Updated Catalan translation from Translation Project

[findutils.git] / doc / find.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename find.info
@settitle Finding Files
@c For double-sided printing, uncomment:
@c @setchapternewpage odd
@c %**end of header

@include version.texi

@iftex
@finalout
@end iftex

@dircategory Basics
@direntry
* Finding files: (find).        Operating on files matching certain criteria.
@end direntry

@dircategory Individual utilities
@direntry
* find: (find)Invoking find.                    Finding and acting on files.
* locate: (find)Invoking locate.                Finding files in a database.
* updatedb: (find)Invoking updatedb.            Building the locate database.
* xargs: (find)Invoking xargs.                  Operating on many files.
@end direntry

@copying

This file documents the GNU utilities for finding files that match
certain criteria and performing various operations on them.

Copyright (C) 1994, 1996, 1998, 2000, 2001, 2003, 2004, 2005 Free
Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.
@end copying

@titlepage
@title Finding Files
@subtitle Edition @value{EDITION}, for GNU @code{find} version @value{VERSION}
@subtitle @value{UPDATED}
@author by David MacKenzie

@page
@vskip 0pt plus 1filll
@insertcopying{}
@end titlepage

@contents

@ifnottex
@node Top, Introduction, , (dir)
@comment  node-name,  next,  previous,  up

This file documents the GNU utilities for finding files that match
certain criteria and performing various actions on them.

This is edition @value{EDITION}, for @code{find} version @value{VERSION}.
@end ifnottex

@c The master menu, created with texinfo-master-menu, goes here.

@menu
* Introduction::                Summary of the tasks this manual describes.
* Finding Files::               Finding files that match certain criteria.
* Actions::                     Doing things to files you have found.
* Databases::                   Maintaining file name databases.
* File Permissions::            How to control access to files.
* Reference::                   Summary of how to invoke the programs.
* Common Tasks::                Solutions to common real-world problems.
* Worked Examples::             Examples demonstrating more complex points.
* Security Considerations::     Security issues relating to findutils.
* Error Messages::              Explanations of some messages you might see.
* Primary Index::               The components of @code{find} expressions.
@end menu

@node Introduction, Finding Files, Top, Top
@chapter Introduction

This manual shows how to find files that meet criteria you specify,
and how to perform various actions on the files that you find.  The
principal programs that you use to perform these tasks are
@code{find}, @code{locate}, and @code{xargs}.  Some of the examples in
this manual use capabilities specific to the GNU versions of those
programs.

GNU @code{find} was originally written by Eric Decker, with
enhancements by David MacKenzie, Jay Plett, and Tim Wood.  GNU
@code{xargs} was originally written by Mike Rendell, with enhancements
by David MacKenzie.  GNU @code{locate} and its associated utilities
were originally written by James Woods, with enhancements by David
MacKenzie.  The idea for @samp{find -print0} and @samp{xargs -0} came
from Dan Bernstein.  The current maintainer of GNU findutils (and this
manual) is James Youngman.  Many other people have contributed bug
fixes, small improvements, and helpful suggestions.  Thanks!

Mail suggestions and bug reports for these programs to
@code{bug-findutils@@gnu.org}.  Please include the version
number, which you can get by running @samp{find --version}.

@menu
* Scope::
* Overview::
* find Expressions::
@end menu

@node Scope
@section Scope

For brevity, the word @dfn{file} in this manual means a regular file,
a directory, a symbolic link, or any other kind of node that has a
directory entry.  A directory entry is also called a @dfn{file name}.
A file name may contain some, all, or none of the directories in a
path that leads to the file.  These are all examples of what this
manual calls ``file names'':

@example
parser.c
README
./budget/may-94.sc
fred/.cshrc
/usr/local/include/termcap.h
@end example

A @dfn{directory tree} is a directory and the files it contains, all
of its subdirectories and the files they contain, etc.  It can also be
a single non-directory file.

These programs enable you to find the files in one or more directory
trees that:

@itemize @bullet
@item
have names that contain certain text or match a certain pattern;
@item
are links to certain files;
@item
were last used during a certain period of time;
@item
are within a certain size range;
@item
are of a certain type (regular file, directory, symbolic link, etc.);
@item
are owned by a certain user or group;
@item
have certain access permissions;
@item
contain text that matches a certain pattern;
@item
are within a certain depth in the directory tree;
@item
or some combination of the above.
@end itemize

Once you have found the files you're looking for (or files that are
potentially the ones you're looking for), you can do more to them than
simply list their names.  You can get any combination of the files'
attributes, or process the files in many ways, either individually or
in groups of various sizes.  Actions that you might want to perform on
the files you have found include, but are not limited to:

@itemize @bullet
@item
view or edit
@item
store in an archive
@item
remove or rename
@item
change access permissions
@item
classify into groups
@end itemize

This manual describes how to perform each of those tasks, and more.

@node Overview
@section Overview

The principal programs used for making lists of files that match given
criteria and running commands on them are @code{find}, @code{locate},
and @code{xargs}.  An additional command, @code{updatedb}, is used by
system administrators to create databases for @code{locate} to use.

@code{find} searches for files in a directory hierarchy and prints
information about the files it found.  It is run like this:

@example
find @r{[}@var{file}@dots{}@r{]} @r{[}@var{expression}@r{]}
@end example

@noindent
Here is a typical use of @code{find}.  This example prints the names
of all files in the directory tree rooted in @file{/usr/src} whose
name ends with @samp{.c} and that are larger than 100 Kilobytes.
@example
find /usr/src -name '*.c' -size +100k -print
@end example

Notice that the wildcard must be enclosed in quotes in order to
protect it from expansion by the shell.

@code{locate} searches special file name databases for file names that
match patterns.  The system administrator runs the @code{updatedb}
program to create the databases.  @code{locate} is run like this:

@example
locate @r{[}@var{option}@dots{}@r{]} @var{pattern}@dots{}
@end example

@noindent
This example prints the names of all files in the default file name
database whose name ends with @samp{Makefile} or @samp{makefile}.
Which file names are stored in the database depends on how the system
administrator ran @code{updatedb}.
@example
locate '*[Mm]akefile'
@end example

The name @code{xargs}, pronounced EX-args, means ``combine
arguments.''  @code{xargs} builds and executes command lines by
gathering together arguments it reads on the standard input.  Most
often, these arguments are lists of file names generated by
@code{find}.  @code{xargs} is run like this:

@example
xargs @r{[}@var{option}@dots{}@r{]} @r{[}@var{command} @r{[}@var{initial-arguments}@r{]}@r{]}
@end example

@noindent
The following command searches the files listed in the file
@file{file-list} and prints all of the lines in them that contain the
word @samp{typedef}.
@example
xargs grep typedef < file-list
@end example

@node find Expressions
@section @code{find} Expressions

The expression that @code{find} uses to select files consists of one
or more @dfn{primaries}, each of which is a separate command line
argument to @code{find}.  @code{find} evaluates the expression each
time it processes a file.  An expression can contain any of the
following types of primaries:

@table @dfn
@item options
affect overall operation rather than the processing of a specific
file;
@item tests
return a true or false value, depending on the file's attributes;
@item actions
have side effects and return a true or false value; and
@item operators
connect the other arguments and affect when and whether they are
evaluated.
@end table

You can omit the operator between two primaries; it defaults to
@samp{-and}.  @xref{Combining Primaries With Operators}, for ways to
connect primaries into more complex expressions.  If the expression
contains no actions other than @samp{-prune}, @samp{-print} is
performed on all files for which the entire expression is true
(@pxref{Print File Name}).

Options take effect immediately, rather than being evaluated for each
file when their place in the expression is reached.  Therefore, for
clarity, it is best to place them at the beginning of the expression.

Many of the primaries take arguments, which immediately follow them in
the next command line argument to @code{find}.  Some arguments are
file names, patterns, or other strings; others are numbers.  Numeric
arguments can be specified as

@table @code
@item +@var{n}
for greater than @var{n},
@item -@var{n}
for less than @var{n},
@item @var{n}
for exactly @var{n}.
@end table

@node Finding Files, Actions, Introduction, Top
@chapter Finding Files

By default, @code{find} prints to the standard output the names of the
files that match the given criteria.  @xref{Actions}, for how to get
more information about the matching files.


@menu
* Name::
* Links::
* Time::
* Size::
* Type::
* Owner::
* Permissions::
* Contents::
* Directories::
* Filesystems::
* Combining Primaries With Operators::
@end menu

@node Name
@section Name

Here are ways to search for files whose name matches a certain
pattern.  @xref{Shell Pattern Matching}, for a description of the
@var{pattern} arguments to these tests.

Each of these tests has a case-sensitive version and a
case-insensitive version, whose name begins with @samp{i}.  In a
case-insensitive comparison, the patterns @samp{fo*} and @samp{F??}
match the file names @file{Foo}, @samp{FOO}, @samp{foo}, @samp{fOo},
etc.

@menu
* Base Name Patterns::
* Full Name Patterns::
* Fast Full Name Search::
* Shell Pattern Matching::      Wildcards used by these programs.
@end menu

@node Base Name Patterns
@subsection Base Name Patterns

@deffn Test -name pattern
@deffnx Test -iname pattern
True if the base of the file name (the path with the leading
directories removed) matches shell pattern @var{pattern}.  For
@samp{-iname}, the match is case-insensitive.  To ignore a whole
directory tree, use @samp{-prune} (@pxref{Directories}).  As an
example, to find Texinfo source files in @file{/usr/local/doc}:

@example
find /usr/local/doc -name '*.texi'
@end example
@end deffn

Notice that the wildcard must be enclosed in quotes in order to
protect it from expansion by the shell.

Patterns for @samp{-name} and @samp{-iname} will match a file name
with a leading @samp{.}.  For example the command @samp{find /tmp
-name \*bar} will match the file @file{/tmp/.foobar}.


@node Full Name Patterns
@subsection Full Name Patterns

@deffn Test -wholename pattern
@deffnx Test -iwholename pattern
True if the entire file name, starting with the command line argument
under which the file was found, matches shell pattern @var{pattern}.
For @samp{-iwholename}, the match is case-insensitive.  To ignore a
whole directory tree, use @samp{-prune} rather than checking every
file in the tree (@pxref{Directories}).  The ``entire file name'' as
used by @code{find} starts with the starting-point specified on the
command line, and is not converted to an absolute pathname, so for
example @code{cd /; find tmp -wholename /tmp} will never match
anything.
@end deffn

@deffn Test -path pattern
@deffnx Test -ipath pattern
These tests are deprecated, but work as for @samp{-wholename} and
@samp{-iwholename}, respectively.  The @samp{-ipath} test is a GNU
extension, but @samp{-path} is also provided by HP-UX @code{find}.
@end deffn

@deffn Test -regex expr
@deffnx Test -iregex expr
True if the entire file name matches regular expression @var{expr}.
This is a match on the whole path, not a search.  For example, to
match a file named @file{./fubar3}, you can use the regular expression
@samp{.*bar.} or @samp{.*b.*3}, but not @samp{f.*r3}.  @xref{Regexps,
, Syntax of Regular Expressions, emacs, The GNU Emacs Manual}, for a
description of the syntax of regular expressions.  For @samp{-iregex},
the match is case-insensitive.  There are several varieties of regular
expressions; by default this test uses POSIX basic regular
expressions, but this can be changed with the option
@samp{-regextype}.
@end deffn

@deffn Option -regextype name
This option controls the variety of regular expression syntax
understood by the @samp{-regex} and @samp{-iregex} tests.  This option
is positional; that is, it only affects regular expressions which
occur later in the command line.  If this option is not given, GNU
Emacs regular expressions are assumed.  Currently-implemented types
are


@table @samp
@item emacs
Regular expressions compatible with GNU Emacs; this is also the
default behaviour if this option is not used.
@item posix-awk
Regular expressions compatible with the POSIX awk command (not GNU awk)
@item posix-basic
POSIX Basic Regular Expressions.
@item posix-egrep
Regular expressions compatible with the POSIX egrep command
@item posix-extended
POSIX Extended Regular Expressions
@end table

@ref{Regular Expressions} for more information on the regular
expression dialects understood by GNU findutils.


@end deffn

@node Fast Full Name Search
@subsection Fast Full Name Search

To search for files by name without having to actually scan the
directories on the disk (which can be slow), you can use the
@code{locate} program.  For each shell pattern you give it,
@code{locate} searches one or more databases of file names and
displays the file names that contain the pattern.  @xref{Shell Pattern
Matching}, for details about shell patterns.

If a pattern is a plain string---it contains no
metacharacters---@code{locate} displays all file names in the database
that contain that string.  If a pattern contains
metacharacters, @code{locate} only displays file names that match the
pattern exactly.  As a result, patterns that contain metacharacters
should usually begin with a @samp{*}, and will most often end with one
as well.  The exceptions are patterns that are intended to explicitly
match the beginning or end of a file name.

If you only want @code{locate} to match against the last component of
the file names (the ``base name'' of the files) you can use the
@samp{--basename} option.  The opposite behaviour is the default, but
can be selected explicitly by using the option @samp{--wholename}.

The command
@example
locate @var{pattern}
@end example

is almost equivalent to
@example
find @var{directories} -name @var{pattern}
@end example

where @var{directories} are the directories for which the file name
databases contain information.  The differences are that the
@code{locate} information might be out of date, and that @code{locate}
handles wildcards in the pattern slightly differently than @code{find}
(@pxref{Shell Pattern Matching}).

The file name databases contain lists of files that were on the system
when the databases were last updated.  The system administrator can
choose the file name of the default database, the frequency with which
the databases are updated, and the directories for which they contain
entries.

Here is how to select which file name databases @code{locate}
searches.  The default is system-dependent.

@table @code
@item --database=@var{path}
@itemx -d @var{path}
Instead of searching the default file name database, search the file
name databases in @var{path}, which is a colon-separated list of
database file names.  You can also use the environment variable
@code{LOCATE_PATH} to set the list of database files to search.  The
option overrides the environment variable if both are used.
@end table

@node Shell Pattern Matching
@subsection Shell Pattern Matching

@code{find} and @code{locate} can compare file names, or parts of file
names, to shell patterns.  A @dfn{shell pattern} is a string that may
contain the following special characters, which are known as
@dfn{wildcards} or @dfn{metacharacters}.

You must quote patterns that contain metacharacters to prevent the
shell from expanding them itself.  Double and single quotes both work;
so does escaping with a backslash.

@table @code
@item *
Matches any zero or more characters.

@item ?
Matches any one character.

@item [@var{string}]
Matches exactly one character that is a member of the string
@var{string}.  This is called a @dfn{character class}.  As a
shorthand, @var{string} may contain ranges, which consist of two
characters with a dash between them.  For example, the class
@samp{[a-z0-9_]} matches a lowercase letter, a number, or an
underscore.  You can negate a class by placing a @samp{!} or @samp{^}
immediately after the opening bracket.  Thus, @samp{[^A-Z@@]} matches
any character except an uppercase letter or an at sign.

@item \
Removes the special meaning of the character that follows it.  This
works even in character classes.
@end table

In the @code{find} tests that do shell pattern matching (@samp{-name},
@samp{-wholename}, etc.), wildcards in the pattern will match a
@samp{.}  at the beginning of a file name.  This is also the case for
@code{locate}.  Thus, @samp{find -name '*macs'} will match a file
named @file{.emacs}, as will @samp{locate '*macs'}.

Slash characters have no special significance in the shell pattern
matching that @code{find} and @code{locate} do, unlike in the shell,
in which wildcards do not match them.  Therefore, a pattern
@samp{foo*bar} can match a file name @samp{foo3/bar}, and a pattern
@samp{./sr*sc} can match a file name @samp{./src/misc}.

If you want to locate some files with the @samp{locate} command but
don't need to see the full list you can use the @samp{--limit} option
to see just a small number of results, or the @samp{--count} option to
display only the total number of matches.

@node Links
@section Links

There are two ways that files can be linked together.  @dfn{Symbolic
links} are a special type of file whose contents are a portion of the
name of another file.  @dfn{Hard links} are multiple directory entries
for one file; the file names all have the same index node
(@dfn{inode}) number on the disk.

@menu
* Symbolic Links::
* Hard Links::
@end menu

@node Symbolic Links
@subsection Symbolic Links

Symbolic links are names that reference other files.  GNU @code{find}
will handle symbolic links in one of two ways; firstly, it can
dereference the links for you - this means that if it comes across a
symbolic link, it examines the file that the link points to, in order
to see if it matches the criteria you have specified.  Secondly, it
can check the link itself in case you might be looking for the actual
link.  If the file that the symbolic link points to is also within the
directory hierarchy you are searching with the @code{find} command,
you may not see a great deal of difference between these two
alternatives.

By default, @code{find} examines symbolic links themselves when it
finds them (and, if it later comes across the linked-to file, it will
examine that, too).  If you would prefer @code{find} to dereference
the links and examine the file that each link points to, specify the
@samp{-L} option to @code{find}.  You can explicitly specify the
default behaviour by using the @samp{-P} option.  The @samp{-H}
option is a half-way-between option which ensures that any symbolic
links listed on the command line are dereferenced, but other symbolic
links are not.

