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@c Copyright (C) 1994, 1996, 1999-2001, 2003-2006, 2008-2011 Free Software
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@c Copyright (C) 1994-2012 Free Software Foundation, Inc.
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@c Permission is granted to copy, distribute and/or modify this document
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@c under the terms of the GNU Free Documentation License, Version 1.3 or
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* Mode Structure:: Structure of file mode bits.
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* Symbolic Modes:: Mnemonic representation of file mode bits.
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* Numeric Modes:: File mode bits as octal numbers.
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* Operator Numeric Modes:: ANDing, ORing, and setting modes octally.
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* Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories.
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The combination @samp{o+s} has no effect. On @acronym{GNU} systems
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The combination @samp{o+s} has no effect. On GNU systems
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the combinations @samp{u+t} and @samp{g+t} have no effect, and
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@samp{o+t} acts like plain @samp{+t}.
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number that represents the mode.
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This number is always interpreted in octal; you do not have to add a
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leading @samp{0}, as you do in C. Mode @samp{0055} is the same as
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mode @samp{55}. (However, modes of five digits or more, such as
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@samp{00055}, are sometimes special. @xref{Directory Setuid and Setgid}.)
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A numeric mode is usually shorter than the corresponding symbolic
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mode, but it is limited in that normally it cannot take into account the
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previous file mode bits; it can only set them absolutely.
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(As discussed in the next section, the set-user-ID and set-group-ID
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bits of directories are an exception to this general limitation.)
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The set-user-ID and set-group-ID bits of directories are an exception
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to this general limitation. @xref{Directory Setuid and Setgid}.
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Also, operator numeric modes can take previous file mode bits into
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account. @xref{Operator Numeric Modes}.
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The permissions granted to the user,
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to other users in the file's group,
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@samp{ug=rw,o=r}. Numeric mode @samp{0} corresponds to symbolic mode
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@node Operator Numeric Modes
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@section Operator Numeric Modes
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An operator numeric mode is a numeric mode that is prefixed by a
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@samp{-}, @samp{+}, or @samp{=} operator, which has the same
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interpretation as in symbolic modes. For example, @samp{+440} enables
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read permission for the file's owner and group, @samp{-1} disables
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execute permission for other users, and @samp{=600} clears all
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permissions except for enabling read-write permissions for the file's
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owner. Operator numeric modes can be combined with symbolic modes by
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separating them with a comma; for example, @samp{=0,u+r} clears all
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permissions except for enabling read permission for the file's owner.
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The commands @samp{chmod =755 @var{dir}} and @samp{chmod 755
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@var{dir}} differ in that the former clears the directory @var{dir}'s
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setuid and setgid bits, whereas the latter preserves them.
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@xref{Directory Setuid and Setgid}.
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Operator numeric modes are a GNU extension.
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@node Directory Setuid and Setgid
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@section Directories and the Set-User-ID and Set-Group-ID Bits
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mechanisms would be less convenient and it would be harder to share
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files. Therefore, a command like @command{chmod} does not affect the
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set-user-ID or set-group-ID bits of a directory unless the user
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specifically mentions them in a symbolic mode, or sets them in
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a numeric mode. For example, on systems that support
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specifically mentions them in a symbolic mode, or uses an operator
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numeric mode such as @samp{=755}, or sets them in a numeric mode, or
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clears them in a numeric mode that has five or more octal digits.
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For example, on systems that support
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set-group-ID inheritance:
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# These commands try to set the set-user-ID
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# and set-group-ID bits of the subdirectories.
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chmod u=rwx,go=rx,a+s H
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chmod u=rwx,go=rx,a+s G
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mkdir -m u=rwx,go=rx,a+s J
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If you want to try to clear these bits, you must mention them
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explicitly in a symbolic mode, e.g.:
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explicitly in a symbolic mode, or use an operator numeric mode, or
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specify a numeric mode with five or more octal digits, e.g.:
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# This command tries to clear the set-user-ID
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# These commands try to clear the set-user-ID
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# and set-group-ID bits of the directory D.
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This behavior is a @acronym{GNU} extension. Portable scripts should
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This behavior is a GNU extension. Portable scripts should
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not rely on requests to set or clear these bits on directories, as
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@acronym{POSIX} allows implementations to ignore these requests.
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POSIX allows implementations to ignore these requests.
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The GNU behavior with numeric modes of four or fewer digits
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is intended for scripts portable to systems that preserve these bits;
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the behavior with numeric modes of five or more digits is for scripts
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portable to systems that do not preserve the bits.
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doc/perm.texi
