On Tue, Apr 22, 2008 at 07:59:07PM +0400, sergio wrote:
% mkdir test
% ls -ld test
drwxr-xr-x 2 sergio sergio 48 Апр 22 19:44 test/
% chmod +s test
% ls -ld test
drwsr-sr-x 2 sergio sergio 48 Апр 22 19:44 test/
% chmod 0755 test
% ls -ld test
drwsr-sr-x 2 sergio sergio 48 Апр 22 19:44 test/
% chmod -s test
% ls -ld test
drwxr-xr-x 2 sergio sergio 48 Апр 22 19:44 test/
Well, it's documented that way (man page says):
chmod preserves a directory’s set-user-ID and set-group-ID bits unless
you explicitly specify otherwise. You can set or clear the bits with
symbolic modes like u+s and g-s, and you can set (but not clear) the
bits with a numeric mode.
This was a change in 6.0 (from NEWS):
chmod, install, and mkdir now preserve a directory's set-user-ID and
set-group-ID bits unless you explicitly request otherwise. E.g.,
`chmod 755 DIR' and `chmod u=rwx,go=rx DIR' now preserve DIR's
set-user-ID and set-group-ID bits instead of clearing them, and
similarly for `mkdir -m 755 DIR' and `mkdir -m u=rwx,go=rx DIR'. To
clear the bits, mention them explicitly in a symbolic mode, e.g.,
`mkdir -m u=rwx,go=rx,-s DIR'. To set them, mention them explicitly
in either a symbolic or a numeric mode, e.g., `mkdir -m 2755 DIR',
`mkdir -m u=rwx,go=rx,g+s' DIR. This change is for convenience on
systems where these bits inherit from parents. Unfortunately other
operating systems are not consistent here, and portable scripts
cannot assume the bits are set, cleared, or preserved, even when the
bits are explicitly mentioned. For example, OpenBSD 3.9 `mkdir -m
777 D' preserves D's setgid bit but `chmod 777 D' clears it.
Conversely, Solaris 10 `mkdir -m 777 D', `mkdir -m g-s D', and
`chmod 0777 D' all preserve D's setgid bit, and you must use
something like `chmod g-s D' to clear it.
I'm not sure how this isn't a POSIX violation, though:
For an octal integer mode operand, the file mode bits shall be set absolutely.
For each bit set in the octal number, the corresponding file permission bit
shown in the following table shall be set; all other file permission bits shall
be cleared. For regular files, for each bit set in the octal number
corresponding to the set-user-ID-on-execution or the set-group-ID-on-execution,
bits shown in the following table shall be set; if these bits are not set in
the octal number, they are cleared. For other file types, it is
implementation-defined whether or not requests to set or clear the
set-user-ID-on-execution or set-group-ID-on-execution bits are honored.