Symbolic links are different to ``hard links'' in the sense that you
need permissions upon the linked-to file in order to be able to
dereference the link.  This can mean that even if you specify the
@samp{-L} option, @code{find} may not be able to determine the
properties of the file that the link points to (because you don't have
sufficient permissions).  In this situation, @code{find} uses the
properties of the link itself.  This also occurs if a symbolic link
exists but points to a file that is missing.

The options controlling the behaviour of @code{find} with respect to
links are as follows :-

@table @samp
@item -P
@code{find} does not dereference symbolic links at all.  This is the
default behaviour.  This option must be specified before any of the
file names on the command line.
@item -H
@code{find} does not dereference symbolic links (except in the case of
file names on the command line, which are dereferenced).  If a
symbolic link cannot be dereferenced, the information for the symbolic
link itself is used.  This option must be specified before any of the
file names on the command line.
@item -L
@code{find} dereferences symbolic links where possible, and where this
is not possible it uses the properties of the symbolic link itself.
This option must be specified before any of the file names on the
command line.  Use of this option also implies the same behaviour as
the @samp{-noleaf} option.  If you later use the @samp{-H} or
@samp{-P} options, this does not turn off @samp{-noleaf}.

@item -follow
This option forms part of the ``expression'' and must be specified
after the file names, but it is otherwise equivalent to @samp{-L}.
@end table

The following differences in behavior occur when the @samp{-L} option
is used:

@itemize @bullet
@item
@code{find} follows symbolic links to directories when searching
directory trees.
@item
@samp{-lname} and @samp{-ilname} always return false (unless they
happen to match broken symbolic links).
@item
@samp{-type} reports the types of the files that symbolic links point
to.
@item
Implies @samp{-noleaf} (@pxref{Directories}).
@end itemize

If the @samp{-L} option or the @samp{-H} option is used,
the file names used as arguments to @samp{-newer}, @samp{-anewer}, and
@samp{-cnewer} are dereferenced and the timestamp from the pointed-to
file is used instead (if possible -- otherwise the timestamp from the
symbolic link is used).

@deffn Test -lname pattern
@deffnx Test -ilname pattern
True if the file is a symbolic link whose contents match shell pattern
@var{pattern}.  For @samp{-ilname}, the match is case-insensitive.
@xref{Shell Pattern Matching}, for details about the @var{pattern}
argument.  If the @samp{-L} option is in effect, this test will always
return false for symbolic links unless they are broken.  So, to list
any symbolic links to @file{sysdep.c} in the current directory and its
subdirectories, you can do:

@example
find . -lname '*sysdep.c'
@end example
@end deffn

@node Hard Links
@subsection Hard Links

Hard links allow more than one name to refer to the same file.  To
find all the names which refer to the same file as NAME, use
@samp{-samefile NAME}.  If you are not using the @samp{-L} option, you
can confine your search to one filesystem using the @samp{-xdev}
option.  This is useful because hard links cannot point outside a
single filesystem, so this can cut down on needless searching.

If the @samp{-L} option is in effect, and NAME is in fact a symbolic
link, the symbolic link will be dereferenced.  Hence you are searching
for other links (hard or symbolic) to the file pointed to by NAME.  If
@samp{-L} is in effect but NAME is not itself a symbolic link, other
symbolic links to the file NAME will be matched.

You can also search for files by inode number.  This can occasionally
be useful in diagnosing problems with filesystems for example, because
@code{fsck} tends to print inode numbers.  Inode numbers also
occasionally turn up in log messages for some types of software, and
are used to support the @code{ftok()} library function.

You can learn a file's inode number and the number of links to it by
running @samp{ls -li} or @samp{find -ls}.

You can search for hard links to inode number NUM by using @samp{-inum
NUM}. If there are any filesystem mount points below the directory
where you are starting the search, use the @samp{-xdev} option unless
you are also using the @samp{-L} option.  Using @samp{-xdev} this
saves needless searching, since hard links to a file must be on the
same filesystem.  @xref{Filesystems}.

@deffn Test -samefile NAME
File is a hard link to the same inode as NAME.  If the @samp{-L}
option is in effect, symbolic links to the same file as NAME points to
are also matched.
@end deffn

@deffn Test -inum n
File has inode number @var{n}.  The @samp{+} and @samp{-} qualifiers
also work, though these are rarely useful.
@end deffn

You can also search for files that have a certain number of links,
with @samp{-links}.  Directories normally have at least two hard
links; their @file{.} entry is the second one.  If they have
subdirectories, each of those also has a hard link called @file{..} to
its parent directory.  The @file{.} and @file{..} directory entries
are not normally searched unless they are mentioned on the @code{find}
command line.

@deffn Test -links n
File has @var{n} hard links.
@end deffn

@deffn Test -links +n
File has more than @var{n} hard links.
@end deffn

@deffn Test -links -n
File has fewer than @var{n} hard links.
@end deffn

@node Time
@section Time

Each file has three time stamps, which record the last time that
certain operations were performed on the file:

@enumerate
@item
access (read the file's contents)
@item
change the status (modify the file or its attributes)
@item
modify (change the file's contents)
@end enumerate

There is no timestamp that indicates when a file was @emph{created}.

You can search for files whose time stamps are within a certain age
range, or compare them to other time stamps.

@menu
* Age Ranges::
* Comparing Timestamps::
@end menu

@node Age Ranges
@subsection Age Ranges

These tests are mainly useful with ranges (@samp{+@var{n}} and
@samp{-@var{n}}).

@deffn Test -atime n
@deffnx Test -ctime n
@deffnx Test -mtime n
True if the file was last accessed (or its status changed, or it was
modified) @var{n}*24 hours ago.  The number of 24-hour periods since
the file's timestamp is always rounded down; therefore 0 means ``less
than 24 hours ago'', 1 means ``between 24 and 48 hours ago'', and so
forth.
@end deffn

@deffn Test -amin n
@deffnx Test -cmin n
@deffnx Test -mmin n
True if the file was last accessed (or its status changed, or it was
modified) @var{n} minutes ago.  These tests provide finer granularity
of measurement than @samp{-atime} et al., but rounding is done in a
similar way.  For example, to list files in @file{/u/bill} that were
last read from 2 to 6 minutes ago:

@example
find /u/bill -amin +2 -amin -6
@end example
@end deffn

@deffn Option -daystart
Measure times from the beginning of today rather than from 24 hours
ago.  So, to list the regular files in your home directory that were
modified yesterday, do

@example
find ~ -daystart -type f -mtime 1
@end example
@end deffn

The @samp{-daystart} option is unlike most other options in that it
has an effect on the way that other tests are performed.  The affected
tests are @samp{-amin}, @samp{-cmin}, @samp{-mmin}, @samp{-atime},
@samp{-ctime} and @samp{-mtime}.

@node Comparing Timestamps
@subsection Comparing Timestamps

As an alternative to comparing timestamps to the current time, you can
compare them to another file's timestamp.  That file's timestamp could
be updated by another program when some event occurs.  Or you could
set it to a particular fixed date using the @code{touch} command.  For
example, to list files in @file{/usr} modified after February 1 of the
current year:

@c Idea from Rick Sladkey.
@example
touch -t 02010000 /tmp/stamp$$
find /usr -newer /tmp/stamp$$
rm -f /tmp/stamp$$
@end example

@deffn Test -anewer file
@deffnx Test -cnewer file
@deffnx Test -newer file
True if the file was last accessed (or its status changed, or it was
modified) more recently than @var{file} was modified.  These tests are
affected by @samp{-follow} only if @samp{-follow} comes before them on
the command line.  @xref{Symbolic Links}, for more information on
@samp{-follow}.  As an example, to list any files modified since
@file{/bin/sh} was last modified:

@example
find . -newer /bin/sh
@end example
@end deffn

@deffn Test -used n
True if the file was last accessed @var{n} days after its status was
last changed.  Useful for finding files that are not being used, and
could perhaps be archived or removed to save disk space.
@end deffn

@node Size
@section Size

@deffn Test -size n@r{[}bckwMG@r{]}
True if the file uses @var{n} units of space, rounding up.  The units
are 512-byte blocks by default, but they can be changed by adding a
one-character suffix to @var{n}:

@table @code
@item b
512-byte blocks (never 1024)
@item c
bytes
@item k
kilobytes (1024 bytes)
@item w
2-byte words
@item M
Megabytes
@item G
Gigabytes
@end table

The `b' suffix always considers blocks to be 512 bytes.  This is not
affected by the setting (or non-setting) of the POSIXLY_CORRECT
environment variable.  This behaviour is different to the behaviour of
the @samp{-ls} action).  If you want to use 1024-byte units, use the
`k' suffix instead.

The number can be prefixed with a `+' or a `-'.  A plus sign indicates
that the test should succeed if the file uses at least @var{n} units
of storage (a common use of this test) and a minus sign
indicates that the test should succeed if the file uses less than
@var{n} units of storage.  There is no `=' prefix, because that's the
default anyway.

The size does not count indirect blocks, but it does count blocks in
sparse files that are not actually allocated.  In other words, it's
consistent with the result you get for @samp{ls -l} or @samp{wc -c}.
This handling of sparse files differs from the output of the @samp{%k}
and @samp{%b} format specifiers for the @samp{-printf} predicate.

@end deffn

@deffn Test -empty
True if the file is empty and is either a regular file or a directory.
This might help determine good candidates for deletion.  This test is
useful with @samp{-depth} (@pxref{Directories}) and @samp{-delete}
(@pxref{Single File}).
@end deffn

@node Type
@section Type

@deffn Test -type c
True if the file is of type @var{c}:

@table @code
@item b
block (buffered) special
@item c
character (unbuffered) special
@item d
directory
@item p
named pipe (FIFO)
@item f
regular file
@item l
symbolic link
@item s
socket
@item D
door (Solaris)
@end table
@end deffn

@deffn Test -xtype c
The same as @samp{-type} unless the file is a symbolic link.  For
symbolic links: if @samp{-follow} has not been given, true if the file
is a link to a file of type @var{c}; if @samp{-follow} has been given,
true if @var{c} is @samp{l}.  In other words, for symbolic links,
@samp{-xtype} checks the type of the file that @samp{-type} does not
check.  @xref{Symbolic Links}, for more information on @samp{-follow}.
@end deffn

@node Owner
@section Owner

@deffn Test -user uname
@deffnx Test -group gname
True if the file is owned by user @var{uname} (belongs to group
@var{gname}).  A numeric ID is allowed.
@end deffn

@deffn Test -uid n
@deffnx Test -gid n
True if the file's numeric user ID (group ID) is @var{n}.  These tests
support ranges (@samp{+@var{n}} and @samp{-@var{n}}), unlike
@samp{-user} and @samp{-group}.
@end deffn

@deffn Test -nouser
@deffnx Test -nogroup
True if no user corresponds to the file's numeric user ID (no group
corresponds to the numeric group ID).  These cases usually mean that
the files belonged to users who have since been removed from the
system.  You probably should change the ownership of such files to an
existing user or group, using the @code{chown} or @code{chgrp}
program.
@end deffn

@node Permissions
@section Permissions

@xref{File Permissions}, for information on how file permissions are
structured and how to specify them.

@deffn Test -perm mode

True if the file's permissions are exactly @var{mode}, which can be
numeric or symbolic.

If @var{mode} starts with @samp{-}, true if
@emph{all} of the permissions set in @var{mode} are set for the file;
permissions not set in @var{mode} are ignored.
If @var{mode} starts with @samp{/}, true if
@emph{any} of the permissions set in @var{mode} are set for the file;
permissions not set in @var{mode} are ignored.
This is a GNU extension.

If you don't use the @samp{/} or @samp{-} form with a symbolic mode
string, you may have to specify a rather complex mode string.  For
example @samp{-perm g=w} will only match files which have mode 0020
(that is, ones for which group write permission is the only permission
set).  It is more likely that you will want to use the @samp{/} or
@samp{-} forms, for example @samp{-perm -g=w}, which matches any file
with group write permission.


@table @samp
@item -perm 664
Match files which have read and write permission for their owner,
and group, but which the rest of the world can read but not write to.
Files which meet these criteria but have other permissions bits set
(for example if someone can execute the file) will not be matched.

@item -perm -664
Match files which have read and write permission for their owner,
and group, but which the rest of the world can read but not write to,
without regard to the presence of any extra permission bits (for
example the executable bit).  This will match a file which has mode
0777, for example.

@item -perm /222
Match files which are writeable by somebody (their owner, or
their group, or anybody else).

@item -perm /022
Match files which are writeable by either their owner or their
group.  The files don't have to be writeable by both the owner and
group to be matched; either will do.

@item -perm /g+w,o+w
As above.

@item -perm /g=w,o=w
As above

@item -perm -022
Search for files which are writeable by both their owner and their
group.

@item -perm -g+w,o+w
As above.
@end table
@end deffn

@node Contents
@section Contents

To search for files based on their contents, you can use the
@code{grep} program.  For example, to find out which C source files in
the current directory contain the string @samp{thing}, you can do:

@example
grep -l thing *.[ch]
@end example

If you also want to search for the string in files in subdirectories,
you can combine @code{grep} with @code{find} and @code{xargs}, like
this:

@example
find . -name '*.[ch]' | xargs grep -l thing
@end example

The @samp{-l} option causes @code{grep} to print only the names of
files that contain the string, rather than the lines that contain it.
The string argument (@samp{thing}) is actually a regular expression,
so it can contain metacharacters.  This method can be refined a little
by using the @samp{-r} option to make @code{xargs} not run @code{grep}
if @code{find} produces no output, and using the @code{find} action
@samp{-print0} and the @code{xargs} option @samp{-0} to avoid
misinterpreting files whose names contain spaces:

@example
find . -name '*.[ch]' -print0 | xargs -r -0 grep -l thing
@end example

For a fuller treatment of finding files whose contents match a
pattern, see the manual page for @code{grep}.

@node Directories
@section Directories

Here is how to control which directories @code{find} searches, and how
it searches them.  These two options allow you to process a horizontal
slice of a directory tree.

@deffn Option -maxdepth levels
Descend at most @var{levels} (a non-negative integer) levels of
directories below the command line arguments.  @samp{-maxdepth 0}
means only apply the tests and actions to the command line arguments.
@end deffn

@deffn Option -mindepth levels
Do not apply any tests or actions at levels less than @var{levels} (a
non-negative integer).  @samp{-mindepth 1} means process all files
except the command line arguments.
@end deffn

@deffn Option -depth
Process each directory's contents before the directory itself.  Doing
this is a good idea when producing lists of files to archive with
@code{cpio} or @code{tar}.  If a directory does not have write
permission for its owner, its contents can still be restored from the
archive since the directory's permissions are restored after its
contents.
@end deffn

@deffn Option -d
This is a deprecated synonym for @samp{-depth}, for compatibility with
Mac OS X, FreeBSD and OpenBSD.  The @samp{-depth} option is a POSIX
feature, so it is better to use that.
@end deffn

@deffn Action -prune
If the file is a directory, do not descend into it.  The result is
true.  For example, to skip the directory @file{src/emacs} and all
files and directories under it, and print the names of the other files
found:

@example
find . -wholename './src/emacs' -prune -o -print
@end example

The above command will not print @file{./src/emacs} among its list of
results.  This however is not due to the effect of the @samp{-prune}
action (which only prevents further descent, it doesn't make sure we
ignore that item).  Instead, this effect is due to the use of
@samp{-o}.  Since the left hand side of the ``or'' condition has
succeeded for @file{./src/emacs}, it is not necessary to evaluate the
right-hand-side (@samp{-print}) at all for this particular file.  If
you wanted to print that directory name you could use either an extra
@samp{-print} action:

@example
find . -wholename './src/emacs' -prune -print -o -print
@end example

or use the comma operator:

@example
find . -wholename './src/emacs' -prune , -print
@end example

If the @samp{-depth} option is in effect, the subdirectories will have
already been visited in any case.  Hence @samp{-prune} has no effect
and returns false.
@end deffn


@deffn Action -quit
Exit immediately (with return value zero if no errors have occurred).
No child processes will be left running, but no more files specified
on the command line will be processed.  For example, @code{find
/tmp/foo /tmp/bar -print -quit} will print only @samp{/tmp/foo}.
@end deffn

@deffn Option -noleaf
Do not optimize by assuming that directories contain 2 fewer
subdirectories than their hard link count.  This option is needed when
searching filesystems that do not follow the Unix directory-link
convention, such as CD-ROM or MS-DOS filesystems or AFS volume mount
points.  Each directory on a normal Unix filesystem has at least 2
hard links: its name and its @file{.}  entry.  Additionally, its
subdirectories (if any) each have a @file{..}  entry linked to that
directory.  When @code{find} is examining a directory, after it has
statted 2 fewer subdirectories than the directory's link count, it
knows that the rest of the entries in the directory are
non-directories (@dfn{leaf} files in the directory tree).  If only the
files' names need to be examined, there is no need to stat them; this
gives a significant increase in search speed.
@end deffn

@deffn Option -ignore_readdir_race
If a file disappears after its name has been read from a directory but
before @code{find} gets around to examining the file with @code{stat},
don't issue an error message.  If you don't specify this option, an
error message will be issued.  This option can be useful in system
scripts (cron scripts, for example) that examine areas of the
filesystem that change frequently (mail queues, temporary directories,
and so forth), because this scenario is common for those sorts of
directories.  Completely silencing error messages from @code{find} is
undesirable, so this option neatly solves the problem.  There is no
way to search one part of the filesystem with this option on and part
of it with this option off, though.
@end deffn

@deffn Option -noignore_readdir_race
This option reverses the effect of the @samp{-ignore_readdir_race}
option.
@end deffn


@node Filesystems
@section Filesystems

A @dfn{filesystem} is a section of a disk, either on the local host or
mounted from a remote host over a network.  Searching network
filesystems can be slow, so it is common to make @code{find} avoid
them.

