I know what setgid does for directories and binary executable files - there is a very nice article about it in Wikipedia:

http://en.wikipedia.org/wiki/Setgid

But what is the effect of setgid (i.e. chmod g+s) on a text file?

Is this defined at all?

Or this simply have no influence on the file?

Thanks!
Alex

You setgid on a directory so that any new files created by a user under that directory will be assigned the setgid group as the group owner. In other words, which group do you want to have rigths to the file, the default group that the creating user belongs to or the group that the directory belongs to? Do you want only the user to be able to modify this file or the user and another group? setgid is only relevant for directories, think about it.

ramram29, thank you for your quick reply.

I am afraid I don't understand your answer since in my understanding it is self-contradicting: On one hand you say:
On the other hand you say:On the third hand, the documentation clearly says that setgid is relevant not only for directories but also for binary executable files.

So there is a contradiction here and despite the excellent article in Wikipedia (and reading the 'man' page, of course), I haven't been able to figure out what setgid does for a text file.

Any idea?

Thanks,
Alex

This is what I know. Let's say you have a directory called /home/manuscripts owned by the group writers. The default group of the user-account 'johnw' (John Wayne) is actors.

When johnw saves any file it is usually saved with permissions johnw.actors unless there is a sticky bit set on the directory.

We want to setup /home/manuscripts with a stickybit so that johnw can create new files in there and the writers can then write to those files he created and so can he. However, he can only modify the files that he created and not all the other files in there - the writers want to be able to edit johnw files but they don't want him to edit their files. How do you do that?

chgrp writers /home/manuscripts
chmod 02775 /home/manuscripts

When he saves the file it will be owned by him and by the group writers. Now john can read all the files in /home/manuscripts but he can only edit his own files, while the writers group can edit all the files including his files too.

If you have a directory with the suid bit set, files copied into it will have the group ownership changed to the group ownership of the directory itself. If you move or create a file there, it will retain the original ownership.
Note how the suid bit set on the directory doesn't change the ownership. Also, suid isn't supported on scripts in Linux.