GNU bug report logs - #25651
Weird output of random files

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#25651: closed (Weird output of random files)
Date: Tue, 07 Feb 2017 22:32:03 +0000

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Your message dated Tue, 7 Feb 2017 14:30:56 -0800
with message-id <12aa46c3-7684-1b69-dde1-ff45211736b2 <at> cs.ucla.edu>
and subject line Re: bug#25651: Weird output of random files
has caused the debbugs.gnu.org bug report #25651,
regarding Weird output of random files
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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25651: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=25651
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From: "rexkogitans <at> gmx.at" <rexkogitans <at> gmx.at>
To: bug-grep <at> gnu.org
Subject: Weird output of random files
Date: Tue, 7 Feb 2017 21:24:32 +0100

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Hello,

I could repeatedly reproduce an unexpected behaviour on several Linux 
systems (two installations of Arch Linux [newest state], a Debian [8.7] 
and a Ubuntu [16.4]).

I ran the following command - when I first ran it, it was accidentially

ps ax | grep -reh

I wanted to grep for processes containing the string "-reh", but forgot 
the "-e" which was necessary due to the leading dash in the string.

On all systems, grep outputs hundreds of (not even matching) lines from 
files of my home directory, not at all related to the output of ps, 
which I piped into grep.

Maybe, the command line is interpreted as:

   * r ... search files recursively in directories. Note, that input is
   read from stdin here.

   * e ... use the following pattern

   * h ... this is the pattern expected by "e"

It may be that all the lines ouput by grep contain "h".

Although expected to be identical, this command works as expected:

ps ax | grep -reh /dev/stdin

Kind regards,

rexkogitans

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: "rexkogitans <at> gmx.at" <rexkogitans <at> gmx.at>, 25651-done <at> debbugs.gnu.org
Subject: Re: bug#25651: Weird output of random files
Date: Tue, 7 Feb 2017 14:30:56 -0800

On 02/07/2017 12:24 PM, rexkogitans <at> gmx.at wrote:
> Maybe, the command line is interpreted as:
>
>    * r ... search files recursively in directories. Note, that input is
>    read from stdin here.
>
>    * e ... use the following pattern
>
>    * h ... this is the pattern expected by "e"

Yes, that's the interpretation that 'grep' uses, except that it reads 
input from ".", not from stdin. That is, 'grep -r PAT' is equivalen to 
'grep -r PAT .'. This is for convenience: as it does not make sense to 
use -r on an input stream, the default input is '.' instead of '-' when 
-r is used.

grep cannot "know" that the user wanted it to read from stdin, for the 
same reason that it does not "know" that the user wanted it to read from 
stdin here:

   ps ax | grep 0:00 xyz

This causes 'grep' to read from the file 'xyz', instead of the desired 
behavior of looking for the pattern '0:00 xyz' in the 'ps ax' output. 
The 'ps ax' output is ignored.

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