There are two ways to avoid searching certain filesystems.  One way is
to tell @code{find} to only search one filesystem:

@deffn Option -xdev
@deffnx Option -mount
Don't descend directories on other filesystems.  These options are
synonyms.
@end deffn

The other way is to check the type of filesystem each file is on, and
not descend directories that are on undesirable filesystem types:

@deffn Test -fstype type
True if the file is on a filesystem of type @var{type}.  The valid
filesystem types vary among different versions of Unix; an incomplete
list of filesystem types that are accepted on some version of Unix or
another is:
@example
ext2 ext3 proc sysfs ufs 4.2 4.3 nfs tmp mfs S51K S52K
@end example
You can use @samp{-printf} with the @samp{%F} directive to see the
types of your filesystems.  The @samp{%D} directive shows the device
number.  @xref{Print File Information}.  @samp{-fstype} is usually
used with @samp{-prune} to avoid searching remote filesystems
(@pxref{Directories}).
@end deffn

@node Combining Primaries With Operators
@section Combining Primaries With Operators

Operators build a complex expression from tests and actions.
The operators are, in order of decreasing precedence:

@table @code
@item @asis{( @var{expr} )}
@findex ()
Force precedence.  True if @var{expr} is true.

@item @asis{! @var{expr}}
@itemx @asis{-not @var{expr}}
@findex !
@findex -not
True if @var{expr} is false.

@item @asis{@var{expr1 expr2}}
@itemx @asis{@var{expr1} -a @var{expr2}}
@itemx @asis{@var{expr1} -and @var{expr2}}
@findex -a
@findex -and
And; @var{expr2} is not evaluated if @var{expr1} is false.

@item @asis{@var{expr1} -o @var{expr2}}
@itemx @asis{@var{expr1} -or @var{expr2}}
@findex -o
@findex -or
Or; @var{expr2} is not evaluated if @var{expr1} is true.

@item @asis{@var{expr1} , @var{expr2}}
@findex ,
List; both @var{expr1} and @var{expr2} are always evaluated.  True if
@var{expr2} is true.  The value of @var{expr1} is discarded.  This
operator lets you do multiple independent operations on one traversal,
without depending on whether other operations succeeded.  The two
operations @var{expr1} and @var{expr2} are not always fully
independent, since @var{expr1} might have side effects like touching
or deleting files, or it might use @samp{-prune} which would also
affect @var{expr2}.
@end table

@code{find} searches the directory tree rooted at each file name by
evaluating the expression from left to right, according to the rules
of precedence, until the outcome is known (the left hand side is false
for @samp{-and}, true for @samp{-or}), at which point @code{find}
moves on to the next file name.

There are two other tests that can be useful in complex expressions:

@deffn Test -true
Always true.
@end deffn

@deffn Test -false
Always false.
@end deffn

@node Actions, Databases, Finding Files, Top
@chapter Actions

There are several ways you can print information about the files that
match the criteria you gave in the @code{find} expression.  You can
print the information either to the standard output or to a file that
you name.  You can also execute commands that have the file names as
arguments.  You can use those commands as further filters to select
files.

@menu
* Print File Name::
* Print File Information::
* Run Commands::
* Delete Files::
* Adding Tests::
@end menu

@node Print File Name
@section Print File Name

@deffn Action -print
True; print the entire file name on the standard output, followed by a
newline.
@end deffn

@deffn Action -fprint file
True; print the entire file name into file @var{file}, followed by a
newline.  If @var{file} does not exist when @code{find} is run, it is
created; if it does exist, it is truncated to 0 bytes.  The file names
@file{/dev/stdout} and @file{/dev/stderr} are handled specially; they
refer to the standard output and standard error output, respectively.
@end deffn

@node Print File Information
@section Print File Information

@deffn Action -ls
True; list the current file in @samp{ls -dils} format on the standard
output.  The output looks like this:

@smallexample
204744   17 -rw-r--r--   1 djm      staff       17337 Nov  2  1992 ./lwall-quotes
@end smallexample

The fields are:

@enumerate
@item
The inode number of the file.  @xref{Hard Links}, for how to find
files based on their inode number.

@item
the number of blocks in the file.  The block counts are of 1K blocks,
unless the environment variable @code{POSIXLY_CORRECT} is set, in
which case 512-byte blocks are used.  @xref{Size}, for how to find
files based on their size.

@item
The file's type and permissions.  The type is shown as a dash for a
regular file; for other file types, a letter like for @samp{-type} is
used (@pxref{Type}).  The permissions are read, write, and execute for
the file's owner, its group, and other users, respectively; a dash
means the permission is not granted.  @xref{File Permissions}, for
more details about file permissions.  @xref{Permissions}, for how to
find files based on their permissions.

@item
The number of hard links to the file.

@item
The user who owns the file.

@item
The file's group.

@item
The file's size in bytes.

@item
The date the file was last modified.

@item
The file's name.  @samp{-ls} quotes non-printable characters in the
file names using C-like backslash escapes.  This may change soon, as
the treatment of unprintable characters is harmonised for @samp{-ls},
@samp{-fls}, @samp{-print}, @samp{-fprint}, @samp{-printf} and
@samp{-fprintf}.
@end enumerate
@end deffn

@deffn Action -fls file
True; like @samp{-ls} but write to @var{file} like @samp{-fprint}
(@pxref{Print File Name}).
@end deffn

@deffn Action -printf format
True; print @var{format} on the standard output, interpreting @samp{\}
escapes and @samp{%} directives.  Field widths and precisions can be
specified as with the @code{printf} C function.  Format flags (like
@samp{#} for example) may not work as you expect because many of the
fields, even numeric ones, are printed with %s.  This means though
that the format flag @samp{-} will work; it forces left-alignment of
the field.  Unlike @samp{-print}, @samp{-printf} does not add a
newline at the end of the string.  If you want a newline at the end of
the string, add a @samp{\n}.
@end deffn

@deffn Action -fprintf file format
True; like @samp{-printf} but write to @var{file} like @samp{-fprint}
(@pxref{Print File Name}).
@end deffn

@menu
* Escapes::
* Format Directives::
* Time Formats::
@end menu

@node Escapes
@subsection Escapes

The escapes that @samp{-printf} and @samp{-fprintf} recognize are:

@table @code
@item \a
Alarm bell.
@item \b
Backspace.
@item \c
Stop printing from this format immediately and flush the output.
@item \f
Form feed.
@item \n
Newline.
@item \r
Carriage return.
@item \t
Horizontal tab.
@item \v
Vertical tab.
@item \\
A literal backslash (@samp{\}).
@item \NNN
The character whose ASCII code is NNN (octal).
@end table

A @samp{\} character followed by any other character is treated as an
ordinary character, so they both are printed, and a warning message is
printed to the standard error output (because it was probably a typo).

@node Format Directives
@subsection Format Directives

@samp{-printf} and @samp{-fprintf} support the following format
directives to print information about the file being processed.  The C
@code{printf} function, field width and precision specifiers are
supported, as applied to string (%s) types. That is, you can specify
"minimum field width"."maximum field width" for each directive.
Format flags (like @samp{#} for example) may not work as you expect
because many of the fields, even numeric ones, are printed with %s.
The format flag @samp{-} does work; it forces left-alignment of the
field.

@samp{%%} is a literal percent sign.  A @samp{%} character followed by
an unrecognised character (i.e. not a known directive or printf field
width and precision specifier), is discarded (but the unrecognised
character is printed), and a warning message is printed to the
standard error output (because it was probably a typo).

@menu
* Name Directives::
* Ownership Directives::
* Size Directives::
* Location Directives::
* Time Directives::
* Formatting Flags::
@end menu

@node Name Directives
@subsubsection Name Directives

@table @code
@item %p
@c supports %-X.Yp
File's name (not the absolute path name, but the name of the file as
it was encountered by @code{find} - that is, as a relative path from
one of the starting points).
@item %f
File's name with any leading directories removed (only the last
element).
@c supports %-X.Yf
@item %h
Leading directories of file's name (all but the last element and the
slash before it).  If the file's name contains no slashes (for example
because it was named on the command line and is in the current working
directory), then ``%h'' expands to ``.''.  This prevents ``%h/%f''
expanding to ``/foo'', which would be surprising and probably not
desirable.
@c supports %-X.Yh
@item %P
File's name with the name of the command line argument under which
it was found removed from the beginning.
@c supports %-X.YP
@item %H
Command line argument under which file was found.
@c supports %-X.YH
@end table

@node Ownership Directives
@subsubsection Ownership Directives

@table @code
@item %g
@c supports %-X.Yg
File's group name, or numeric group ID if the group has no name.
@item %G
@c supports %-X.Yg
@c TODO: Needs to support # flag and 0 flag
File's numeric group ID.
@item %u
@c supports %-X.Yu
File's user name, or numeric user ID if the user has no name.
@item %U
@c supports %-X.Yu
@c TODO: Needs to support # flag
File's numeric user ID.
@item %m
@c full support, including # and 0.
File's permissions (in octal).  If you always want to have a leading
zero on the number, use the '#' format flag, for example '%#m'.
@end table

@node Size Directives
@subsubsection Size Directives

@table @code
@item %k
The amount of disk space used for this file in 1K blocks. Since disk
space is allocated in multiples of the filesystem block size this is
usually greater than %s/1024, but it can also be smaller if the file
is a sparse file (that is, it has ``holes'').
@item %b
The amount of disk space used for this file in 512-byte blocks. Since
disk space is allocated in multiples of the filesystem block size this
is usually greater than %s/1024, but it can also be smaller if the
file is a sparse file (that is, it has ``holes'').
@item %s
File's size in bytes.
@end table

@node Location Directives
@subsubsection Location Directives

@table @code
@item %d
File's depth in the directory tree (depth below a file named on the
command line, not depth below the root directory).  Files named on the
command line have a depth of 0.  Subdirectories immediately below them
have a depth of 1, and so on.
@item %D
The device number on which the file exists (the @code{st_dev} field of
@code{struct stat}), in decimal.
@item %F
Type of the filesystem the file is on; this value can be used for
@samp{-fstype} (@pxref{Directories}).
@item %l
Object of symbolic link (empty string if file is not a symbolic link).
@item %i
File's inode number (in decimal).
@item %n
Number of hard links to file.
@item %y
Type of the file as used with @samp{-type}.  If the file is a symbolic
link, @samp{l} will be printed.
@item %Y
Type of the file as used with @samp{-type}.  If the file is a symbolic
link, it is dereferenced.  If the file is a broken symbolic link,
@samp{N} is printed.

@end table

@node Time Directives
@subsubsection Time Directives

Some of these directives use the C @code{ctime} function.  Its output
depends on the current locale, but it typically looks like

@example
Wed Nov  2 00:42:36 1994
@end example

@table @code
@item %a
File's last access time in the format returned by the C @code{ctime}
function.
@item %A@var{k}
File's last access time in the format specified by @var{k}
(@pxref{Time Formats}).
@item %c
File's last status change time in the format returned by the C
@code{ctime} function.
@item %C@var{k}
File's last status change time in the format specified by @var{k}
(@pxref{Time Formats}).
@item %t
File's last modification time in the format returned by the C
@code{ctime} function.
@item %T@var{k}
File's last modification time in the format specified by @var{k}
(@pxref{Time Formats}).
@end table

@node Time Formats
@subsection Time Formats

Below are the formats for the directives @samp{%A}, @samp{%C}, and
@samp{%T}, which print the file's timestamps.  Some of these formats
might not be available on all systems, due to differences in the C
@code{strftime} function between systems.

@menu
* Time Components::
* Date Components::
* Combined Time Formats::
@end menu

@node Time Components
@subsubsection Time Components

The following format directives print single components of the time.

@table @code
@item H
hour (00..23)
@item I
hour (01..12)
@item k
hour ( 0..23)
@item l
hour ( 1..12)
@item p
locale's AM or PM
@item Z
time zone (e.g., EDT), or nothing if no time zone is determinable
@item M
minute (00..59)
@item S
second (00..61)
@item @@
seconds since Jan. 1, 1970, 00:00 GMT.
@end table

@node Date Components
@subsubsection Date Components

The following format directives print single components of the date.

@table @code
@item a
locale's abbreviated weekday name (Sun..Sat)
@item A
locale's full weekday name, variable length (Sunday..Saturday)
@item b
@itemx h
locale's abbreviated month name (Jan..Dec)
@item B
locale's full month name, variable length (January..December)
@item m
month (01..12)
@item d
day of month (01..31)
@item w
day of week (0..6)
@item j
day of year (001..366)
@item U
week number of year with Sunday as first day of week (00..53)
@item W
week number of year with Monday as first day of week (00..53)
@item Y
year (1970@dots{})
@item y
last two digits of year (00..99)
@end table

@node Combined Time Formats
@subsubsection Combined Time Formats

The following format directives print combinations of time and date
components.

@table @code
@item r
time, 12-hour (hh:mm:ss [AP]M)
@item T
time, 24-hour (hh:mm:ss)
@item X
locale's time representation (H:M:S)
@item c
locale's date and time (Sat Nov 04 12:02:33 EST 1989)
@item D
date (mm/dd/yy)
@item x
locale's date representation (mm/dd/yy)
@item +
Date and time, separated by '+', for example `2004-04-28+22:22:05'.
The time is given in the current timezone (which may be affected by
setting the TZ environment variable).  This is a GNU extension.
@end table

@node Formatting Flags
@subsubsection Formatting Flags

The @samp{%m} and @samp{%d} directives support the @samp{#}, @samp{0}
and @samp{+} flags, but the other directives do not, even if they
print numbers.  Numeric directives that do not support these flags
include

@samp{G},
@samp{U},
@samp{b},
@samp{D},
@samp{k} and
@samp{n}.

All fields support the format flag @samp{-}, which makes fields
left-aligned.  That is, if the field width is greater than the actual
contents of the field, the requisite number of spaces are printed
after the field content instead of before it.

@node Run Commands
@section Run Commands

You can use the list of file names created by @code{find} or
@code{locate} as arguments to other commands.  In this way you can
perform arbitrary actions on the files.

@menu
* Single File::
* Multiple Files::
* Querying::
@end menu

@node Single File
@subsection Single File

Here is how to run a command on one file at a time.

@deffn Action -execdir command ;
Execute @var{command}; true if zero status is returned.  @code{find}
takes all arguments after @samp{-exec} to be part of the command until
an argument consisting of @samp{;} is reached.  It replaces the string
@samp{@{@}} by the current file name being processed everywhere it
occurs in the command.  Both of these constructions need to be escaped
(with a @samp{\}) or quoted to protect them from expansion by the
shell.  The command is executed in the directory in which @code{find}
was run.

For example, to compare each C header file in the current directory
with the file @file{/tmp/master}:

@example
find . -name '*.h' -execdir diff -u '@{@}' /tmp/master ';'
@end example
@end deffn


Another similar option, @samp{-exec} is supported, but is less secure.
@xref{Security Considerations}, for a discussion of the security
problems surrounding @samp{-exec}.


@deffn Action -exec command ;
This insecure variant of the @samp{-execdir} action is specified by
POSIX.  The main difference is that the command is executed in the
directory from which @code{find} was invoked, meaning that @samp{@{@}}
is expanded to a relative path starting with the name of one of the
starting directories, rather than just the basename of the matched
file.
@end deffn


@node Multiple Files
@subsection Multiple Files

Sometimes you need to process files one of the time.  But usually this
is not necessary, and, it is faster to run a command on as many files
as possible at a time, rather than once per file.  Doing this saves on
the time it takes to start up the command each time.

The @samp{-execdir} and @samp{-exec} actions have variants that build
command lines containing as many matched files as possible.

@deffn Action -execdir command @{@} +
This works as for @samp{-execdir command ;}, except that the
@samp{@{@}} at the end of the command is expanded to a list of names
of matching files.  This expansion is done in such a way as to avoid
exceeding the maximum command line length available on the system.
Only one @samp{@{@}} is allowed within the command, and it must appear
at the end, immediately before the @samp{+}.  A @samp{+} appearing in
any position other than immediately after @samp{@{@}} is not
considered to be special (that is, it does not terminate the command).
@end deffn


@deffn Action -exec command @{@} +
This insecure variant of the @samp{-execdir} action is specified by
POSIX.  The main difference is that the command is executed in the
directory from which @code{find} was invoked, meaning that @samp{@{@}}
is expanded to a relative path starting with the name of one of the
starting directories, rather than just the basename of the matched
file.
@end deffn

Before @code{find} exits, any partially-built command lines are
executed.  This happens even if the exit was caused by the
@samp{-quit} action.  However, some types of error (for example not
being able to invoke @code{stat()} on the current directory) can cause
an immediate fatal exit.  In this situation, any partially-built
command lines will not be invoked (this prevents possible infinite
loops).

