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 chmod or
chown to share new files.
These convenience mechanisms rely on the set-user-ID and set-group-ID
bits of directories. If commands like chmod and
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 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 ‘=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:
# 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
If you want to try to set these bits, you must mention them explicitly in the symbolic or numeric modes, e.g.:
# 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
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.:
# 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
This behavior is a GNU extension. Portable scripts should not rely on requests to set or clear these bits on directories, as POSIX allows implementations to ignore these requests. The 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.