build: add numerous gnulib modules, mostly for improved portability

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@c File mode bits

@c Copyright (C) 1994-2012 Free Software Foundation, Inc.

@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.3 or
@c any later version published by the Free Software Foundation; with no
@c Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
@c Texts.  A copy of the license is included in the ``GNU Free
@c Documentation License'' file as part of this distribution.

Each file has a set of @dfn{file mode bits} that control the kinds of
access that users have to that file.  They can be represented either in
symbolic form or as an octal number.

@menu
* Mode Structure::              Structure of file mode bits.
* Symbolic Modes::              Mnemonic representation of file mode bits.
* Numeric Modes::               File mode bits as octal numbers.
* Operator Numeric Modes::      ANDing, ORing, and setting modes octally.
* Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories.
@end menu

@node Mode Structure
@section Structure of File Mode Bits

The file mode bits have two parts: the @dfn{file permission bits},
which control ordinary access to the file, and @dfn{special mode
bits}, which affect only some files.

There are three kinds of permissions that a user can have for a file:

@enumerate
@item
@cindex read permission
permission to read the file.  For directories, this means permission to
list the contents of the directory.
@item
@cindex write permission
permission to write to (change) the file.  For directories, this means
permission to create and remove files in the directory.
@item
@cindex execute/search permission
permission to execute the file (run it as a program).  For directories,
this means permission to access files in the directory.
@end enumerate

There are three categories of users who may have different permissions
to perform any of the above operations on a file:

@enumerate
@item
the file's owner;
@item
other users who are in the file's group;
@item
everyone else.
@end enumerate

@cindex owner, default
@cindex group owner, default
Files are given an owner and group when they are created.  Usually the
owner is the current user and the group is the group of the directory
the file is in, but this varies with the operating system, the
file system the file is created on, and the way the file is created.  You
can change the owner and group of a file by using the @command{chown} and
@command{chgrp} commands.

In addition to the three sets of three permissions listed above, the
file mode bits have three special components, which affect only
executable files (programs) and, on most systems, directories:

@enumerate
@item
@cindex set-user-ID
@cindex setuid
Set the process's effective user ID to that of the file upon execution
(called the @dfn{set-user-ID bit}, or sometimes the @dfn{setuid bit}).
For directories on a few systems, give files created in the directory
the same owner as the directory, no matter who creates them, and set
the set-user-ID bit of newly-created subdirectories.
@item
@cindex set-group-ID
@cindex setgid
Set the process's effective group ID to that of the file upon execution
(called the @dfn{set-group-ID bit}, or sometimes the @dfn{setgid bit}).
For directories on most systems, give files created in the directory
the same group as the directory, no matter what group the user who
creates them is in, and set the set-group-ID bit of newly-created
subdirectories.
@item
@cindex sticky
@cindex swap space, saving text image in
@cindex text image, saving in swap space
@cindex restricted deletion flag
Prevent unprivileged users from removing or renaming a file in a directory
unless they own the file or the directory; this is called the
@dfn{restricted deletion flag} for the directory, and is commonly
found on world-writable directories like @file{/tmp}.

For regular files on some older systems, save the program's text image on the
swap device so it will load more quickly when run; this is called the
@dfn{sticky bit}.
@end enumerate

In addition to the file mode bits listed above, there may be file attributes
specific to the file system, e.g., access control lists (ACLs), whether a
file is compressed, whether a file can be modified (immutability), and whether
a file can be dumped.  These are usually set using programs
specific to the file system.  For example:
@c should probably say a lot more about ACLs... someday

@table @asis
@item ext2
On @acronym{GNU} and @acronym{GNU}/Linux the file attributes specific to
the ext2 file system are set using @command{chattr}.

@item FFS
On FreeBSD the file flags specific to the FFS
file system are set using @command{chflags}.
@end table

Even if a file's mode bits allow an operation on that file,
that operation may still fail, because:

@itemize
@item
the file-system-specific attributes or flags do not permit it; or

@item
the file system is mounted as read-only.
@end itemize

For example, if the immutable attribute is set on a file,
it cannot be modified, regardless of the fact that you
may have just run @code{chmod a+w FILE}.

@node Symbolic Modes
@section Symbolic Modes

@cindex symbolic modes
@dfn{Symbolic modes} represent changes to files' mode bits as
operations on single-character symbols.  They allow you to modify either
all or selected parts of files' mode bits, optionally based on
their previous values, and perhaps on the current @code{umask} as well
(@pxref{Umask and Protection}).

The format of symbolic modes is:

@example
@r{[}ugoa@dots{}@r{][}+-=@r{]}@var{perms}@dots{}@r{[},@dots{}@r{]}
@end example

@noindent
where @var{perms} is either zero or more letters from the set
@samp{rwxXst}, or a single letter from the set @samp{ugo}.

