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Comment by iagooar | Hacker News Reader

This is what I get on a Precise 32bit Ubuntu:

  stuff$ du -sh big.log
  2.8G	big.log

  stuff$ time grep -i e big.log > /dev/null
  real	0m30.228s
  user	0m12.213s
  sys	0m3.228s

  stuff$ time LANG=C grep -i e big.log > /dev/null
  real	0m30.130s
  user	0m12.105s
  sys	0m3.308s

What is LANG=C supposed to do?

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Maybe your default locale is already C?

I'm still surprised that TFA can claim such a speedup, I would have thought IO speed was the bottleneck when you grep through a big amount of data.

As an other poster mentioned I wonder if the speedup is not mainly disk caching in RAM during the 2nd run.

FWIW my locale is en_GB.utf-8 and I also get no difference (in fact, the version with locale is slightly faster than without), with GNU grep 2.14 on OSX 10.9.

The built-in BSD grep (2.5.1-FreeBSD) also runs in 30% of the time GNU grep does.

I just did 'strings /dev/urandom > stuff' for 100M. Got it in memory first with untimed grep, then:

    $ time grep -i blah stuff
    bLAH
    blAH

    real	0m7.227s
    user	0m7.194s
    sys	0m0.011s

    $ time LANG=C grep -i blah stuff
    bLAH
    blAH

    real	0m0.486s
    user	0m0.467s
    sys	0m0.016s

So a pretty big difference. My default lang is en_US.utf8.

Sets the process's locale to C rather than whatever the system's default is (if the process is locale-aware, as "C" is the default locale for C programs). The main change[0] for this case is to also disable encoding (and thus decoding), all text is considered to be ASCII rather than whatever the locale specifies (usually UTF8 these days)

[0] the locale should have an impact on what character ranges match. See http://stackoverflow.com/questions/6799872/how-to-make-grep-... for an example

It's historical and sets your local character set. The default value on FreeBSD is POSIX which is an alias the historically value which is C. Desktop unix's like OSX or ubuntu set it to utf8.

Another note is that this is triggering fgrep which is already fast due to it's fixed length expression (i.e. no recursion is involved)

>What is LANG=C supposed to do?

It changes the charset to do not use utf-8.

Not supporting Unicode often drive me crazy/sad, especially for a diff/patch tool.

It doesn't actually change the charset, it changes the behaviour of a few libc functions and other things that use it (like perl). Sane programs ignore libc locales, as you do not want the behaviour of your program changing randomly based on some environment variables, that way madness lies.

Having control makes me happy/sane. Having unicode set to (on|off) does not affect either diff/patch.

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