Creating new files with group id of dir

How do you create a file in a directory in such
a way that the file gets the same group id as the

(hal:~/tmp): ls -la
total 2
drwxrwxr-x   2 mag      foobar       1024 Feb 10 14:22 .
drwxrwxr-x   3 mag      mag          1024 Feb 10 14:22 ..

(hal:~/tmp): touch foo
(hal:~/tmp): ls -la
total 2
drwxrwxr-x   2 mag      foobar       1024 Feb 10 14:22 .
drwxrwxr-x   3 mag      mag          1024 Feb 10 14:22 ..
-rw-rw-r--   1 mag      mag             0 Feb 10 14:22 foo

This is what happens, but I want the group to be "foobar"
not "mag".

That is, I want:
(hal:~/tmp): ls -la
total 2
drwxrwxr-x   2 mag      foobar       1024 Feb 10 14:22 .
drwxrwxr-x   3 mag      mag          1024 Feb 10 14:22 ..
-rw-rw-r--   1 mag      foobar          0 Feb 10 14:22 foo

So how do I get this to happen WITHOUT type chgrp after the file has been created?

Thank You!
Who is Participating?
dhmConnect With a Mentor Commented:
On many versions of unix, if the directory has the setgid bit set, then any files created in the directory will be created with the group ID set the same as the directory's group.  For example, here's an excerpt from a session on a Solaris-2.5.1 machine:

dhm@sweet[71]$ ls -l
total 16
drwxrwxr-x   2 dhm      eng            69 Feb 10 16:25 dir1/
drwxrwsr-x   2 dhm      sys            69 Feb 10 16:25 dir2/

[Note that dir2 has the setgid bit set, and that its group is sys, rather than eng (my default group)]

dhm@sweet[72]$ touch dir1/foo
dhm@sweet[73]$ touch dir2/foo
dhm@sweet[74]$ ls -l dir1
total 0
-rw-rw-r--   1 dhm      eng             0 Feb 10 16:26 foo
dhm@sweet[75]$ ls -l dir2
total 0
-rw-rw-r--   1 dhm      sys             0 Feb 10 16:26 foo

The file created in dir1 has my default group, but the file created in dir2 has the same group as the directory.  This works even if the group of dir2 is one that the user has no connection with whatsoever (but, of course, the directory must be world-writable for the user to be able to create a file there).
You either have to ask your admin to change your default group
ID to be foobar in the /etc/passwd file or just create a script
touch $1
chgrp foobar $1

So if you named that script "create_foobar" by running
create_foobar foo, it will create a file called foo with the group id of foobar.

Minh Lai
mag062397Author Commented:
Okay, sorry, I should be more clear.

1) I don't want to change the default group.  I want someone to be in several groups and still be able to create files with the current directories group id.

2) The purpose of "touch" was just to make the example simple. I want group id to be set correctly for any file that gets created in the directory.

Is there no way to do this?

dhm's solution works on Linux, IRIX, HP-UX and AIX too.
Keep in mind that the s-bit for the group must be set.
mag062397Author Commented:
Thank you.  I'm using Linux and Solaris 2.5.1 too.  It
worked perfect on both.  It's nice to finally know how
to do this--something I should have asked along time ago.
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