Skip grep, use awk • Hacker News Board

Re: Skip grep, use awk

#51

Post by hiq » Tue, Jul 04, 2017, 7:13 AM UTC

The author mentions that using awk instead of grep -v is not a good idea, what about:

  awk '! /something/'

Doesn't it reproduce the same behaviour as the following?

  grep -v 'something'

Re: Skip grep, use awk

#52

Post by rflrob » Tue, Jul 04, 2017, 7:23 AM UTC

bluetomcat wrote: post #47

AWK is the general-purpose programmatic filter and reporting tool in the Unix pipeline. Sed, grep and cut are specializations for specific use cases whose implementations might have better performance. Perl and Python are probably too general-purpose for writing compact one-liners in a pipeline.

Even though it's more characters, I usually use perl -pe 's/search/replace/' instead of sed in pipelines because it understands /n (and other escape characters I don't remember). Because all it takes is to get burned a couple times for it to be worth sticking with what you know will work.

Re: Skip grep, use awk

#53

Post by OskarS » Tue, Jul 04, 2017, 7:29 AM UTC

emmelaich wrote: post #40

thaumaturgy wrote: post #19

Earlier quoted context omitted.

Serious question: what would be a better way to do this?

As per other users, you can use pkill but it's not entirely portable in meaning between OSs. This software had to run on just about every possible Unix out there. Solaris, HP-UX, AIX, many others ... My main point is that using perl and grep, (multiple thereof!) is nuts when you can do it all in perl. Also crazy was using -ef (all processes) when the process of interest was using a known user. So ps -u would be more…

If it's something you need to do on a semi-regular basis, using a pidfile seems to me to be the best solution. Instead of trying to be clever about pipelines, just write the pid(s) to a file and use that.

Re: Skip grep, use awk

#54

Post by usgroup » Tue, Jul 04, 2017, 7:47 AM UTC

Except "grep | awk" is often way quicker because it does not need to split the record before it does anything and I usually want to span more processes because awk will usually eat a whole cpu if it's doing something gnarly.

Re: Skip grep, use awk

#55

Post by cyphar » Tue, Jul 04, 2017, 8:44 AM UTC

bewuethr wrote: post #50

cyphar wrote: post #45

Earlier quoted context omitted.

That's what I was referencing when I said > Pulling out a field from a line is what most people use awk for, but it's honestly the least interesting part of awk. In fact, if cut supported regular expressions for specifying the field and record separators people wouldn't be using awk for that purpose (because that's all that $n does). There's nothing magical about awk's default FS. It's literally just /\s+/. If cut's…

The default FS throws away leading blanks, though, which doesn't happen if you set it explicitly to \s+, so a tiny little bit of magic does go on after all.

Fair enough, though ultimately you could emulate it with sed:

    some-cmd | sed -E '{ s/^\s+//g ; s/\s+/ /g }' | cut -f$n

Re: Skip grep, use awk

#56

Post by lkirk » Tue, Jul 04, 2017, 8:54 AM UTC

hiq wrote: post #51

The author mentions that using awk instead of grep -v is not a good idea, what about: awk '! /something/' Doesn't it reproduce the same behaviour as the following? grep -v 'something'

Yes, these are the same

Re: Skip grep, use awk

#57

Post by lkirk » Tue, Jul 04, 2017, 8:56 AM UTC

thaumaturgy wrote: post #19

emmelaich wrote: post #13

My favourite bad example of using grep was from a big enterprise software vendor to kill one of their processes. It looked something like ps -ef| grep SomeDaemon | grep -v grep | grep -v perl | perl -e ' '

Serious question: what would be a better way to do this?

A pid file is the best IMO

Re: Skip grep, use awk

#58

Post by torrent-of-ions » Tue, Jul 04, 2017, 9:11 AM UTC

This is basic awk. But grep is much faster if that's all you're doing. fgrep even more so if that's what you want.

Re: Skip grep, use awk

#59

Post by thinkMOAR » Tue, Jul 04, 2017, 9:12 AM UTC

is there any awk equivalent for grep "matcha\|matchb" ?

Re: Skip grep, use awk

#60

Post by dozzie » Tue, Jul 04, 2017, 9:14 AM UTC

vacri wrote: post #20

lathiat wrote: post #16

Earlier quoted context omitted.

grep -v grep.. I wish I could admit to using this less often than I do; but it's always manual not automated. At some point I'll figure out a better alternative than this or pgrep which often misses processes.

Just do 'grep [S]omeDaemon', and you won't see your grep line.

No, please don't do that. Use filtering capabilities of ps, so grep becomes completely unnecessary.