[wahern]

I just recalled an interesting difference between BSD and SysV (including Linux[1]) regarding group ownership.

On BSD when you create a file the group owner is set to the group owner of the directory. From the BSD open(2) manual page:

  When a new file is created it is given the group of the
  directory which contains it.

But on Linux the [default] behavior is to set the group owner to the creating process' effective group. From the Linux open(2) man page:

  The  group  ownership  (group ID) is set either to the
  effective group ID of the process or to the group ID of the
  parent  directory  (depending  on file system type and
  mount options, and the mode of the parent directory, see
  the  mount  options  bsdgroups and sysvgroups described in
  mount(8)).

The BSD behavior is more convenient when it comes to using shared supplemental groups because you can often invoke existing programs with a umask of 0002 or 0007 and files they create (without subsequently modifying permissions) will usually have the desirable permissions--specifically, writable by any process which has a supplemental group of the directory they're located in. For example, with BSD semantics Git technically wouldn't have needed a special core.sharedRepository setting. But because of SysV semantics on Linux Git had to be modified to explicitly read the group of the parent directory and explicitly change the group owner of newly created files.