Another, but less secure, way to run a command on more than one file
at once, is to use the @code{xargs} command, which is invoked like
this:

@example
xargs @r{[}@var{option}@dots{}@r{]} @r{[}@var{command} @r{[}@var{initial-arguments}@r{]}@r{]}
@end example

@code{xargs} normally reads arguments from the standard input.  These
arguments are delimited by blanks (which can be protected with double
or single quotes or a backslash) or newlines.  It executes the
@var{command} (default is @file{/bin/echo}) one or more times with any
@var{initial-arguments} followed by arguments read from standard
input.  Blank lines on the standard input are ignored.

Instead of blank-delimited names, it is safer to use @samp{find
-print0} or @samp{find -fprint0} and process the output by giving the
@samp{-0} or @samp{--null} option to GNU @code{xargs}, GNU @code{tar},
GNU @code{cpio}, or @code{perl}.  The @code{locate} command also has a
@samp{-0} or @samp{--null} option which does the same thing.

You can use shell command substitution (backquotes) to process a list
of arguments, like this:

@example
grep -l sprintf `find $HOME -name '*.c' -print`
@end example

However, that method produces an error if the length of the @samp{.c}
file names exceeds the operating system's command line length limit.
@code{xargs} avoids that problem by running the command as many times
as necessary without exceeding the limit:

@example
find $HOME -name '*.c' -print | xargs grep -l sprintf
@end example

However, if the command needs to have its standard input be a terminal
(@code{less}, for example), you have to use the shell command
substitution method or use the @samp{--arg-file} option of
@code{xargs}.

The @code{xargs} command will process all its input, building command
lines and executing them, unless one of the commands exits with a
status of 255 (this will cause xargs to issue an error message and
stop) or it reads a line contains the end of file string specified
with the @samp{--eof} option.

@menu
* Unsafe File Name Handling::
* Safe File Name Handling::
* Unusual Characters in File Names::
* Limiting Command Size::
* Interspersing File Names::
@end menu

@node Unsafe File Name Handling
@subsubsection Unsafe File Name Handling

Because file names can contain quotes, backslashes, blank characters,
and even newlines, it is not safe to process them using @code{xargs}
in its default mode of operation.  But since most files' names do not
contain blanks, this problem occurs only infrequently.  If you are
only searching through files that you know have safe names, then you
need not be concerned about it.

@c This example is adapted from:
@c From: pfalstad@stone.Princeton.EDU (Paul John Falstad)
@c Newsgroups: comp.unix.shell
@c Subject: Re: Beware xargs security holes
@c Date: 16 Oct 90 19:12:06 GMT
@c
In many applications, if @code{xargs} botches processing a file
because its name contains special characters, some data might be lost.
The importance of this problem depends on the importance of the data
and whether anyone notices the loss soon enough to correct it.
However, here is an extreme example of the problems that using
blank-delimited names can cause.  If the following command is run
daily from @code{cron}, then any user can remove any file on the
system:

@example
find / -name '#*' -atime +7 -print | xargs rm
@end example

For example, you could do something like this:

@example
eg$ echo > '#
vmunix'
@end example

@noindent
and then @code{cron} would delete @file{/vmunix}, if it ran
@code{xargs} with @file{/} as its current directory.

To delete other files, for example @file{/u/joeuser/.plan}, you could
do this:

@example
eg$ mkdir '#
'
eg$ cd '#
'
eg$ mkdir u u/joeuser u/joeuser/.plan'
'
eg$ echo > u/joeuser/.plan'
/#foo'
eg$ cd ..
eg$ find . -name '#*' -print | xargs echo
./# ./# /u/joeuser/.plan /#foo
@end example

@node Safe File Name Handling
@subsubsection Safe File Name Handling

Here is how to make @code{find} output file names so that they can be
used by other programs without being mangled or misinterpreted.  You
can process file names generated this way by giving the @samp{-0} or
@samp{--null} option to GNU @code{xargs}, GNU @code{tar}, GNU
@code{cpio}, or @code{perl}.

@deffn Action -print0
True; print the entire file name on the standard output, followed by a
null character.
@end deffn

@deffn Action -fprint0 file
True; like @samp{-print0} but write to @var{file} like @samp{-fprint}
(@pxref{Print File Name}).
@end deffn

As of findutils version 4.2.4, the @code{locate} program also has a
@samp{--null} option which does the same thing.  For similarity with
@code{xargs}, the short form of the option @samp{-0} can also be used.

If you want to be able to handle file names safely but need to run
commands which want to be connected to a terminal on their input, you
can use the @samp{--arg-file} option to @code{xargs} like this:

@example
find / -name xyzzy -print0 > list
xargs --null --arg-file=list munge
@end example

The example above runs the @code{munge} program on all the files named
@file{xyzzy} that we can find, but @code{munge}'s input will still be
the terminal (or whatever the shell was using as standard input).  If
your shell has the ``process substitution'' feature @samp{<(...)}, you
can do this in just one step:

@example
xargs --null --arg-file=<(find / -name xyzzy -print0) munge
@end example

@node Unusual Characters in File Names
@subsubsection Unusual Characters in File Names
As discussed above, you often need to be careful about how the names
of files are handled by @code{find} and other programs.  If the output
of @code{find} is not going to another program but instead is being
shown on a terminal, this can still be a problem.  For example, some
character sequences can reprogram the function keys on some terminals.
@xref{Security Considerations}, for a discussion of other security
problems relating to @code{find}.

Unusual characters are handled differently by various
actions, as described below.

@table @samp
@item -print0
@itemx -fprint0
Always print the exact file name, unchanged, even if the output is
going to a terminal.
@item -ok
@itemx -okdir
Always print the exact file name, unchanged.  This will probably
change in a future release.
@item -ls
@itemx -fls
Unusual characters are always escaped.  White space, backslash, and
double quote characters are printed using C-style escaping (for
example @samp{\f}, @samp{\"}).  Other unusual characters are printed
using an octal escape.  Other Printable characters (for @samp{-ls} and
@samp{-fls} these are the characters between octal 041 and 0176) are
printed as-is.
@item -printf
@itemx -fprintf
If the output is not going to a terminal, it is printed as-is.
Otherwise, the result depends on which directive is in use:

@table @asis
@item %D, %F, %H, %Y, %y
These expand to values which are not under control of files' ownwers,
and so are printed as-is.
@item  %a, %b, %c, %d, %g, %G, %i, %k, %m, %M, %n, %s, %t, %u, %U
These have values which are under the control of files' ownwers but
which cannot be used to send arbitrary data to the terminal, and so
these are printed as-is.
@item %f, %h, %l, %p, %P
The output of these directives is quoted if the output is going to a
terminal.

This quoting is performed in the same way as for GNU @code{ls}.  This
is not the same quoting mechanism as the one used for @samp{-ls} and
@samp{fls}.  If you are able to decide what format to use for the
output of @code{find} then it is normally better to use @samp{\0} as a
terminator than to use newline, as file names can contain white space
and newline characters.
@end table
@item -print
@itemx -fprint
Quoting is handled in the same way as for the @samp{%p} directive of
@samp{-printf} and @samp{-fprintf}.  If you are using @code{find} in a
script or in a situation where the matched files might have arbitrary
names, you should consider using @samp{-print0} instead of
@samp{-print}.
@end table


The @code{locate} program quotes and escapes unusual characters in
file names in the same way as @code{find}'s @samp{-print} action.

The behaviours described above may change soon, as the treatment of
unprintable characters is harmonised for @samp{-ls}, @samp{-fls},
@samp{-print}, @samp{-fprint}, @samp{-printf} and @samp{-fprintf}.

@node Limiting Command Size
@subsubsection Limiting Command Size

@code{xargs} gives you control over how many arguments it passes to
the command each time it executes it.  By default, it uses up to
@code{ARG_MAX} - 2k, or 128k, whichever is smaller, characters per
command.  It uses as many lines and arguments as fit within that
limit.  The following options modify those values.

@table @code
@item --no-run-if-empty
@itemx -r
If the standard input does not contain any nonblanks, do not run the
command.  By default, the command is run once even if there is no
input.

@item --max-lines@r{[}=@var{max-lines}@r{]}
@itemx -L@r{[}@var{max-lines}@r{]}
@itemx -l@r{[}@var{max-lines}@r{]}
Use at most @var{max-lines} nonblank input lines per command line;
@var{max-lines} defaults to 1 if omitted.  Trailing blanks cause an
input line to be logically continued on the next input line, for the
purpose of counting the lines.  Implies @samp{-x}.  The preferred
name for this option is @samp{-L} as this is specified by POSIX.  This
option should be compatible with @samp{-I} but currently it is not.

@item --max-args=@var{max-args}
@itemx -n @var{max-args}
Use at most @var{max-args} arguments per command line.  Fewer than
@var{max-args} arguments will be used if the size (see the @samp{-s}
option) is exceeded, unless the @samp{-x} option is given, in which
case @code{xargs} will exit.

@item --max-chars=@var{max-chars}
@itemx -s @var{max-chars}
Use at most @var{max-chars} characters per command line, including the
command initial arguments and the terminating nulls at the ends of
the argument strings.  If you specify a value for this option which is
too large or small, a warning message is printed and the appropriate
upper or lower limit is used instead.

@item --max-procs=@var{max-procs}
@itemx -P @var{max-procs}
Run up to @var{max-procs} processes at a time; the default is 1.  If
@var{max-procs} is 0, @code{xargs} will run as many processes as
possible at a time.  Use the @samp{-n}, @samp{-s}, or @samp{-L} option
with @samp{-P}; otherwise chances are that the command will be run
only once.
@end table

@node Interspersing File Names
@subsubsection Interspersing File Names

@code{xargs} can insert the name of the file it is processing between
arguments you give for the command.  Unless you also give options to
limit the command size (@pxref{Limiting Command Size}), this mode of
operation is equivalent to @samp{find -exec} (@pxref{Single File}).

@table @code
@item --replace@r{[}=@var{replace-str}@r{]}
@itemx -I@r{[}@var{replace-str}@r{]}
@itemx -i@r{[}@var{replace-str}@r{]}
Replace occurrences of @var{replace-str} in the initial arguments with
names read from the input.  Also, unquoted blanks do not
terminate arguments; instead, the input is split at newlines only.  If
@var{replace-str} is omitted, it defaults to @samp{@{@}} (like for
@samp{find -exec}).  Implies @samp{-x} and @samp{-l 1}.  @samp{-i} is
depreceated in favour of @samp{-I}. As an
example, to sort each file in the @file{bills} directory, leaving the
output in that file name with @file{.sorted} appended, you could do:

@example
find bills -type f | xargs -IXX sort -o XX.sorted XX
@end example

@noindent
The equivalent command using @samp{find -execdir} is:

@example
find bills -type f -execdir sort -o '@{@}.sorted' '@{@}' ';'
@end example
@end table

@node Querying
@subsection Querying

To ask the user whether to execute a command on a single file, you can
use the @code{find} primary @samp{-okdir} instead of @samp{-execdir},
and the @code{find} primary @samp{-ok} instead of @samp{-exec}:

@deffn Action -okdir command ;
Like @samp{-execdir} (@pxref{Single File}), but ask the user first (on
the standard input); if the response does not start with @samp{y} or
@samp{Y}, do not run the command, and return false.  If the command is
run, its standard input is redirected from @file{/dev/null}.
@end deffn

@deffn Action -ok command ;
This insecure variant of the @samp{-okdir} action is specified by
POSIX.  The main difference is that the command is executed in the
directory from which @code{find} was invoked, meaning that @samp{@{@}}
is expanded to a relative path starting with the name of one of the
starting directories, rather than just the basename of the matched
file.  If the command is run, its standard input is redirected from
@file{/dev/null}.
@end deffn

When processing multiple files with a single command, to query the
user you give @code{xargs} the following option.  When using this
option, you might find it useful to control the number of files
processed per invocation of the command (@pxref{Limiting Command
Size}).

@table @code
@item --interactive
@itemx -p
Prompt the user about whether to run each command line and read a line
from the terminal.  Only run the command line if the response starts
with @samp{y} or @samp{Y}.  Implies @samp{-t}.
@end table

@node Delete Files
@section Delete Files

@deffn Action -delete
Delete files or directories; true if removal succeeded.  If the
removal failed, an error message is issued.

The use of the @samp{-delete} action on the command line automatically
turns on the @samp{-depth} option (@pxref{find Expressions}).
@end deffn

@node Adding Tests
@section Adding Tests

You can test for file attributes that none of the @code{find} builtin
tests check.  To do this, use @code{xargs} to run a program that
filters a list of files printed by @code{find}.  If possible, use
@code{find} builtin tests to pare down the list, so the program run by
@code{xargs} has less work to do.  The tests builtin to @code{find}
will likely run faster than tests that other programs perform.

For reasons of efficiency it is often useful to limit the number of
times an external program has to be run.  For this reason, it is often
a good idea to implement ``extended'' tests by using @code{xargs}.

For example, here is a way to print the names of all of the unstripped
binaries in the @file{/usr/local} directory tree.  Builtin tests avoid
running @code{file} on files that are not regular files or are not
executable.

@example
find /usr/local -type f -perm /a=x | xargs file |
  grep 'not stripped' | cut -d: -f1
@end example

@noindent
The @code{cut} program removes everything after the file name from the
output of @code{file}.

However, using @code{xargs} can present important security problems
(@pxref{Security Considerations}).  These can be avoided by using
@samp{-execdir}.  The @samp{-execdir} action is also a useful way of
putting your own test in the middle of a set of other tests or actions
for @code{find} (for example, you might want to use @samp{-prune}).

@c Idea from Martin Weitzel.
To place a special test somewhere in the middle of a @code{find}
expression, you can use @samp{-execdir} (or, less securely,
@samp{-exec}) to run a program that performs the test.  Because
@samp{-execdir} evaluates to the exit status of the executed program,
you can use a program (which can be a shell script) that tests for a
special attribute and make it exit with a true (zero) or false
(non-zero) status.  It is a good idea to place such a special test
@emph{after} the builtin tests, because it starts a new process which
could be avoided if a builtin test evaluates to false.

Here is a shell script called @code{unstripped} that checks whether
its argument is an unstripped binary file:

@example
#! /bin/sh
file "$1" | grep -q "not stripped"
@end example


This script relies on the shell exiting with the status of
the last command in the pipeline, in this case @code{grep}.  The
@code{grep} command exits with a true status if it found any matches,
false if not.  Here is an example of using the script (assuming it is
in your search path).  It lists the stripped executables (and shell
scripts) in the file @file{sbins} and the unstripped ones in
@file{ubins}.

@example
find /usr/local -type f -perm /a=x \
  \( -execdir unstripped '@{@}' \; -fprint ubins -o -fprint sbins \)
@end example


@node Databases, File Permissions, Actions, Top
@chapter File Name Databases

The file name databases used by @code{locate} contain lists of files
that were in particular directory trees when the databases were last
updated.  The file name of the default database is determined when
@code{locate} and @code{updatedb} are configured and installed.  The
frequency with which the databases are updated and the directories for
which they contain entries depend on how often @code{updatedb} is run,
and with which arguments.

You can obtain some statistics about the databases by using
@samp{locate --statistics}.

@menu
* Database Locations::
* Database Formats::
* Newline Handling::
@end menu


@node Database Locations
@section Database Locations

There can be multiple file name databases.  Users can select which
databases @code{locate} searches using the @code{LOCATE_PATH}
environment variable or a command line option.  The system
administrator can choose the file name of the default database, the
frequency with which the databases are updated, and the directories
for which they contain entries.  File name databases are updated by
running the @code{updatedb} program, typically nightly.

In networked environments, it often makes sense to build a database at
the root of each filesystem, containing the entries for that
filesystem.  @code{updatedb} is then run for each filesystem on the
fileserver where that filesystem is on a local disk, to prevent
thrashing the network.

@xref{Invoking updatedb},
for the description of the options to @code{updatedb}, which specify
which directories would each database contain entries for.


@node Database Formats
@section Database Formats

The file name databases contain lists of files that were in particular
directory trees when the databases were last updated.  The file name
database format changed starting with GNU @code{locate} version 4.0 to
allow machines with different byte orderings to share the databases.
The new GNU @code{locate} can read both the old and new database
formats.  However, old versions of @code{locate} and @code{find}
produce incorrect results if given a new-format database.

If you run @samp{locate --statistics}, the resulting summary indicates
the type of each @code{locate} database.


@menu
* New Database Format::
* Sample Database::
* Old Database Format::
@end menu

@node New Database Format
@subsection New Database Format

@code{updatedb} runs a program called @code{frcode} to
@dfn{front-compress} the list of file names, which reduces the
database size by a factor of 4 to 5.  Front-compression (also known as
incremental encoding) works as follows.

