When could or should I use chmod g+s on a file or directory?

Question

In deploying to a new (Solaris 9) environment recently, one of the steps was to copy a set of files and directories to their new location and then to apply the group UID bit (us

Answer 1

For files it means that the file is executed as the group that owns the file, not the group user that executes the file belongs to. It is usable when you want to allow user to do something which for which he does not have the privilege. For example, for one DBMS I use, it is common to allow everybody to backup databases. Although only the 'dbms' group has read/write access to database file, the backup program has g+s set to allow anyone to access the database through it, but not directly.

For directories, it means that newly created directories will be owned by the group that owns the directory, not the group user that created the file belongs to. A good example for this is web space of sourceforge.net project. Imagine 3 developers maintaining the project website. Now, if one of them creates a file, only he can write to it (by default). To work around this, all users on the same project are in the same group as well, and directory has rws privilege for that group, so whoever creates the file, it gets created as readable and writable to the group.