bug#29390: chmod man page - clear setuid/setgid with numerical value

[Pádraig Brady]

On 21/11/17 18:46, Oliver Isaac wrote:
> Hello,
> 
> There is an incorrect assertion made in the chmod man page:
> 
> Under the setuid/setgid header it says, "you can set (but not clear) the bits 
> with a numeric mode"
> 
> This is not entirely true. If you prefix your numeric mode with a 0 then it 
> will work:
> 
> e.g.:
> 
> touch myfile
> chmod 644 myfile
> ls -l myfile
> chmod g+s myfile
> Is -l myfile
> chmod 00644 myfile
> is -l myfile
> 
> 
> This simple test shows that you can clear the uid/gid bits with numeric mode 
> which is especially useful in scripts.
> 
> An update to the man page to clarify this would be helpful. 

Yes we should update the man page to be consistent.

The current text was added in COREUTILS-6_9-89-gf4a5097
It was correct at the time, but slightly ambiguous as it
might be missed that the paragraph pertains only to directories,
and not files.

Then http://debbugs.gnu.org/8391 (v8.15-64-g8931cdb) changed things
to allow leading 00 to clear the setuid and setgid bits
of _directories_ with numeric modes.  BTW I notice solaris
accepts 00755 as a mode but does _not_ clear these bits
for directories.

I.E. the most portable and simplest way to access this functionality
is to use `chmod -s dir`

Proposed patch attached.

cheers,
Pádraig

chmod-setgid-man.patch
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