The database entries are a sorted list (case-insensitively, for users'
convenience).  Since the list is sorted, each entry is likely to share
a prefix (initial string) with the previous entry.  Each database
entry begins with an offset-differential count byte, which is the
additional number of characters of prefix of the preceding entry to
use beyond the number that the preceding entry is using of its
predecessor.  (The counts can be negative.)  Following the count is a
null-terminated ASCII remainder---the part of the name that follows
the shared prefix.

If the offset-differential count is larger than can be stored in a
byte (+/-127), the byte has the value 0x80 and the count follows in a
2-byte word, with the high byte first (network byte order).

Every database begins with a dummy entry for a file called
@file{LOCATE02}, which @code{locate} checks for to ensure that the
database file has the correct format; it ignores the entry in doing
the search.

Databases cannot be concatenated together, even if the first (dummy)
entry is trimmed from all but the first database.  This is because the
offset-differential count in the first entry of the second and
following databases will be wrong.

In the output of @samp{locate --statistics}, the new database format
is referred to as @samp{LOCATE02}.

@node Sample Database
@subsection Sample Database

Sample input to @code{frcode}:
@c with nulls changed to newlines:

@example
/usr/src
/usr/src/cmd/aardvark.c
/usr/src/cmd/armadillo.c
/usr/tmp/zoo
@end example

Length of the longest prefix of the preceding entry to share:

@example
0 /usr/src
8 /cmd/aardvark.c
14 rmadillo.c
5 tmp/zoo
@end example

Output from @code{frcode}, with trailing nulls changed to newlines
and count bytes made printable:

@example
0 LOCATE02
0 /usr/src
8 /cmd/aardvark.c
6 rmadillo.c
-9 tmp/zoo
@end example

(6 = 14 - 8, and -9 = 5 - 14)

@node Old Database Format
@subsection Old Database Format

The old database format is used by Unix @code{locate} and @code{find}
programs and earlier releases of the GNU ones.  @code{updatedb}
produces this format if given the @samp{--old-format} option.

@code{updatedb} runs programs called @code{bigram} and @code{code} to
produce old-format databases.  The old format differs from the new one
in the following ways.  Instead of each entry starting with an
offset-differential count byte and ending with a null, byte values
from 0 through 28 indicate offset-differential counts from -14 through
14.  The byte value indicating that a long offset-differential count
follows is 0x1e (30), not 0x80.  The long counts are stored in host
byte order, which is not necessarily network byte order, and host
integer word size, which is usually 4 bytes.  They also represent a
count 14 less than their value.  The database lines have no
termination byte; the start of the next line is indicated by its first
byte having a value <= 30.

In addition, instead of starting with a dummy entry, the old database
format starts with a 256 byte table containing the 128 most common
bigrams in the file list.  A bigram is a pair of adjacent bytes.
Bytes in the database that have the high bit set are indexes (with the
high bit cleared) into the bigram table.  The bigram and
offset-differential count coding makes these databases 20-25% smaller
than the new format, but makes them not 8-bit clean.  Any byte in a
file name that is in the ranges used for the special codes is replaced
in the database by a question mark, which not coincidentally is the
shell wildcard to match a single character.

The old format therefore cannot faithfully store entries with
non-ASCII characters. It therefore should not be used in
internationalized environments.

The output of @samp{locate --statistics} will give an incorrect count
of the number of file names containing newlines or high-bit characters
for old-format databases.

@node Newline Handling
@section Newline Handling

Within the database, file names are terminated with a null character.
This is the case for both the old and the new format.

When the new database format is being used, the compression technique
used to generate the database though relies on the ability to sort the
list of files before they are presented to @code{frcode}.

If the system's sort command allows its input list of files to be
separated with null characters via the @samp{-z} option, this option
is used and therefore @code{updatedb} and @code{locate} will both
correctly handle file names containing newlines.  If the @code{sort}
command lacks support for this, the list of files is delimited with
the newline character, meaning that parts of file names containing
newlines will be incorrectly sorted.  This can result in both
incorrect matches and incorrect failures to match.

On the other hand, if you are using the old database format, file
names with embedded newlines are not correctly handled.  There is no
technical limitation which enforces this, it's just that the
@code{bigram} program has not been updated to support lists of file
names separated by nulls.

So, if you are using the new database format (this is the default) and
your system uses GNU @code{sort}, newlines will be correctly handled
at all times.  Otherwise, newlines may not be correctly handled.

@node File Permissions, Reference, Databases, Top
@chapter File Permissions

@include perm.texi

@node Reference, Common Tasks, File Permissions, Top
@chapter Reference

Below are summaries of the command line syntax for the programs
discussed in this manual.

@menu
* Invoking find::
* Invoking locate::
* Invoking updatedb::
* Invoking xargs::
* Regular Expressions::
@end menu

@node Invoking find, Invoking locate, , Reference
@section Invoking @code{find}

@example
find @r{[-H] [-L] [-P]} @r{[}@var{file}@dots{}@r{]} @r{[}@var{expression}@r{]}
@end example

@code{find} searches the directory tree rooted at each file name
@var{file} by evaluating the @var{expression} on each file it finds in
the tree.

The options @samp{-H}, @samp{-L} or @samp{-P} may be specified at the
start of the command line (if none of these is specified, @samp{-P} is
assumed).  The arguments after these are a list of files or
directories that should be searched.

This list of files to search is followed by a list of expressions
describing the files we wish to search for.  The first part of the
expression is recognised by the fact that it begins with @samp{-},
@samp{(}, @samp{)}, @samp{,}, or @samp{!}.  Any arguments after it are
the rest of the expression.  If no files are given, the current
directory is used.  If no expression is given, the expression
@samp{-print} is used.

@code{find} exits with status zero if all files matched are processed
successfully, greater than 0 if errors occur.

Three options can precede the list of files.  They determine the
way that symbolic links are handled.

@table @code
@item -P
Never follow symbolic links (this is the default), except in the case
of the @samp{-xtype} predicate.
@item -L
Always follow symbolic links, except in the case of the @samp{-xtype}
predicate.
@item -H
Follow symbolic links specified in the list of files to search, or
which are otherwise specified on the command line.
@end table

If @code{find} would follow a symbolic link, but cannot for any reason
(for example, because it has insufficient permissions or the link is
broken), it falls back on using the properties of the symbolic link
itself.  @ref{Symbolic Links} for a more complete description of how
symbolic links are handled.

@xref{Primary Index}, for a summary of all of the tests, actions, and
options that the expression can contain.  If the expression is
missing, @samp{-print} is assumed.


@code{find} also recognizes two options for administrative use:

@table @code
@item --help
Print a summary of the command line usage and exit.
@item --version
Print the version number of @code{find} and exit.
@end table


@menu
* Warning Messages::
@end menu


@node Warning Messages,,, Invoking find
@subsection Warning Messages

If there is an error on the @code{find} command line, an error message
is normally issued.  However, there are some usages that are
inadvisable but which @code{find} should still accept.  Under these
circumstances, @code{find} may issue a warning message.  By default,
warnings are enabled only if @code{find} is being run interactively
(specifically, if the standard input is a terminal).  Warning messages
can be controlled explicitly by the use of options on the command
line:

@table @code
@item -warn
Issue warning messages where appropriate.
@item -nowarn
Do not issue warning messages.
@end table

These options take effect at the point on the command line where they
are specified.  Therefore if you specify @samp{-nowarn} at the end of
the command line, you will not see warning messages for any problems
occurring before that.  The warning messages affected by the above
options are triggered by:

@itemize @minus
@item
Use of the @samp{-d} option which is deprecated; please use
@samp{-depth} instead, since the latter is POSIX-compliant.
@item
Use of the @samp{-ipath} option which is deprecated; please use
@samp{-iwholename} instead.
@item
Specifying an option (for example @samp{-mindepth}) after a non-option
(for example @samp{-type} or @samp{-print}) on the command line.
@end itemize


The default behaviour above is designed to work in that way so that
existing shell scripts which use such constructs don't generate
spurious errors, but people will be made aware of the problem.

Some warning messages are issued for less common or more serious
problems, and consequently cannot be turned off:

@itemize @minus
@item
Use of an unrecognised backslash escape sequence with @samp{-fprintf}
@item
Use of an unrecognised formatting directive with @samp{-fprintf}
@end itemize

@node Invoking locate, Invoking updatedb, Invoking find, Reference
@section Invoking @code{locate}

@example
locate @r{[}@var{option}@dots{}@r{]} @var{pattern}@dots{}
@end example

For each @var{pattern} given @code{locate} searches one or more file
name databases returning each match of @var{pattern}.

For each @var{pattern} given @code{locate} searches one or more file
name databases returning each match of @var{pattern}.

@table @code
@item --all
@itemx -A
Print only names which match all non-option arguments, not those
matching one or more non-option arguments.

@item --basename
@itemx -b
The specified pattern is matched against just the last component of
the name of a file in the @code{locate} database.  This last
component is also called the ``base name''.  For example, the base
name of @file{/tmp/mystuff/foo.old.c} is @file{foo.old.c}.  If the
pattern contains metacharacters, it must match the base name exactly.
If not, it must match part of the base name.

@item --count
@itemx -c
Instead of printing the matched file names, just print the total
number of matches found, unless @samp{--print} (@samp{-p}) is also
present.


@item --database=@var{path}
@itemx -d @var{path}
Instead of searching the default @code{locate} database, @code{locate} search the file
name databases in @var{path}, which is a colon-separated list of
database file names.  You can also use the environment variable
@code{LOCATE_PATH} to set the list of database files to search.  The
option overrides the environment variable if both are used.  Empty
elements in @var{path} (that is, a leading or trailing colon, or two
colons in a row) are taken to stand for the default database.
A database can be supplied on stdin, using @samp{-} as an element
of @samp{path}. If more than one element of @samp{path} is @samp{-},
later instances are ignored (but a warning message is printed).

@item --existing
@itemx -e
Only print out such names which currently exist (instead of such names
which existed when the database was created).  Note that this may slow
down the program a lot, if there are many matches in the database.
The way in which broken symbolic links are treated is affected by the
@samp{-L}, @samp{-P} and @samp{-H} options.

@item --non-existing
@itemx -E
Only print out such names which currently do not exist (instead of
such names which existed when the database was created).  Note that
this may slow down the program a lot, if there are many matches in the
database.  The way in which broken symbolic links are treated is
affected by the @samp{-L}, @samp{-P} and @samp{-H} options.

@item --follow
@itemx -L
If testing for the existence of files (with the @samp{-e} or @samp{-E}
options), consider broken symbolic links to be non-existing.  This is
the default behaviour.


@item --nofollow
@itemx -P
@itemx -H
If testing for the existence of files (with the @samp{-e} or @samp{-E}
options), treat broken symbolic links as if they were existing files.
The @samp{-H} form of this option is provided purely for similarity
with @code{find}; the use of @samp{-P} is recommended over @samp{-H}.

@item --ignore-case
@itemx -i
Ignore case distinctions in both the pattern and the file names.

@item --limit=N
@itemx -l N
Limit the number of results printed to N.  When used with the
@samp{--count} option, the value printed will never be larger than
this limit.

@item --mmap
@itemx -m
Accepted but does nothing.  The option is supported only to provide
compatibility with BSD's @code{locate}.

@item --null
@itemx -0
Results are separated with the ASCII NUL character rather than the
newline character.  To get the full benefit of the use of this option,
use the new @code{locate} database format (that is the default
anyway).

@item --print
@itemx -p
Print search results when they normally would not, because of the
presence of @samp{--statistics} (@samp{-S}) or @samp{--count}
(@samp{-c}).

@item --wholename
@itemx -w
The specified pattern is matched against the whole name of the file in
the @code{locate} database.  If the pattern contains metacharacters,
it must match exactly.  If not, it must match part of the whole file
name.  This is the default behaviour.

@item --regex
@itemx -r
Instead of using substring or shell glob matching, the pattern
specified on the command line is understood to be a regular
expression.  GNU Emacs-style regular expressions are assumed unless
the @samp{--regextype} option is also given.  File names from the
@code{locate} database are matched using the specified regular
expression.  If the @samp{-i} flag is also given, matching is
case-insensitive.  Matches are performed against the whole path name,
and so by default a pathname will be matched if any part of it matches
the specified regular expression.  The regular expression may use
@samp{^} or @samp{$} to anchor a match at the beginning or end of a
pathname.

@item --regextype
This option changes the regular expression dialect used.  Dialects
understood are described in @ref{Regular Expressions}.



@item --regextype
This option changes the regular expression syntax and behaviour used
by the @samp{--regex} option.  @ref{Regular Expressions} for more
information on the regular expression dialects understood by GNU
findutils.

@item --stdio
@itemx -s
Accepted but does nothing.  The option is supported only to provide
compatibility with BSD's @code{locate}.

@item --statistics
@itemx -S
Print some summary information for each @code{locate} database.  No
search is performed unless non-option arguments are given.

@item --help
Print a summary of the command line usage for @code{locate} and exit.

@item --version
Print the version number of @code{locate} and exit.
@end table

@node Invoking updatedb, Invoking xargs, Invoking locate, Reference
@section Invoking @code{updatedb}

@example
updatedb @r{[}@var{option}@dots{}@r{]}
@end example

@code{updatedb} creates and updates the database of file names used by
@code{locate}.  @code{updatedb} generates a list of files similar to
the output of @code{find} and then uses utilities for optimizing the
database for performance.  @code{updatedb} is often run periodically
as a @code{cron} job and configured with environment variables or
command options.  Typically, operating systems have a shell script
that ``exports'' configurations for variable definitions and uses
another schell script that ``sources'' the configuration file into the
environment and then executes @code{updatedb} in the environment.

@code{updatedb} creates and updates the database of file names used by
@code{locate}.  @code{updatedb} generates a list of files similar to
the output of @code{find} and then uses utilities for optimizing the
database for performance.  @code{updatedb} is often run periodically
as a @code{cron} job and configured with environment variables or
command options.  Typically, operating systems have a shell script
that ``exports'' configurations for variable definitions and uses
another schell script that ``sources'' the configuration file into the
environment and then executes @code{updatedb} in the environment.

@table @code
@item --findoptions='@var{OPTION}@dots{}'
Global options to pass on to @code{find}.
The environment variable @code{FINDOPTIONS} also sets this value.
Default is none.

@item --localpaths='@var{path}@dots{}'
Non-network directories to put in the database.
Default is @file{/}.

@item --netpaths='@var{path}@dots{}'
Network (NFS, AFS, RFS, etc.) directories to put in the database.
The environment variable @code{NETPATHS} also sets this value.
Default is none.

@item --prunepaths='@var{path}@dots{}'
Directories to omit from the database, which would otherwise be
included.  The environment variable @code{PRUNEPATHS} also sets this
value.  Default is @file{/tmp /usr/tmp /var/tmp /afs}.  The paths are
used as regular expressions (with @code{find ... -regex}, so you need
to specify these paths in the same way that @code{find} will encounter
them.  This means for example that the paths must not include trailing
slashes.

@item --prunefs='@var{path}@dots{}'
Filesystems to omit from the database, which would otherwise be
included.  Note that files are pruned when a filesystem is reached;
Any filesystem mounted under an undesired filesystem will be ignored.
The environment variable @code{PRUNEFS} also sets this value.  Default
is @file{nfs NFS proc}.

@item --output=@var{dbfile}
The database file to build.  Default is system-dependent, but
typically @file{/usr/local/var/locatedb}.

@item --localuser=@var{user}
The user to search the non-network directories as, using @code{su}.
Default is to search the non-network directories as the current user.
You can also use the environment variable @code{LOCALUSER} to set this user.

@item --netuser=@var{user}
The user to search network directories as, using @code{su}.  Default
@code{user} is @code{daemon}.  You can also use the environment variable
@code{NETUSER} to set this user.

@item --old-format
Generate a @code{locate} database in the old format, for compatibility
with versions of @code{locate} other than GNU @code{locate}.  Using
this option means that @code{locate} will not be able to properly
handle non-ASCII characters in file names (that is, file names
containing characters which have the eighth bit set, such as many of
the characters from the ISO-8859-1 character set).
@item --help
Print a summary of the command line usage and exit.
@item --version
Print the version number of @code{updatedb} and exit.
@end table

@node Invoking xargs, Regular Expressions,  Invoking updatedb, Reference
@section Invoking @code{xargs}

@example
xargs @r{[}@var{option}@dots{}@r{]} @r{[}@var{command} @r{[}@var{initial-arguments}@r{]}@r{]}
@end example

@code{xargs} exits with the following status:

@table @asis
@item 0
if it succeeds
@item 123
if any invocation of the command exited with status 1-125
@item 124
if the command exited with status 255
@item 125
if the command is killed by a signal
@item 126
if the command cannot be run
@item 127
if the command is not found
@item 1
if some other error occurred.
@end table

@table @code
@item --arg-file@r{=@var{inputfile}}
@itemx -a o@r{@var{inputfile}}
Read names from the file @var{inputfile} instead of standard input.

@item --null
@itemx -0
Input file names are terminated by a null character instead of by
whitespace, and any quotes and backslash characters are not considered special (every
character is taken literally).  Disables the end of file string, which
is treated like any other argument.

