If you work on a UNIX system with lots of users, you may be taking
advantage of UNIX
group permissions (
to let users in one group write to files in a directory - but not let
people in other groups write there.
How does UNIX determine what group should own
the files you create?
There are three ways:
On most System V-based systems, the
effective group ID of the process
determines the ownership of the files you create.
(Your effective GID is your
primary group membership
unless you're running a
On most BSD UNIXes, files are owned by
the group that
owns the directory in which you create the file
The rules under SunOS 4.
and System V Release 4 are more complicated.
The system administrator decides which of the two above methods a filesystem
will use for group ownership.
There are other wrinkles, too.
A good place to look for the gory details
is your system's
but it's probably easier to just
create an empty new file (
and then check the group ownership with
You may be able to use the
set group ID
to control group ownership.
In those cases, if the bit is set, the BSD rules apply.
if the bit is not set, the System V rules apply.
To set and remove the setgid bit, use the commands
You can use the
command to change a file's group.
However, you must own the file. And you must also be a member of the
file's new group.
If you've reset directory mode bits, it's possible to
wind up with
permissions that have an uppercase "S",
(It's often a mistake.)
The directory's setgid bit is set, but the execute bit isn't set.
If you want the directory to be group-accessible,
add execute permission with
Otherwise, you may want to clear the setgid bit with