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RE: grep command doc - point out that you can chain now

From: Nick Roberts
Subject: RE: grep command doc - point out that you can chain now
Date: Tue, 18 Jul 2006 17:02:32 +1200

Drew Adams writes:
 >      >     I don't think it should be pointed out in the manual
 >      >     because you would expect it to work.
 >      >
 >      > It never occurred to me, so I think it is worth mentioning in the
 >      > Emacs Manual.  I will add it to the manual.
 >     Probably because you knew it didn't work in the past.
 > Yes, I said that myself.

You predicted Richard's reponse?

 > And, I added, also because what you type is apparently not the arguments to
 > the Unix/GNU-Linux `grep' command, but something possibly much richer. Even
 > if someone were not used to Emacs 20 `grep' behavior, if s?he were used to
 > the Unix/GNU-Linux `grep' command, then s?he would expect to be inputting
 > just the `grep' args, no?

Grep uses start-process just like shell-command.  You saw it as natural to
use the latter with a pipe so I'm surprised that you feel it needs
explaining for grep.

Grep failed in the past because grep-use-null-device was automatically t (or
more accurately "/dev/null" was always appended to the end of the command) By
saying that I would expect it to work, I mean I can't see why it wouldn't.
Could you see a reason?  M-x grep says:

Run grep (like this): grep -nH -e 


Run grep (with the following arguments): -nH -e

 >     It still doesn't work for users with versions of grep which don't
 >     have the -H option, so they will be confused by your description.
 > Do you mean Eli's description? I didn't describe it, beyond mentioning
 > piping. Eli gave a real description of what's going on, presumably.

I mean the person to whom the e-mail was addressed (Richard in this case).
Since you only post to the list, I'm often not sure who you're talking to.

 > I didn't know that either (-H), BTW. That too is worth pointing out. IOW,
 > what you type is different for different `grep's - in one case it is the
 > `grep' args; in another case it is "_any_ shell command whose output
 > resembles that of Grep."
 > Those behavior differences are certainly worth pointing out, unless they are
 > part of the `grep -H' doc. IOW, is what you type the expected arguments for
 > `grep -H', or is it something special that Emacs (only) allows? Does
 > `grep -H'  _itself_ allow "_any_ shell command whose output resembles that
 > of Grep."?
 > If this is part of the behavior of `grep -H' itself, then I guess it
 > wouldn't need to be documented in Emacs. It's not clear to me what the story
 > is. When I do `M-x man RET grep', and look for -H, it says "Print the
 > filename for each match." That can't be what this is about, can it? Yet,
 > with that same `grep' (it says "GNU Project 2002/01/22" in the footer), I
 > get the magical piping. Isn't some explanation necessary? Is this an Emacs
 > `grep' feature or a Unix/GNU-Linux `grep' feature that Emacs manifests?

Behaviour depends on the version of grep and the value of grep-command but
ultimately the value of grep-use-null-device.  It has to do with the fact
that without "-H" grep doesn't output the filename if only one is given
in the input (hence the dummy extra: /dev/null).  I don't which versions
of grep have "-H" or when it was added to GNU/Linux.

Nick                                           http://www.inet.net.nz/~nickrob

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