@item --eof@r{[}=@var{eof-str}@r{]}
@itemx -e@r{[}@var{eof-str}@r{]}
Set the end of file string to @var{eof-str}.  If the end of file
string occurs as a line of input, the rest of the input is ignored.
If @var{eof-str} is omitted, there is no end of file string.  If this
option is not given, the end of file string defaults to @samp{_}.

@item --help
Print a summary of the options to @code{xargs} and exit.

@item --replace@r{[}=@var{replace-str}@r{]}
@itemx -I@r{[}@var{replace-str}@r{]}
@itemx -i@r{[}@var{replace-str}@r{]}
Replace occurrences of @var{replace-str} in the initial arguments with
names read from standard input.  Also, unquoted blanks do not
terminate arguments; instead, the input is split at newlines only.
If @var{replace-str} is omitted, it defaults to @samp{@{@}}
(like for @samp{find -exec}).  Implies @samp{-x} and @samp{-l 1}.  @samp{-i} is
depreceated in favour of @samp{-I}.

@item --max-lines@r{[}=@var{max-lines}@r{]}
@itemx -l@r{[}@var{max-lines}@r{]}
Use at most @var{max-lines} non-blank input lines per command line;
@var{max-lines} defaults to 1 if omitted.  Trailing blanks cause an
input line to be logically continued on the next input line, for the
purpose of counting the lines.  Implies @samp{-x}.

@item --max-args=@var{max-args}
@itemx -n @var{max-args}
Use at most @var{max-args} arguments per command line.  Fewer than
@var{max-args} arguments will be used if the size (see the @samp{-s}
option) is exceeded, unless the @samp{-x} option is given, in which
case @code{xargs} will exit.

@item --interactive
@itemx -p
Prompt the user about whether to run each command line and read a line
from the terminal.  Only run the command line if the response starts
with @samp{y} or @samp{Y}.  Implies @samp{-t}.

@item --no-run-if-empty
@itemx -r
If the standard input is completely empty, do not run the
command.  By default, the command is run once even if there is no
input.

@item --max-chars=@var{max-chars}
@itemx -s @var{max-chars}
Use at most @var{max-chars} characters per command line, including the
command, initial arguments and any terminating nulls at the ends of
the argument strings.

@item --verbose
@itemx -t
Print the command line on the standard error output before executing
it.

@item --version
Print the version number of @code{xargs} and exit.

@item --exit
@itemx -x
Exit if the size (see the @samp{-s} option) is exceeded.


@item --max-procs=@var{max-procs}
@itemx -P @var{max-procs}
Run simultaneously up to @var{max-procs} processes at once; the default is 1.  If
@var{max-procs} is 0, @code{xargs} will run as many processes as
possible simultaneously.
@end table


@node Regular Expressions,, Invoking xargs, Reference
@section Regular Expressions

The @samp{-regex} and @samp{-iregex} tests of @code{find} allow
matching by regular expression, as does the @samp{--regex} option of
@code{locate}.  There are many different types of Regular Expression,
but the type used by @code{find} and @code{locate} is the same as is
used in GNU Emacs.  Both programs provide an option which allows you
to select an alternative regular expression syntax; for @code{find}
this is the @samp{-regextype} option, and for @code{locate} this is
the @samp{--regextype} option.

These options take a single argument, which indicates the specific
regular expression syntax and behaviour that should be used.  This
should be one of the following:

@include regexprops.texi


@node Common Tasks, Worked Examples, Reference, Top
@chapter Common Tasks

The sections that follow contain some extended examples that both give
a good idea of the power of these programs, and show you how to solve
common real-world problems.

@menu
* Viewing And Editing::
* Archiving::
* Cleaning Up::
* Strange File Names::
* Fixing Permissions::
* Classifying Files::
@end menu

@node Viewing And Editing
@section Viewing And Editing

To view a list of files that meet certain criteria, simply run your
file viewing program with the file names as arguments.  Shells
substitute a command enclosed in backquotes with its output, so the
whole command looks like this:

@example
less `find /usr/include -name '*.h' | xargs grep -l mode_t`
@end example

@noindent
You can edit those files by giving an editor name instead of a file
viewing program:

@example
emacs `find /usr/include -name '*.h' | xargs grep -l mode_t`
@end example

Because there is a limit to the length of any individual command line,
there is a limit to the number of files that can be handled in this
way.  We can get around this difficulty by using xargs like this:

@example
find /usr/include -name '*.h' | xargs grep -l mode_t > todo
xargs --arg-file=todo emacs
@end example

Here, @code{xargs} will run @code{emacs} as many times as necessary to
visit all of the files listed in the file @file{todo}.

@node Archiving
@section Archiving

You can pass a list of files produced by @code{find} to a file
archiving program.  GNU @code{tar} and @code{cpio} can both read lists
of file names from the standard input---either delimited by nulls (the
safe way) or by blanks (the lazy, risky default way).  To use
null-delimited names, give them the @samp{--null} option.  You can
store a file archive in a file, write it on a tape, or send it over a
network to extract on another machine.

One common use of @code{find} to archive files is to send a list of
the files in a directory tree to @code{cpio}.  Use @samp{-depth} so if
a directory does not have write permission for its owner, its contents
can still be restored from the archive since the directory's
permissions are restored after its contents.  Here is an example of
doing this using @code{cpio}; you could use a more complex @code{find}
expression to archive only certain files.

@example
find . -depth -print0 |
  cpio --create --null --format=crc --file=/dev/nrst0
@end example

You could restore that archive using this command:

@example
cpio --extract --null --make-dir --unconditional \
  --preserve --file=/dev/nrst0
@end example

Here are the commands to do the same things using @code{tar}:

@example
find . -depth -print0 |
  tar --create --null --files-from=- --file=/dev/nrst0

tar --extract --null --preserve-perm --same-owner \
  --file=/dev/nrst0
@end example

@c Idea from Rick Sladkey.
Here is an example of copying a directory from one machine to another:

@example
find . -depth -print0 | cpio -0o -Hnewc |
  rsh @var{other-machine} "cd `pwd` && cpio -i0dum"
@end example

@node Cleaning Up
@section Cleaning Up

@c Idea from Jim Meyering.
This section gives examples of removing unwanted files in various
situations.  Here is a command to remove the CVS backup files created
when an update requires a merge:

@example
find . -name '.#*' -print0 | xargs -0r rm -f
@end example

The command above works, but the following is safer:

@example
find . -name '.#*' -depth -delete
@end example

@c Idea from Franc,ois Pinard.
You can run this command to clean out your clutter in @file{/tmp}.
You might place it in the file your shell runs when you log out
(@file{.bash_logout}, @file{.logout}, or @file{.zlogout}, depending on
which shell you use).

@example
find /tmp -depth -user "$LOGNAME" -type f -delete
@end example

If your @code{find} command removes directories, you may find that
you get a spurious error message when @code{find} tries to recurse
into a directory that has now been removed.  Using the @samp{-depth}
option will normally resolve this problem.

@c Idea from Noah Friedman.
To remove old Emacs backup and auto-save files, you can use a command
like the following.  It is especially important in this case to use
null-terminated file names because Emacs packages like the VM mailer
often create temporary file names with spaces in them, like
@file{#reply to David J. MacKenzie<1>#}.

@example
find ~ \( -name '*~' -o -name '#*#' \) -print0 |
  xargs --no-run-if-empty --null rm -vf
@end example

Removing old files from @file{/tmp} is commonly done from @code{cron}:

@c Idea from Kaveh Ghazi.
@example
find /tmp /var/tmp -not -type d -mtime +3 -delete
find /tmp /var/tmp -depth -mindepth 1 -type d -empty -delete
@end example

The second @code{find} command above uses @samp{-depth} so it cleans
out empty directories depth-first, hoping that the parents become
empty and can be removed too.  It uses @samp{-mindepth} to avoid
removing @file{/tmp} itself if it becomes totally empty.

@node Strange File Names
@section Strange File Names

@c Idea from:
@c From: tmatimar@isgtec.com (Ted Timar)
@c Newsgroups: comp.unix.questions,comp.unix.shell,comp.answers,news.answers
@c Subject: Unix - Frequently Asked Questions (2/7) [Frequent posting]
@c Subject: How do I remove a file with funny characters in the filename ?
@c Date: Thu Mar 18 17:16:55 EST 1993
@code{find} can help you remove or rename a file with strange
characters in its name.  People are sometimes stymied by files whose
names contain characters such as spaces, tabs, control characters, or
characters with the high bit set.  The simplest way to remove such
files is:

@example
rm -i @var{some*pattern*that*matches*the*problem*file}
@end example

@code{rm} asks you whether to remove each file matching the given
pattern.  If you are using an old shell, this approach might not work
if the file name contains a character with the high bit set; the shell
may strip it off.  A more reliable way is:

@example
find . -maxdepth 1 @var{tests} -okdir rm '@{@}' \;
@end example

@noindent
where @var{tests} uniquely identify the file.  The @samp{-maxdepth 1}
option prevents @code{find} from wasting time searching for the file
in any subdirectories; if there are no subdirectories, you may omit
it.  A good way to uniquely identify the problem file is to figure out
its inode number; use

@example
ls -i
@end example

Suppose you have a file whose name contains control characters, and
you have found that its inode number is 12345.  This command prompts
you for whether to remove it:

@example
find . -maxdepth 1 -inum 12345 -okdir rm -f '@{@}' \;
@end example

If you don't want to be asked, perhaps because the file name may
contain a strange character sequence that will mess up your screen
when printed, then use @samp{-execdir} instead of @samp{-okdir}.

If you want to rename the file instead, you can use @code{mv} instead
of @code{rm}:

@example
find . -maxdepth 1 -inum 12345 -okdir mv '@{@}' @var{new-file-name} \;
@end example

@node Fixing Permissions
@section Fixing Permissions

Suppose you want to make sure that everyone can write to the
directories in a certain directory tree.  Here is a way to find
directories lacking either user or group write permission (or both),
and fix their permissions:

@example
find . -type d -not -perm -ug=w | xargs chmod ug+w
@end example

@noindent
You could also reverse the operations, if you want to make sure that
directories do @emph{not} have world write permission.

@node Classifying Files
@section Classifying Files

@c Idea from:
@c From: martin@mwtech.UUCP (Martin Weitzel)
@c Newsgroups: comp.unix.wizards,comp.unix.questions
@c Subject: Advanced usage of 'find' (Re: Unix security automating script)
@c Date: 22 Mar 90 15:05:19 GMT
If you want to classify a set of files into several groups based on
different criteria, you can use the comma operator to perform multiple
independent tests on the files.  Here is an example:

@example
find / -type d \( -perm -o=w -fprint allwrite , \
  -perm -o=x -fprint allexec \)

echo "Directories that can be written to by everyone:"
cat allwrite
echo ""
echo "Directories with search permissions for everyone:"
cat allexec
@end example

@code{find} has only to make one scan through the directory tree
(which is one of the most time consuming parts of its work).

@node Worked Examples, Security Considerations, Common Tasks, Top
@chapter Worked Examples

The tools in the findutils package, and in particular @code{find},
have a large number of options.  This means that quite often,
there is more than one way to do things.  Some of the options
and facilities only exist for compatibility with other tools, and
findutils provides improved ways of doing things.

This chapter describes a number of useful tasks that are commonly
performed, and compares the different ways of achieving them.

@menu
* Deleting Files::
* Updating A Timestamp File::
@end menu

@node Deleting Files
@section Deleting Files

One of the most common tasks that @code{find} is used for is locating
files that can be deleted.  This might include:

@itemize
@item 
Files last modified more than 3 years ago which haven't been accessed
for at least 2 years
@item
Files belonging to a certain user
@item
Temporary files which are no longer required
@end itemize

This example concentrates on the actual deletion task rather than on
sophisticated ways of locatng the files that need to be deleted.
We'll assume that the files we want to delete are old files underneath
@file{/var/tmp/stuff}.

@subsection The Traditional Way

The traditional way to delete files in @file{var/tmp/stuff} that have
not been modified in over 90 days would have been:

@smallexample
find /var/tmp/stuff -mtime +90 -exec /bin/rm @{@} \;
@end smallexample

The above command uses @samp{-exec} to run the @code{/bin/rm} command
to remove each file.  This approach works and in fact would have
worked in Version 7 Unix in 1979.  However, there are a number of
problems with this approach.


The most obvious problem with the approach above is that it causes
@code{find} to fork every time it finds a file that needs to delete,
and the child process then has to use the @code{exec} system call to
launch @code{/bin/rm}.   All this is quite inefficient.  If we are
going to use @code{/bin/rm} to do this job, it is better to make it
delete more than one file at a time.  

The most obvious way of doing this is to use the shell's command
expansion feature:

@smallexample
/bin/rm `find /var/tmp/stuff -mtime +90 -print`
@end smallexample
or you could use the more modern form
@smallexample
/bin/rm $(find /var/tmp/stuff -mtime +90 -print)
@end smallexample

The commands above are much more efficient than the first attempt.
However, there is a problem with them.  The shell has a maximum
command length which is imposed by the operating system (the actual
limit varies between systems).  This means that while the command
expansion technique will usually work, it will suddenly fail when
there are lots of files to delete.  Since the task is to delete
unwanted files, this is precisely the time we don't want things to go
wrong.

@subsection Making Use of xargs

So, is there a way to be more efficient in the use of @code{fork()}
and @code{exec()} without running up against this limit?
Yes, we can be almost optimally efficient by making use
of the @code{xargs} command.  The @code{xargs} command reads arguments
from its standard input and builds them into command lines.  We can
use it like this:

@smallexample
find /var/tmp/stuff -mtime +90 -print | xargs /bin/rm 
@end smallexample

For example if the files found by @code{find} are
@file{/var/tmp/stuff/A}, 
@file{/var/tmp/stuff/B} and 
@file{/var/tmp/stuff/C} then @code{xargs} might issue the commands 

@smallexample
/bin/rm /var/tmp/stuff/A /var/tmp/stuff/B
/bin/rm /var/tmp/stuff/C
@end smallexample

The above assumes that @code{xargs} has a very small maximum command
line length.  The real limit is much larger but the idea is that
@code{xargs} will run @code{/bin/rm} as many times as necessary to get
the job done, given the limits on command line length.

This usage of @code{xargs} is pretty efficient, and the @code{xargs}
command is widely implemented (all modern versions of Unix offer it).
So far then, the news is all good.  However, there is bad news too.

@subsection Unusual characters in filenames

Unix-like systems allow any characters to appear in file names with
the exception of the ASCII NUL character and the backslash.
Backslashes can occur in path names (as the directory separator) but
not in the names of actual directory entries.  This means that the
list of files that @code{xargs} reads could in fact contain white space
characters --- spaces, tabs and newline characters.  Since by default,
@code{xargs} assumes that the list of files it is reading uses white
space as an argument separator, it cannot correctly handle the case
where a filename actually includes white space.  This makes the
default behaviour of @code{xargs} almost useless for handling
arbitrary data.

To solve this problem, GNU findutils introduced the @samp{-print0}
action for @code{find}.  This uses the ASCII NUL character to separate
the entries in the file list that it produces.  This is the ideal
choice of separator since it is the only character that cannot appear
within a path name.  The @samp{-0} option to @code{xargs} makes it
assume that arguments are separated with ASCII NUL instead of white
space.  It also turns off another misfeature in the default behaviour
of @code{xargs}, which is that it pays attention to quote characters
in its input.  Some versions of @code{xargs} also terminate when they
see a lone @samp{_} in the input, but GNU @code{find} no longer does
that (since it has become an optional behaviour in the Unix standard).

So, putting @code{find -print0} together with @code{xargs -0} we get
this command:

@smallexample
find /var/tmp/stuff -mtime +90 -print0 | xargs -0 /bin/rm 
@end smallexample

The result is an efficient way of proceeding that
correctly handles all the possible characters that could appear in the
list of files to delete.  This is good news.  However, there is, as
I'm sure you're expecting, also more bad news.  The problem is that
this is not a portable construct; although other versions of Unix
(notable BSD-derived ones) support @samp{-print0}, it's not
universal.  So, is there a more universal mechanism?

@subsection Going back to -exec

There is indeed a more universal mechanism, which is a slight
modification to the @samp{-exec} action.  The normal @samp{-exec}
action assumes that the command to run is terminated with a semicolon
(the semicolon normally has to be quoted in order to protect it from
interpretation as the shell command separator).  The SVR4 edition of
Unix introduced a slight variation, which involves terminating the
command with @samp{+} instead:

@smallexample
find /var/tmp/stuff -mtime +90 -exec /bin/rm @{@} \+
@end smallexample

The above use of @samp{-exec} causes @code{find} to build up a long
command line and then issue it.  This can be less efficient than some
uses of @code{xargs}; for example @code{xargs} allows new command
lines to be built up while the previous command is still executing, and
allows you to specify a number of commands to run in parallel.
However, the @code{find @dots{} -exec @dots{} +} construct has the advantage
of wide portability.  GNU findutils did not support @samp{-exec @dots{} +}
until version 4.2.12; one of the reasons for this is that it already
had the @samp{-print0} action in any case.


