SGID (set group ID) on a directory: in this special case every file created in the directory will have the same group owner as the directory itself (while normal behavior would be that new files are owned by the users who create them). This way, users don't need to worry about file ownership when sharing directories:
mimi:~> ls -ld /opt/docs
drwxrws--- 4 root users 4096 Jul 25 2001 docs/
mimi:~> ls -l /opt/docs
-rw-rw---- 1 mimi users 345672 Aug 30 2001-Council.doc
This is the standard way of sharing files in UNIX.
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http://tldp.org/LDP/intro-linux/html/sect_03_04.html
