25.3.1 About umask and the setgid and Restricted Deletion Bits

Users whose primary group is not a UPG have a umask of 0022 set by /etc/profile or /etc/bashrc, which prevents other users, including other members of the primary group, from modifying any file that the user owns.

A user whose primary group is a UPG has a umask of 0002. It is assumed that no other user has the same group.

To grant users in the same group write access to files within the same directory, change the group ownership on the directory to the group, and set the setgid bit on the directory:

# chgrp groupname directory
# chmod g+s directory

Files created in such a directory have their group set to that of the directory rather than the primary group of the user who creates the file.

The restricted deletion bit prevents unprivileged users from removing or renaming a file in the directory unless they own either the file or the directory.

To set the restricted deletion bit on a directory:

# chmod a+t directory

For more information, see the chmod(1) manual page.