@subsection A more secure version of -exec

The command above seems to be efficient and portable.  However,
within it lurks a security problem.  The problem is shared with
all the commands we've tried in this worked example so far, too.  The
security problem is a race condition; that is, if it is possible for
somebody to manipulate the filesystem that you are searching while you
are searching it, it is possible for them to persuade your @code{find}
command to cause the deletion of a file that you can delete but they
normally cannot.  

The problem occurs because the @samp{-exec} action is defined by the
@acronym{POSIX} standard to invoke its command with the same working directory
as @code{find} had when it was started.  This means that the arguments
which replace the @{@} include a relative path from @code{find}'s
starting point down the file that needs to be deleted.  For example,

@smallexample
find /var/tmp/stuff -mtime +90 -exec /bin/rm @{@} \+
@end smallexample

might actually issue the command:

@smallexample
/bin/rm /var/tmp/stuff/A /var/tmp/stuff/B /var/tmp/stuff/passwd
@end smallexample

Notice the file @file{/var/tmp/stuff/passwd}.  Likewise, the command:

@smallexample
cd /var/tmp && find stuff -mtime +90 -exec /bin/rm @{@} \+
@end smallexample

might actually issue the command:

@smallexample
/bin/rm stuff/A stuff/B stuff/passwd
@end smallexample

If an attacker can rename @file{stuff} to something else (making use
of their write permissions in @file{/var/tmp}) they can replace it
with a symbolic link to @file{/etc}.  That means that the
@code{/bin/rm} command will be invoked on @file{/etc/passwd}.  If you
are running your @code{find} command as root, the attacker has just managed
to delete a vital file.  All they needed to do to achieve this was
replace a subdirectory with a symbolic link at the vital moment.

There is however, a simple solution to the problem.  This is an action
which works a lot like @code{-exec} but doesn't need to traverse a
chain of directories to reach the file that it needs to work on.  This
is the @samp{-execdir} action, which was introduced by the BSD family
of operating systems.   The command,

@smallexample
find /var/tmp/stuff -mtime +90 -execdir /bin/rm @{@} \+
@end smallexample

might delete a set of files by performing these actions:

@enumerate
@item 
Change directory to /var/tmp/stuff/foo
@item 
Invoke @code{/bin/rm ./file1 ./file2 ./file3}
@item
Change directory to /var/tmp/stuff/bar
@item 
Invoke @code{/bin/rm ./file99 ./file100 ./file101}
@end enumerate

This is a much more secure method.  We are no longer exposed to a race
condition.  For many typical uses of @code{find}, this is the best
strategy.   It's reasonably efficient, but the length of the command
line is limited not just by the operating system limits, but also by
how many files we actually need to delete from each directory.

Is it possible to do any better?   In the case of general file
processing, no.  However, in the specific case of deleting files it is
indeed possible to do better.  

@subsection Using the -delete action

The most efficient and secure method of solving this problem is to use
the @samp{-delete} action:

@smallexample
find /var/tmp/stuff -mtime +90 -delete
@end smallexample

This alternative is more efficient than any of the @samp{-exec} or
@samp{-execdir} actions, since it entirely avoids the overhead of
forking a new process and using @code{exec} to run @code{/bin/rm}.  It
is also normally more efficient than @code{xargs} for the same
reason.   The file deletion is performed from the directory containing
the entry to be deleted, so the @samp{-delete} action has the same
security advantages as the @samp{-execdir} action has.  

The @samp{-delete} action was introduced by the BSD family of
operating systems.

@subsection Improving things still further

Is it possible to improve things still further?  Not without either
modifying the system library to the operating system or having more specific
knowledge of the layout of the filesystem and disk I/O subsystem, or
both.

The @code{find} command traverses the filesystem, reading
directories.  It then issues a separate system call for each file to
be deleted.  If we could modify the operating system, there are
potential gains that could be made:

@itemize
@item
We could have a system call to which we pass more than one filename
for deletion
@item
Alternatively, we could pass in a list of inode numbers (on GNU/Linux
systems, @code{readdir()} also returns the inode number of each
directory entry) to be deleted.
@end itemize

The above possibilities sound interesting, but from the kernel's point
of view it is difficult to enforce standard Unix access controls for
such processing by inode number.  Such a facility would probably
need to be restricted to the superuser.

Another way of improving performance would be to increase the
parallelism of the process.  For example if the directory hierarchy we
are searching is actually spread across a number of disks, we might
somehow be able to arrange for @code{find} to process each disk in
parallel.  In practice GNU @code{find} doesn't have such an intimate
understanding of the system's filesystem layout and disk I/O
subsystem.

However, since the system administrator can have such an understanding
they can take advantage of it like so:

@smallexample
find /var/tmp/stuff1 -mtime +90 -delete &
find /var/tmp/stuff2 -mtime +90 -delete &
find /var/tmp/stuff3 -mtime +90 -delete &
find /var/tmp/stuff4 -mtime +90 -delete &
wait
@end smallexample

In the example above, four spearate instances of @code{find} are used
to search four subdirectories in parallel.  The @code{wait} command
simply waits for all of these to complete.  Whether this approach is
more or less efficient than a single instance of @code{find} depends
on a number of things:

@itemize
@item
Are the directories being searched in parallel actually on separate
disks?  If not, this parallel search might just result in a lot of
disk head movement and so the speed might even be slower.
@item
Other activity - are other programs also doing things on those disks?
@end itemize


@subsection Conclusion

The fastest and most secure way to delete files with the help of
@code{find} is to use @samp{-delete}.  Using @code{xargs -0 -P N} can
also make effective use of the disk, but it is not as secure.

In the case where we're doing things other than deleting files, the
most secure alternative is @samp{-execdir @dots{} +}, but this is not as
portable as the insecure action @samp{-exec @dots{} +}.

The @samp{-delete} action is not completely portable, but the only
other possiblility which is as secure (@samp{-execdir}) is no more
portable.  The most efficient portable alternative is @samp{-exec
@dots{}+}, but this is insecure and isn't supported by versions of GNU
findutils prior to 4.2.12.


@node Updating A Timestamp File
@section Updating A Timestamp File

Suppose we have a directory full of files which is maintained with a
set of automated tools; perhaps one set of tools updates them and
another set of tools uses the result.  In this situation, it might be
useful for the second set of tools to know if the files have recently
been changed.  It might be useful, for example, to have a 'timestamp'
file which gives the timestamp on the newest file in the collection.

We can use @code{find} to achieve this, but there are several
different ways to do it.

@subsection Updating the Timestamp The Wrong Way

The obvious but wrong answer is just to use @samp{-newer}:-

@smallexample
find subdir -newer timestamp -exec touch -r @{@} timestamp \; 
@end smallexample

This does the right sort of thing but has a bug.  Suppose that two
files in the subdirectory have been updated, and that these are called
@file{file1} and @file{file2}.  The command above will update
@file{timestamp} with the modification time of @file{file1} or that of
@file{file2}, but we don't know which one.  Since the timestamps on
@file{file1} and @file{file2} will in general be different, this could
well be the wrong value.

One solution to this problem is to modify @code{find} to recheck the
modification time of @file{timestamp} every time a file is to be
compared against it, but that will reduce the performence of
@code{find}.

@subsection Using the test utlity to compare timestamps

The @code{test} command can be used to compare timestamps:

@smallexample
find subdir -exec test @{@} -nt timestamp \; -exec touch -r @{@} timestamp \; 
@end smallexample

This will ensure that any changes made to the modification time of
@file{timestamp} that take place during the execution of @code{find}
are taken into account.  This resolves our earlier problem, but
unfortunately this runs much more slowly.

@subsection A combined approach

We can of course still use @samp{-newer} to cut down on the number of
calls to @code{test}:

@smallexample
find subdir -newer timestamp -a \
     -exec test @{@} -nt timestamp \; -a \
     -exec touch -r @{@} timestamp \; 
@end smallexample

Here, the @samp{-newer} test excludes all he files which are
definitely older than the timestamp, but all the files which are newer
than the old value of the timestamp are compared against the current
updated timestamp.

This is indeed faster in general, but the speed difference will depend
on how many updated files there are.

@subsection Using -printf and sort to compare timestamps

It is possible to use the @samp{-printf} action to abandon the use of
@code{test} entirely:

@smallexample
newest=$(find subdir -newer timestamp -printf "%A@:%p\n" | 
           sort -n | 
           tail -1 | 
           cut -d: -f2- ) 
touch -r "$@{newest:-timestamp@}" timestamp
@end smallexample

The command above works by generating a list of the timestamps and
names of all the files which are newer than the timestamp.  The
@code{sort}, @code{tail} and @code{cut} comands simply pull out the
name of the file with the largest timestamp value (that is, the latest
file).  The @code{touch} command is then used to update the timestamp,

The @code{"$@{newest:-timestamp@}"} expression simply expands to the
value of @code{$newest} if that variable is set, but to
@file{timestamp} otherwise.  This ensures that an argument is always
given to the @samp{-r} option of the @code{touch} command.

This approach seems quite efficient, but unfortunately it has a bug.
Many operating systems now keep file modification time information at
a granularity which is finer than one second.  Unfortunately the
@samp{%A@@} format for @samp{-printf} only prints a whole-number value
currently; that is, these values are at a one-second granularity.
This means that in our example above, @samp{$newest} wil be the name
of a file which is no more than one second older than the newest file,
but may indeed be older.

It would be possible to solve this problem with some kind of loop:

@smallexample
while true; do
        newest=$(find subdir -newer timestamp -printf "%A@@:%p\n" | 
           sort -n | 
           tail -1 | 
           cut -d: -f2- ) 
        if test -z "$newest" ; then
                break
        else
                touch -r "$newest" timestamp
        fi
done
@end smallexample

A better fix for this problem would be to allow the @samp{%A@@} format
to produce a result having a fractional part, too.  While this is
planned for GNU @code{find}, it hasn't been done yet.

@subsection Coping with sub-second timestamp resolution

Another tool which often works with timestamps is @code{make}.  We can
use @code{find} to generate a @file{Makefile} file on the fly and then
use @code{make} to update the timestamps:

@smallexample
makefile=$(mktemp)
find subdir \
        \( \! -xtype l \) \
        -newer timestamp \
        -printf "timestamp:: %p\n\ttouch -r %p timestamp\n\n" > "$makefile"
make -f "$makefile"
rm   -f "$makefile"
@end smallexample

Unfortunately although the solution above is quite elegant, it fails
to cope with white space within file names, and adjusting it to do so
would require a rather complex shell script.


@subsection Coping with odd filenames too

We can fix both of these problems (looping and problems with white
space), and do things more efficiently too.  The following command
works with newlines and doesn't need to sort the list of filenames.

@smallexample
find subdir -newer timestamp -printf "%A@@:%p\0" | 
   perl -0 newest.pl |
   xargs --no-run-if-empty --null -i \
      find @{@} -maxdepth 0 -newer timestamp -exec touch -r @{@} timestamp \;
@end smallexample

The first @code{find} command generates a list of files which are
newer than the original timestamp file, and prints a list of them with
their timestamps.  The @file{newest.pl} script simply filters out all
the filenames which have timestamps which are older than whatever the
newest file is:-

@smallexample
@verbatim
#! /usr/bin/perl -0
my @newest = ();
my $latest_stamp = undef;
while (<>) {
    my ($stamp, $name) = split(/:/);
    if (!defined($latest_stamp) || ($tstamp > $latest_stamp)) {
        $latest_stamp = $stamp;
        @newest = ();
    }
    if ($tstamp >= $latest_stamp) {
        push @newest, $name;
    }
}
print join("\0", @newest);
@end verbatim
@end smallexample

This prints a list of zero or more files, all of which are newer than
the original timestamp file, and which have the same timestamp as each
other, to the nearest second.  The second @code{find} command takes
each resulting file one at a time, and if that is newer than the
timestamp file, the timestamp is updated.

@node Security Considerations, Error Messages, Worked Examples, Top
@chapter Security Considerations

Security considerations are important if you are using @code{find} or
@code{xargs} to search for or process files that don't belong to you
or which other people have control.  Security considerations
relating to @code{locate} may also apply if you have files which you
do not want others to see.

The most severe forms of security problems affecting
@code{find} and related programs are when third parties bring
about a situation allowing them to do something
they would normally not be able to accomplish.  This is called @emph{privilege
elevation}.  This might include deleting files they would not normally
be able to delete.  It is common for the operating system to periodically
invoke @code{find} for self-maintenance purposes.  These invocations of
@code{find} are particularly problematic from a security point of view
as these are often invoked by the superuser and search the entire
filesystem hierarchy.  Generally, the severity of any associated problem depends
on what the system is going to do with the files found by @code{find}.

@menu
* Levels of Risk::      What is your level of exposure to security problems?
* Security Considerations for find::  Security problems with find
* Security Considerations for xargs:: Security problems with xargs
* Security Considerations for locate:: Security problems with locate
* Security Summary:: That was all very complex, what does it boil down to?
@end menu


@node Levels of Risk
@section Levels of Risk

There are some security risks inherent in the use of @code{find},
@code{xargs} and (to a lesser extent) @code{locate}.  The severity of
these risks depends on what sort of system you are using:

@table @strong
@item High risk
Multi-user systems where you do not control (or trust) the other
users, and on which you execute @code{find}, including areas where
those other users can manipulate the filesystem (for example beneath
@file{/home} or @file{/tmp}).

@item Medium Risk
Systems where the actions of other users can create file names chosen
by them, but to which they don't have access while @code{find} is
being run.  This access might include leaving programs running (shell
background jobs, @code{at} or @code{cron} tasks, for example).  On
these sorts of systems, carefully written commands (avoiding use of
@samp{-print} for example) should not expose you to a high degree of
risk.  Most systems fall into this category.

@item Low Risk
Systems to which untrusted parties do not have access, cannot create
file names of their own choice (even remotely) and which contain no
security flaws which might enable an untrusted third party to gain
access.  Most systems do not fall into this category because there are
many ways in which external parties can affect the names of files that
are created on your system.  The system on which I am writing this for
example automatically downloads software updates from the Internet;
the names of the files in which these updates exist are chosen by
third parties@footnote{Of course, I trust these parties to a large
extent anyway, because I install software provided by them; I choose
to trust them in this way, and that's a deliberate choice}.
@end table

In the discussion above, ``risk'' denotes the likelihood that someone
can cause @code{find}, @code{xargs}, @code{locate} or some other
program which is controlled by them to do something you did not
intend.  The levels of risk suggested do not take any account of the
consequences of this sort of event.  That is, if you operate a ``low
risk'' type system, but the consequences of a security problem are
disastrous, then you should still give serious thought to all the
possible security problems, many of which of course will not be
discussed here -- this section of the manual is intended to be
informative but not comprehensive or exhaustive.

If you are responsible for the operation of a system where the
consequences of a security problem could be very important, you should
do two things:-

@enumerate
@item Define a security policy which defines who is allowed to do what
on your system.
@item Seek competent advice on how to enforce your policy, detect
breaches of that policy, and take account of any potential problems
that might fall outside the scope of your policy.
@end enumerate


@node Security Considerations for find
@section Security Considerations for @code{find}


Some of the actions @code{find} might take have a direct effect;
these include @code{-exec} and @code{-delete}.  However, it is also
common to use @code{-print} explicitly or implicitly, and so if
@code{find} produces the wrong list of file names, that can also be a
security problem; consider the case for example where @code{find} is
producing a list of files to be deleted.

We normally assume that the @code{find} command line expresses the
file selection criteria and actions that the user had in mind -- that
is, the command line is ``trusted'' data.

From a security analysis point of view, the output of @code{find}
should be correct; that is, the output should contain only the names
of those files which meet the user's criteria specified on the command
line.  This applies for the @code{-exec} and @code{-delete} actions;
one can consider these to be part of the output.

On the other hand, the contents of the filesystem can be manipulated
by other people, and hence we regard this as ``untrusted'' data.  This
implies that the @code{find} command line is a filter which converts
the untrusted contents of the filesystem into a correct list of output
files.

The filesystem will in general change while @code{find} is searching
it; in fact, most of the potential security problems with @code{find}
relate to this issue in some way.

@dfn{Race conditions} are a general class of security problem where the
relative ordering of actions taken by @code{find} (for example) and
something else are critically important in getting the correct and expected result@footnote{This is more or less the
definition of the term ``race condition''} .

For @code{find}, an attacker might move or rename files or directories in
the hope that an action might be taken against a file which was not
normally intended to be affected.  Alternatively, this sort of attack
might be intended to persuade @code{find} to search part of the
filesystem which would not normally be included in the search
(defeating the @code{-prune} action for example).

@menu
* Changing the Current Working Directory::
* Race Conditions with -exec::
* Race Conditions with -print and -print0::
@end menu


@node Changing the Current Working Directory
@subsection Changing the Current Working Directory

As @code{find} searches the filesystem, it finds subdirectories and
then searches within them by changing its working directory.  First,
@code{find} reaches and recognizes a subdirectory.  It then decides if that
subdirectory meets the criteria for being searched; that is, any
@samp{-xdev} or @samp{-prune} expressions are taken into account.  The
@code{find} program will then change working directory and proceed to
search the directory.

A race condition attack might take the form that once the checks
relevant to @samp{-xdev} and @samp{-prune} have been done, an attacker
might rename the directory that was being considered, and put in its
place a symbolic link that actually points somewhere else.