The following sections describe the operators and other details of
symbolic modes.

@menu
* Setting Permissions::          Basic operations on permissions.
* Copying Permissions::          Copying existing permissions.
* Changing Special Mode Bits::   Special mode bits.
* Conditional Executability::    Conditionally affecting executability.
* Multiple Changes::             Making multiple changes.
* Umask and Protection::              The effect of the umask.
@end menu

@node Setting Permissions
@subsection Setting Permissions

The basic symbolic operations on a file's permissions are adding,
removing, and setting the permission that certain users have to read,
write, and execute or search the file.  These operations have the following
format:

@example
@var{users} @var{operation} @var{permissions}
@end example

@noindent
The spaces between the three parts above are shown for readability only;
symbolic modes cannot contain spaces.

The @var{users} part tells which users' access to the file is changed.
It consists of one or more of the following letters (or it can be empty;
@pxref{Umask and Protection}, for a description of what happens then).  When
more than one of these letters is given, the order that they are in does
not matter.

@table @code
@item u
@cindex owner of file, permissions for
the user who owns the file;
@item g
@cindex group, permissions for
other users who are in the file's group;
@item o
@cindex other permissions
all other users;
@item a
all users; the same as @samp{ugo}.
@end table

The @var{operation} part tells how to change the affected users' access
to the file, and is one of the following symbols:

@table @code
@item +
@cindex adding permissions
to add the @var{permissions} to whatever permissions the @var{users}
already have for the file;
@item -
@cindex removing permissions
@cindex subtracting permissions
to remove the @var{permissions} from whatever permissions the
@var{users} already have for the file;
@item =
@cindex setting permissions
to make the @var{permissions} the only permissions that the @var{users}
have for the file.
@end table

The @var{permissions} part tells what kind of access to the file should
be changed; it is normally zero or more of the following letters.  As with the
@var{users} part, the order does not matter when more than one letter is
given.  Omitting the @var{permissions} part is useful only with the
@samp{=} operation, where it gives the specified @var{users} no access
at all to the file.

@table @code
@item r
@cindex read permission, symbolic
the permission the @var{users} have to read the file;
@item w
@cindex write permission, symbolic
the permission the @var{users} have to write to the file;
@item x
@cindex execute/search permission, symbolic
the permission the @var{users} have to execute the file,
or search it if it is a directory.
@end table

For example, to give everyone permission to read and write a regular file,
but not to execute it, use:

@example
a=rw
@end example

To remove write permission for all users other than the file's
owner, use:

@example
go-w
@end example

@noindent
The above command does not affect the access that the owner of
the file has to it, nor does it affect whether other users can
read or execute the file.

To give everyone except a file's owner no permission to do anything with
that file, use the mode below.  Other users could still remove the file,
if they have write permission on the directory it is in.

@example
go=
@end example

@noindent
Another way to specify the same thing is:

@example
og-rwx
@end example

@node Copying Permissions
@subsection Copying Existing Permissions

@cindex copying existing permissions
@cindex permissions, copying existing
You can base a file's permissions on its existing permissions.  To do
this, instead of using a series of @samp{r}, @samp{w}, or @samp{x}
letters after the
operator, you use the letter @samp{u}, @samp{g}, or @samp{o}.  For
example, the mode

@example
o+g
@end example

@noindent
adds the permissions for users who are in a file's group to the
permissions that other users have for the file.  Thus, if the file
started out as mode 664 (@samp{rw-rw-r--}), the above mode would change
it to mode 666 (@samp{rw-rw-rw-}).  If the file had started out as mode
741 (@samp{rwxr----x}), the above mode would change it to mode 745
(@samp{rwxr--r-x}).  The @samp{-} and @samp{=} operations work
analogously.

@node Changing Special Mode Bits
@subsection Changing Special Mode Bits

@cindex changing special mode bits
In addition to changing a file's read, write, and execute/search permissions,
you can change its special mode bits.  @xref{Mode Structure}, for a
summary of these special mode bits.

To change the file mode bits to set the user ID on execution, use
@samp{u} in the @var{users} part of the symbolic mode and
@samp{s} in the @var{permissions} part.

To change the file mode bits to set the group ID on execution, use
@samp{g} in the @var{users} part of the symbolic mode and
@samp{s} in the @var{permissions} part.

To set both user and group ID on execution, omit the @var{users} part
of the symbolic mode (or use @samp{a}) and use @samp{s} in the
@var{permissions} part.

