grep.texi

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grep 'hello\>' *
@end example

@noindent
searches only for words ending in @samp{hello}, so it matches the word
@samp{Othello}.

@item
How do I output context around the matching lines?

@example
grep -C 2 'hello' *
@end example

@noindent
prints two lines of context around each matching line.

@item
How do I force grep to print the name of the file?

Append @file{/dev/null}:

@example
grep 'eli' /etc/passwd /dev/null
@end example

gets you:

@smallexample
/etc/passwd:eli:DNGUTF58.IMe.:98:11:Eli Smith:/home/do/eli:/bin/bash
@end smallexample

@item
Why do people use strange regular expressions on @command{ps} output?

@example
ps -ef | grep '[c]ron'
@end example

If the pattern had been written without the square brackets, it would
have matched not only the @command{ps} output line for @command{cron},
but also the @command{ps} output line for @command{grep}.
Note that some platforms @command{ps} limit the ouput to the width
of the screen, grep does not have any limit on the length of a line
except the available memory.

@item
Why does @command{grep} report ``Binary file matches''?

If @command{grep} listed all matching ``lines'' from a binary file, it
would probably generate output that is not useful, and it might even
muck up your display.  So @sc{gnu} @command{grep} suppresses output from
files that appear to be binary files.  To force @sc{gnu} @command{grep}
to output lines even from files that appear to be binary, use the
@samp{-a} or @samp{--binary-files=text} option.  To eliminate the
``Binary file matches'' messages, use the @samp{-I} or
@samp{--binary-files=without-match} option.

@item
Why doesn't @samp{grep -lv} print nonmatching file names?

@samp{grep -lv} lists the names of all files containing one or more
lines that do not match.  To list the names of all files that contain no
matching lines, use the @samp{-L} or @samp{--files-without-match}
option.

@item
I can do @sc{or} with @samp{|}, but what about @sc{and}?

@example
grep 'paul' /etc/motd | grep 'franc,ois'
@end example

@noindent
finds all lines that contain both @samp{paul} and @samp{franc,ois}.

@item
How can I search in both standard input and in files?

Use the special file name @samp{-}:

@example
cat /etc/passwd | grep 'alain' - /etc/motd
@end example

@item
@cindex palindromes
How to express palindromes in a regular expression?

It can be done by using the back referecences, for example a palindrome
of 4 chararcters can be written in BRE.

@example
grep -w -e '\(.\)\(.\).\2\1' file
@end example

It matches the word "radar" or "civic".

Guglielmo Bondioni proposed a single RE that finds all the palindromes up to 19
characters long.

@example
egrep -e '^(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?).?\9\8\7\6\5\4\3\2\1$' file
@end example

Note this is done by using GNU ERE extensions, it might not be portable on
other greps.

@item
Why are my expressions whith the vertical bar fail?

@example
/bin/echo "ba" | egrep '(a)\1|(b)\1'
@end example

The first alternate branch fails then the first group was not in the match
this will make the second alternate branch fails.  For example, "aaba" will
match, the first group participate in the match and can be reuse in the
second branch.

@item
What do @command{grep, fgrep, egrep} stand for ?

grep comes from the way line editing was done on Unix.  For example,
@command{ed} uses this syntax to print a list of matching lines on the screen.

@example
global/regular expression/print
g/re/p
@end example

@command{fgrep} stands for Fixed @command{grep}, @command{egrep} Extended
@command{grep}.

@end enumerate

@node Reporting Bugs, Copying, Usage, Top
@chapter Reporting bugs

@cindex Bugs, reporting
Email bug reports to @email{bug-gnu-utils@@gnu.org}.
Be sure to include the word ``grep'' somewhere in the ``Subject:'' field.

Large repetition counts in the @samp{@{n,m@}} construct may cause
@command{grep} to use lots of memory.  In addition, certain other
obscure regular expressions require exponential time and
space, and may cause grep to run out of memory.
Back-references are very slow, and may require exponential time.

@node Copying, GNU General Public License, Reporting Bugs, Top
@chapter Copying
@cindex Copying
GNU grep is licensed under the GNU GPL, which makes it @dfn{free
software}.

Please note that ``free'' in ``free software'' refers to liberty, not
price.  As some GNU project advocates like to point out, think of ``free
speech'' rather than ``free beer''.  The exact and legally binding
distribution terms are spelled out below; in short, you have the right
(freedom) to run and change grep and distribute it to other people, and
even---if you want---charge money for doing either.  The important
restriction is that you have to grant your recipients the same rights
and impose the same restrictions.

This method of licensing software is also known as @dfn{open source}
because, among other things, it makes sure that all recipients will
receive the source code along with the program, and be able to improve
it.  The GNU project prefers the term ``free software'' for reasons
outlined at
@url{http://www.gnu.org/philosophy/free-software-for-freedom.html}.

The exact license terms are defined by this paragraph and the GNU
General Public License it refers to:

@quotation
GNU grep is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2 of the License, or (at your
option) any later version.

GNU grep is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

A copy of the GNU General Public License is included as part of this
manual; if you did not receive it, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
@end quotation

In addition to this, this manual is free in the same sense:

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``GNU Free
Documentation License'', with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the section
entitled ``GNU Free Documentation License''.
@end quotation

@c #### Maybe we should wrap these licenses in ifinfo?  Stallman says
@c that the GFDL needs to be present in the manual, and to me it would
@c suck to include the license for the manual and not the license for
@c the program.

The full texts of the GNU General Public License and of the GNU Free
Documentation License are available below.

@menu
* GNU General Public License:: GNU GPL
* GNU Free Documentation License:: GNU FDL
@end menu

@node GNU General Public License, GNU Free Documentation License, Copying, Copying
@section GNU General Public License
@center Version 2, June 1991
@cindex GPL, GNU General Public License

@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

  Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

  The precise terms and conditions for copying, distribution and
modification follow.

@iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of