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

Setting directories g+s makes all new files created in said directory have their group set to the directory's group.

This can actually be really handy for collaborative purposes if you have the umask set so that files have group write by default.

Note: This is the way it works in Linux, it could work completely differently in Solaris.