To change the file mode bits to set the restricted deletion flag or sticky bit,
omit the @var{users} part of the symbolic mode (or use @samp{a}) and use
@samp{t} in the @var{permissions} part.

For example, to set the set-user-ID mode bit of a program,
you can use the mode:

@example
u+s
@end example

To remove both set-user-ID and set-group-ID mode bits from
it, you can use the mode:

@example
a-s
@end example

To set the restricted deletion flag or sticky bit, you can use
the mode:

@example
+t
@end example

The combination @samp{o+s} has no effect.  On @acronym{GNU} systems
the combinations @samp{u+t} and @samp{g+t} have no effect, and
@samp{o+t} acts like plain @samp{+t}.

The @samp{=} operator is not very useful with special mode bits.
For example, the mode:

@example
o=t
@end example

@noindent
does set the restricted deletion flag or sticky bit, but it also
removes all read, write, and execute/search permissions that users not in the
file's group might have had for it.

@xref{Directory Setuid and Setgid}, for additional rules concerning
set-user-ID and set-group-ID bits and directories.

@node Conditional Executability
@subsection Conditional Executability

@cindex conditional executability
There is one more special type of symbolic permission: if you use
@samp{X} instead of @samp{x}, execute/search permission is affected only if the
file is a directory or already had execute permission.

For example, this mode:

@example
a+X
@end example

@noindent
gives all users permission to search directories, or to execute files if
anyone could execute them before.

@node Multiple Changes
@subsection Making Multiple Changes

@cindex multiple changes to permissions
The format of symbolic modes is actually more complex than described
above (@pxref{Setting Permissions}).  It provides two ways to make
multiple changes to files' mode bits.

The first way is to specify multiple @var{operation} and
@var{permissions} parts after a @var{users} part in the symbolic mode.

For example, the mode:

@example
og+rX-w
@end example

@noindent
gives users other than the owner of the file read permission and, if
it is a directory or if someone already had execute permission
to it, gives them execute/search permission; and it also denies them write
permission to the file.  It does not affect the permission that the
owner of the file has for it.  The above mode is equivalent to
the two modes:

@example
og+rX
og-w
@end example

The second way to make multiple changes is to specify more than one
simple symbolic mode, separated by commas.  For example, the mode:

@example
a+r,go-w
@end example

@noindent
gives everyone permission to read the file and removes write
permission on it for all users except its owner.  Another example:

@example
u=rwx,g=rx,o=
@end example

@noindent
sets all of the permission bits for the file explicitly.  (It
gives users who are not in the file's group no permission at all for
it.)

The two methods can be combined.  The mode:

@example
a+r,g+x-w
@end example

@noindent
gives all users permission to read the file, and gives users who are in
the file's group permission to execute/search it as well, but not permission
to write to it.  The above mode could be written in several different
ways; another is:

@example
u+r,g+rx,o+r,g-w
@end example

@node Umask and Protection
@subsection The Umask and Protection

@cindex umask and modes
@cindex modes and umask
If the @var{users} part of a symbolic mode is omitted, it defaults to
@samp{a} (affect all users), except that any permissions that are
@emph{set} in the system variable @code{umask} are @emph{not affected}.
The value of @code{umask} can be set using the
@code{umask} command.  Its default value varies from system to system.

@cindex giving away permissions
Omitting the @var{users} part of a symbolic mode is generally not useful
with operations other than @samp{+}.  It is useful with @samp{+} because
it allows you to use @code{umask} as an easily customizable protection
against giving away more permission to files than you intended to.

As an example, if @code{umask} has the value 2, which removes write
permission for users who are not in the file's group, then the mode:

@example
+w
@end example

@noindent
adds permission to write to the file to its owner and to other users who
are in the file's group, but @emph{not} to other users.  In contrast,
the mode:

@example
a+w
@end example

@noindent
ignores @code{umask}, and @emph{does} give write permission for
the file to all users.

@node Numeric Modes
@section Numeric Modes

@cindex numeric modes
@cindex file mode bits, numeric
@cindex octal numbers for file modes
As an
alternative to giving a symbolic mode, you can give an octal (base 8)
number that represents the mode.
This number is always interpreted in octal; you do not have to add a
leading @samp{0}, as you do in C.  Mode @samp{0055} is the same as
mode @samp{55}.  (However, modes of five digits or more, such as
@samp{00055}, are sometimes special.  @xref{Directory Setuid and Setgid}.)

A numeric mode is usually shorter than the corresponding symbolic
mode, but it is limited in that normally it cannot take into account the
previous file mode bits; it can only set them absolutely.
The set-user-ID and set-group-ID bits of directories are an exception
to this general limitation; @xref{Directory Setuid and Setgid}.
Also, operator numeric modes can take previous file mode bits into
account; @xref{Operator Numeric Modes}.

