 22:10 +0200
    Re: grep: show matching line from pattern file Greg Wooledge <greg@wooledge.org> - 2022-05-28 23:20 +0200
      Re: grep: show matching line from pattern file The Wanderer <wanderer@fastmail.fm> - 2022-05-28 23:30 +0200
      Re: grep: show matching line from pattern file Jim Popovitch <jim@k4vqc.com> - 2022-05-29 19:50 +0200
      Re: grep: show matching line from pattern file Will Mengarini <seldon@eskimo.com> - 2022-06-03 00:30 +0200
        Re: grep: show matching line from pattern file David Christensen <dpchrist@holgerdanske.com> - 2022-06-03 01:00 +0200
          Re: grep: show matching line from pattern file Will Mengarini <seldon@eskimo.com> - 2022-06-03 02:20 +0200
  Re: grep: show matching line from pattern file Jörg-Volker Peetz <jvpeetz@web.de> - 2022-05-29 15:10 +0200
    Re: grep: show matching line from pattern file David Wright <deblis@lionunicorn.co.uk> - 2022-05-29 15:50 +0200
      Re: grep: show matching line from pattern file duh <fill_in_the_blanks@email.com> - 2022-06-02 21:20 +0200
        Re: grep: show matching line from pattern file Greg Wooledge <greg@wooledge.org> - 2022-06-02 21:20 +0200
          Re: grep: show matching line from pattern file Richard Hector <richard@walnut.gen.nz> - 2022-06-04 05:00 +0200

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