The idea behind this attack is to fool @code{find} into going into the
wrong directory.  This would leave @code{find} with a working
directory chosen by an attacker, bypassing any protection apparently
provided by @samp{-xdev} and @samp{-prune}, and any protection
provided by being able to @emph{not} list particular directories on
the @code{find} command line.  This form of attack is particularly
problematic if the attacker can predict when the @code{find} command
will be run, as is the case with @code{cron} tasks for example.

GNU @code{find} has specific safeguards to prevent this general class
of problem.  The exact form of these safeguards depends on the
properties of your system.

@menu
* O_NOFOLLOW::                     Safely changing directory using fchdir().
* Systems without O_NOFOLLOW::     Checking for symbolic links after chdir().
@end menu

@node O_NOFOLLOW
@subsubsection O_NOFOLLOW

If your system supports the O_NOFOLLOW flag @footnote{GNU/Linux
(kernel version 2.1.126 and later) and FreeBSD (3.0-CURRENT and later)
support this} to the @code{open(2)} system call, @code{find} uses it
when safely changing directory.  The target directory is first opened
and then @code{find} changes working directory with the
@code{fchdir()} system call.  This ensures that symbolic links are not
followed, preventing the sort of race condition attack in which use
is made of symbolic links.

If for any reason this approach does not work, @code{find} will fall
back on the method which is normally used if O_NOFOLLOW is not
supported.

You can tell if your system supports O_NOFOLLOW by running

@example
find --version
@end example

This will tell you the version number and which features are enabled.
For example, if I run this on my system now, this gives:
@example
GNU find version 4.2.18-CVS
Features enabled: D_TYPE O_NOFOLLOW(enabled)
@end example

Here, you can see that I am running a version of @code{find} which was
built from the development (CVS) code prior to the release of
findutils-4.2.18, and that the D_TYPE and O_NOFOLLOW features are
present.  O_NOFOLLOW is qualified with ``enabled''.  This simply means
that the current system seems to support O_NOFOLLOW.  This check is
needed because it is possible to build @code{find} on a system that
defines O_NOFOLLOW and then run it on a system that ignores the
O_NOFOLLOW flag.  We try to detect such cases at startup by checking
the operating system and version number; when this happens you will
see ``O_NOFOLLOW(disabled)'' instead.

@node Systems without O_NOFOLLOW
@subsubsection Systems without O_NOFOLLOW

The strategy for preventing this type of problem on systems that lack
support for the O_NOFOLLOW flag is more complex.  Each time
@code{find} changes directory, it examines the directory it is about
to move to, issues the @code{chdir()} system call, and then checks
that it has ended up in the subdirectory it expected.  If all is as
expected, processing continues as normal.  However, there are two main
reasons why the directory might change: the use of an automounter and
the someone removing the old directory and replacing it with something
else while @code{find} is trying to descend into it.

Where a filesystem ``automounter'' is in use it can be the case that
the use of the @code{chdir()} system call can itself cause a new
filesystem to be mounted at that point.  On systems that do not
support O_NOFOLLOW, this will cause @code{find}'s security check to
fail.

However, this does not normally represent a security problem, since
the automounter configuration is normally set up by the system
administrator.  Therefore, if the @code{chdir()} sanity check fails,
@code{find} will make one more attempt.  If that succeeds, execution
carries on as normal.  This is the usual case for automounters.

Where an attacker is trying to exploit a race condition, the problem
may not have gone away on the second attampt.  If this is the case,
@code{find} will issue a warning message and then ignore that
subdirectory.  When this happens, actions such as @samp{-exec} or
@samp{-print} may already have taken place for the problematic
subdirectory.  This is because @code{find} applies tests and actions
to directories before searching within them (unless @samp{-depth} was
specified).

Because of the nature of the directory-change operation and security
check, in the worst case the only things that @code{find} would have
done with the directory are to move into it and back out to the
original parent.  No operations would have been performed within that
directory.

@node Race Conditions with -exec
@subsection Race Conditions with -exec

The @samp{-exec} action causes another program to be run.  It passes
to the program the name of the file which is being considered at the
time.  The invoked program will typically then perform some action
on that file.  Once again, there is a race condition which can be
exploited here.  We shall take as a specific example the command

@example
find /tmp -path /tmp/umsp/passwd -exec /bin/rm
@end example

In this simple example, we are identifying just one file to be deleted
and invoking @code{/bin/rm} to delete it.  A problem exists because
there is a time gap between the point where @code{find} decides that
it needs to process the @samp{-exec} action and the point where the
@code{/bin/rm} command actually issues the @code{unlink()} system
call to delete the file from the filesystem.  Within this time period, an attacker can rename the
@file{/tmp/umsp} directory, replacing it with a symbolic link to
@file{/etc}.  There is no way for @code{/bin/rm} to determine that it
is working on the same file that @code{find} had in mind.  Once the
symbolic link is in place, the attacker has persuaded @code{find} to
cause the deletion of the @file{/etc/passwd} file, which is not the
effect intended by the command which was actually invoked.

One possible defence against this type of attack is to modify the
behaviour of @samp{-exec} so that the @code{/bin/rm} command is run
with the argument @file{./passwd} and a suitable choice of working
directory.  This would allow the normal sanity check that @code{find}
performs to protect against this form of attack too.  Unfortunately,
this strategy cannot be used as the POSIX standard specifies that the
current working directory for commands invoked with @samp{-exec} must
be the same as the current working directory from which @code{find}
was invoked.  This means that the @samp{-exec} action is inherently
insecure and can't be fixed.

GNU @code{find} implements a more secure variant of the @samp{-exec}
action, @samp{-execdir}.  The @samp{-execdir} action
ensures that it is not necessary to dereference subdirectories to
process target files.  The current directory used to invoke programs
is the same as the directory in which the file to be processed exists
(@file{/tmp/umsp} in our example, and only the basename of the file to
be processed is passed to the invoked command, with a @samp{./}
prepended (giving @file{./passwd} in our example).

The @samp{-execdir} action refuses to do anything if the current
directory is included in the @var{$PATH} environment variable.  This
is necessary because @samp{-execdir} runs programs in the same
directory in which it finds files -- in general, such a directory
might be writable by untrusted users.  For similar reasons,
@samp{-execdir} does not allow @samp{@{@}} to appear in the name of
the command to be run.

@node Race Conditions with -print and -print0
@subsection Race Conditions with -print and -print0

The @samp{-print} and @samp{-print0} actions can be used to produce a
list of files matching some criteria, which can then be used with some
other command, perhaps with @code{xargs}.  Unfortunately, this means
that there is an unavoidable time gap between @code{find} deciding
that one or more files meet its criteria and the relevant command
being executed.  For this reason, the @samp{-print} and @samp{-print0}
actions are just as insecure as @samp{-exec}.

In fact, since the construction

@example
find @dots{}  -print | xargs @enddots{}
@end example

does not cope correctly with newlines or other ``white space'' in
file names, and copes poorly with file names containing quotes, the
@samp{-print} action is less secure even than @samp{-print0}.


@comment  node-name,  next,  previous,  up
@comment @node Security Considerations for xargs
@node Security Considerations for xargs
@section Security Considerations for @code{xargs}

The description of the race conditions affecting the @samp{-print}
action of @code{find} shows that @code{xargs} cannot be secure if it
is possible for an attacker to modify a filesystem after @code{find}
has started but before @code{xargs} has completed all its actions.

However, there are other security issues that exist even if it is not
possible for an attacker to have access to the filesystem in real
time.  Firstly, if it is possible for an attacker to create files with
names of their choice on the filesystem, then @code{xargs} is
insecure unless the @samp{-0} option is used.  If a file with the name
@file{/home/someuser/foo/bar\n/etc/passwd} exists (assume that
@samp{\n} stands for a newline character), then @code{find @dots{} -print}
can be persuaded to print three separate lines:

@example
/home/someuser/foo/bar

/etc/passwd
@end example

If it finds a blank line in the input, @code{xargs} will ignore it.
Therefore, if some action is to be taken on the basis of this list of
files, the @file{/etc/passwd} file would be included even if this was
not the intent of the person running find.  There are circumstances in
which an attacker can use this to their advantage.  The same
consideration applies to file names containing ordinary spaces rather
than newlines, except that of course the list of file names will no
longer contain an ``extra'' newline.

This problem is an unavoidable consequence of the default behaviour of
the @code{xargs} command, which is specified by the POSIX standard.
The only ways to avoid this problem are either to avoid all use of
@code{xargs} in favour for example of @samp{find -exec} or (where
available) @samp{find -execdir}, or to use the @samp{-0} option, which
ensures that @code{xargs} considers file names to be separated by
ASCII NUL characters rather than whitespace.  However, useful as this
option is, the POSIX standard does not make it mandatory.

@comment  node-name,  next,  previous,  up
@node Security Considerations for locate
@section Security Considerations for @code{locate}

It is fairly unusual for the output of @code{locate} to be fed into
another command.  However, if this were to be done, this would raise
the same set of security issues as the use of @samp{find @dots{} -print}.
Although the problems relating to whitespace in file names can be
resolved by using @code{locate}'s @samp{-0} option, this still leaves
the race condition problems associated with @samp{find @dots{} -print0}.
There is no way to avoid these problems in the case of @code{locate}.

@node Security Summary
@section Summary

Where untrusted parties can create files on the system, or affect the
names of files that are created, all uses for @code{find},
@code{locate} and @code{xargs} have known security problems except the
following:

@table @asis
@item Informational use only
Uses where the programs are used to prepare lists of file names upon
which no further action will ever be taken.

@item @samp{-delete}
Use of the @samp{-delete} action with @code{find} to delete files
which meet specified criteria

@item @samp{-execdir}
Use of the @samp{-execdir} action with @code{find} where the
@env{PATH} environment variable contains directories which contain
only trusted programs.
@end table

@comment  node-name,  next,  previous,  up
@node Error Messages, Primary Index, Security Considerations, Top
@chapter Error Messages

This section describes some of the error messages sometimes made by
@code{find}, @code{xargs}, or @code{locate}, explains them and in some
cases provides advice as to what you should do about this.

This manual is written in English.  The GNU findutils software
features translations of error messages for many languages.  For this
reason the error messages produced by
the programs are made to be as self-explanatory as possible.  This approach avoids leaving people to
figure out which test an English-language error message
corresponds to. Error messages which are self-explanatory
will not normally be mentioned in this document.  For
those messages mentioned in this document, only the
English-language version of the message will be listed.

@menu
* Error Messages From find::
* Error Messages From xargs::
* Error Messages From locate::
* Error Messages From updatedb::
@end menu

@node Error Messages From find, Error Messages From xargs, , Error Messages
@section Error Messages From @code{find}

@table @samp
@item invalid predicate `-foo'
This means that the @code{find} command line included something that
started with a dash or other special character.  The @code{find}
program tried to interpret this as a test, action or option, but
didn't recognise it.  If it was intended to be a test, check what was
specified against the documentation.  If, on the other hand, the
string is the name of a file which has been expanded from a wildcard
(for example because you have a @samp{*} on the command line),
consider using @samp{./*} or just @samp{.} instead.

@item unexpected extra predicate
This usually happens if you have an extra bracket on the command line
(for example @samp{find . -print \)}).

@item Warning: filesystem /path/foo has recently been mounted
@itemx Warning: filesystem /path/foo has recently been unmounted
These messages might appear when @code{find} moves into a directory
and finds that the device number and inode are different to what it
expected them to be.  If the directory @code{find} has moved into is
on an network filesystem (NFS), it will not issue this message, because
@code{automount} frequently mounts new filesystems on directories as
you move into them (that is how it knows you want to use the
filesystem).  So, if you do see this message, be wary ---
@code{automount} may not have been responsible.  Consider the
possibility that someone else is manipulating the filesystem while
@code{find} is running.  Some people might do this in order to mislead
@code{find} or persuade it to look at one set of files when it thought
it was looking at another set.

@item /path/foo changed during execution of find (old device number 12345, new device number 6789, filesystem type is <whatever>) [ref XXX]
This message is issued when @code{find} moves into a directory and ends up
somewhere it didn't expect to be.  This happens in one of two
circumstances.  Firstly, this happens when @code{automount} intervenes
on a system where @code{find} doesn't know how to determine what
the current set of mounted filesystems is.

Secondly, this can happen when the device number of a directory
appears to change during a change of current directory, but
@code{find} is moving up the filesystem hierarchy rather than down into it.
In order to prevent @code{find} wandering off into some unexpected
part of the filesystem, we stop it at this point.

@item Don't know how to use getmntent() to read `/etc/mtab'.  This is a bug.
This message is issued when a problem similar to the above occurs on a
system where @code{find} doesn't know how to figure out the current
list of mount points.  Ask for help on @email{bug-findutils@@gnu.org}.

@item /path/foo/bar changed during execution of find (old inode number 12345, new inode number 67893, filesystem type is <whatever>) [ref XXX]"),
This message is issued when @code{find} moves into a directory and
discovers that the inode number of that directory
is different from the inode number that it obtained when it examined the
directory previously.  This usually means that while
@code{find} was deep in a directory hierarchy doing a
time consuming operation, somebody has moved one of the parent directories to
another location in the same filesystem.  This may or may not have been done
maliciously.  In any case, @code{find} stops at this point
to avoid traversing parts of the filesystem that it wasn't
intended.  You can use @code{ls -li} or @code{find /path -inum
12345 -o -inum 67893} to find out more about what has happened.

@item sanity check of the fnmatch() library function failed.
Please submit a bug report.  You may well be asked questions about
your system, and if you compiled the @code{findutils} code yourself,
you should keep your copy of the build tree around.  The likely
explanation is that your system has a buggy implementation of
@code{fnmatch} that looks enough like the GNU version to fool
@code{configure}, but which doesn't work properly.

@item cannot fork
This normally happens if you use the @code{-exec} action or
something similar (@code{-ok} and so forth) but the system has run out
of free process slots.  This is either because the system is very busy
and the system has reached its maximum process limit, or because you
have a resource limit in place and you've reached it.  Check the
system for runaway processes (with @code{ps}, if possible).  Some process
slots are normally reserved for use by @samp{root}.

@item some-program terminated by signal 99
Some program which was launched with @code{-exec} or similar was killed
with a fatal signal.  This is just an advisory message.
@end table


@node Error Messages From xargs, Error Messages From locate, Error Messages From find, Error Messages
@section Error Messages From xargs

@table @samp
@item environment is too large for exec
This message means that you have so many environment variables set (or
such large values for them) that there is no room within the
system-imposed limits on program command line argument length to
invoke any program.  This is an unlikely situation and is more likely
result of an attempt to test the limits of @code{xargs}, or break it.
Please try unsetting some environment variables, or exiting the
current shell.

@item can not fit single argument within argument list size limit
You are using the @samp{-I} option and @code{xargs} doesn't have
enough space to build a command line because it has read a really
large item and it doesn't fit.  You can probably work around this
problem with the @samp{-s} option, but the default size is pretty
large.  This is a rare situation and is more likely an attempt to test
the limits of @code{xargs}, or break it.  Otherwise, you will need to
try to shorten the problematic argument or not use @code{xargs}.

@item cannot fork
See the description of the similar message for @code{find}.

@item <program>: exited with status 255; aborting
When a command run by @code{xargs} exits with status 255, @code{xargs}
is supposed to stop.  If this is not what you intended, wrap the
program you are trying to invoke in a shell script which doesn't
return status 255.

@item <program>: terminated by signal 99
See the description of the similar message for @code{find}.
@end table

@node Error Messages From locate, Error Messages From updatedb, Error Messages From xargs, Error Messages
@section Error Messages From @code{locate}

@table @samp
@item warning: database `/usr/local/var/locatedb' is more than 8 days old
The @code{locate} program relies on a database which is periodically
built by the @code{updatedb} program.  That hasn't happened in a long
time.  To fix this problem, run @code{updatedb} manually.  This can
often happen on systems that are generally not left on, so the
periodic ``cron'' task which normally does this doesn't get a chance
to run.

@item locate database `/usr/local/var/locatedb' is corrupt or invalid
This should not happen.  Re-run @code{updatedb}.  If that works, but
@code{locate} still produces this error, run @code{locate --version}
and @code{updatedb --version}.  These should produce the same output.
If not, you are using a mixed toolset; check your @samp{$PATH}
environment variable and your shell aliases (if you have any).  If
both programs claim to be GNU versions, this is a bug; all versions of
these programs should interoperate without problem.  Ask for help on
@email{bug-findutils@@gnu.org}.
@end table


@node Error Messages From updatedb, , Error Messages From locate, Error Messages
@section Error Messages From updatedb

The @code{updatedb} program (and the programs it invokes) do issue
error messages, but none seem to be candidates for guidance.  If
you are having a problem understanding one of these, ask for help on
@email{bug-findutils@@gnu.org}.

@node Primary Index, , Error Messages, Top
@unnumbered @code{find} Primary Index

This is a list of all of the primaries (tests, actions, and options)
that make up @code{find} expressions for selecting files.  @xref{find
Expressions}, for more information on expressions.

@printindex fn

@bye

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