Re: OT: grep for \x00 = NUL

Re: OT: grep for \x00 = NUL

[Brian Dessent]

fergus wrote:

>         grep $'\x0d' # e.g. equivalent to grep "^M"

Using that method you're passing the literal character to grep through a
bash quoting mechanism.  You can't pass a literal NULL as part of a
command line because argv[] consists of NULL-delimited strings, as do
most C string functions.

Why not just tell grep to look for a NULL rather than trying to feed it
an actual literal NULL character?

$ grep -P '\000'

In pcre regexps you can use \nnn to match any character represented by
octal nnn.  This would work for any character, and it doesn't rely on a
bash-specific shell feature.  But it does rely on grep supporting -P for
pcre, which is not universal.  If that cannot be relied on then you can
use

$ perl -ne 'print if m/\000/'

or

$ awk '/\000/ { print }'

Brian

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fftw library

[javier sacristan]

Hi, and thanks for your help, I would like to know if someone has already
installed a FFTW library to perform Fourier transforms. If so, could you
explain me how to do it?

Thank you


Javier



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OT: grep for \x00 = NUL

[fergus]

Sorry, probably off topic but so tantalising and so easily stated I thought
I'd try you. Using grep to locate hex characters in files I've used the
syntax

	grep $'\x0d' # e.g. equivalent to grep "^M"
	grep $'\x1a' # e.g. equivalent to grep "^Z"

and found this to work for all \x01 to \xff, that's 255 of the 256 one might
be interested in looking for. (Well, I haven't tried them all, but a lot of
them.) What I still can't do (actually, what I most wanted to do) is search
for the NUL \x00. Nothing in info bash indicated that this approach would
not work for \x00, but as far as I can tell, it doesn't: files that do not
contain \x00 are listed as containing it (try grep $'\x00' *).

Is it a bug?

Any ideas? (This has cropped up before but sorry I can't seem to find it.
Various suggestions included piping output from od to grep, but this
approach just involving grep seems much more direct if it can be made to
work.)

Fergus


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