The permissions granted to the user,
to other users in the file's group,
and to other users not in the file's group each require three
bits, which are represented as one octal digit.  The three special
mode bits also require one bit each, and they are as a group
represented as another octal digit.  Here is how the bits are arranged,
starting with the lowest valued bit:

@example
Value in  Corresponding
Mode      Mode Bit

          Other users not in the file's group:
   1      Execute/search
   2      Write
   4      Read

          Other users in the file's group:
  10      Execute/search
  20      Write
  40      Read

          The file's owner:
 100      Execute/search
 200      Write
 400      Read

          Special mode bits:
1000      Restricted deletion flag or sticky bit
2000      Set group ID on execution
4000      Set user ID on execution
@end example

For example, numeric mode @samp{4755} corresponds to symbolic mode
@samp{u=rwxs,go=rx}, and numeric mode @samp{664} corresponds to symbolic mode
@samp{ug=rw,o=r}.  Numeric mode @samp{0} corresponds to symbolic mode
@samp{a=}.

@node Operator Numeric Modes
@section Operator Numeric Modes

An operator numeric mode is a numeric mode that is prefixed by a
@samp{-}, @samp{+}, or @samp{=} operator, which has the same
interpretation as in symbolic modes.  For example, @samp{+440} enables
read permission for the file's owner and group, @samp{-1} disables
execute permission for other users, and @samp{=600} clears all
permissions except for enabling read-write permissions for the file's
owner.  Operator numeric modes can be combined with symbolic modes by
separating them with a comma; for example, @samp{=0,u+r} clears all
permissions except for enabling read permission for the file's owner.

The commands @samp{chmod =755 @var{dir}} and @samp{chmod 755
@var{dir}} differ in that the former clears the directory @var{dir}'s
setuid and setgid bits, whereas the latter preserves them.
@xref{Directory Setuid and Setgid}.

Operator numeric modes are a @acronym{GNU} extension.

@node Directory Setuid and Setgid
@section Directories and the Set-User-ID and Set-Group-ID Bits

On most systems, if a directory's set-group-ID bit is set, newly
created subfiles inherit the same group as the directory, and newly
created subdirectories inherit the set-group-ID bit of the parent
directory.  On a few systems, a directory's set-user-ID bit has a
similar effect on the ownership of new subfiles and the set-user-ID
bits of new subdirectories.  These mechanisms let users share files
more easily, by lessening the need to use @command{chmod} or
@command{chown} to share new files.

These convenience mechanisms rely on the set-user-ID and set-group-ID
bits of directories.  If commands like @command{chmod} and
@command{mkdir} routinely cleared these bits on directories, the
mechanisms would be less convenient and it would be harder to share
files.  Therefore, a command like @command{chmod} does not affect the
set-user-ID or set-group-ID bits of a directory unless the user
specifically mentions them in a symbolic mode, or uses an operator
numeric mode such as @samp{=755}, or sets them in a numeric mode, or
clears them in a numeric mode that has five or more octal digits.
For example, on systems that support
set-group-ID inheritance:

@example
# These commands leave the set-user-ID and
# set-group-ID bits of the subdirectories alone,
# so that they retain their default values.
mkdir A B C
chmod 755 A
chmod 0755 B
chmod u=rwx,go=rx C
mkdir -m 755 D
mkdir -m 0755 E
mkdir -m u=rwx,go=rx F
@end example

If you want to try to set these bits, you must mention them
explicitly in the symbolic or numeric modes, e.g.:

@example
# These commands try to set the set-user-ID
# and set-group-ID bits of the subdirectories.
mkdir G
chmod 6755 G
chmod +6000 G
chmod u=rwx,go=rx,a+s G
mkdir -m 6755 H
mkdir -m +6000 I
mkdir -m u=rwx,go=rx,a+s J
@end example

If you want to try to clear these bits, you must mention them
explicitly in a symbolic mode, or use an operator numeric mode, or
specify a numeric mode with five or more octal digits, e.g.:

@example
# These commands try to clear the set-user-ID
# and set-group-ID bits of the directory D.
chmod a-s D
chmod -6000 D
chmod =755 D
chmod 00755 D
@end example

This behavior is a @acronym{GNU} extension.  Portable scripts should
not rely on requests to set or clear these bits on directories, as
@acronym{POSIX} allows implementations to ignore these requests.
The @acronym{GNU} behavior with numeric modes of four or fewer digits
is intended for scripts portable to systems that preserve these bits;
the behavior with numeric modes of five or more digits is for scripts
portable to systems that do not preserve the bits.