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Full Discussion: Unix File Permissions
Top Forums UNIX for Beginners Questions & Answers Answers to Frequently Asked Questions Tips and Tutorials Unix File Permissions Post 73755 by Perderabo on Friday 3rd of June 2005 07:20:56 PM
Old 06-03-2005
Set GID Bit on Directories

We briefly mentioned that files have a user and group associated with them. Originally, it was just the user and group of whoever created them. But originally, a user could be in only one group at a time. BSD introduced the concept that a user could be in multiple groups simutaneously. So in BSD, which group was used? BSD decided to use the group of the directory that contained the newly created file.

Many modern versions of unix try to have it both ways. A newly created file gets the group of the user unless the directory has the setgid bit. In that case, the newly created file gets the group of the directory.

And there is an exception to that! Changing the owner or group of a file has security concerns. For that reason, some versions of unix will, optionally, prohibit a user other than root from changing the owner of a file. Additionally, a user is prohibited from changing the group of a file unless he is a member of the new group. This restriction will override the setgid bit on a directory if needed.
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CHMOD(1)						      General Commands Manual							  CHMOD(1)

NAME
chmod - change mode SYNOPSIS
chmod mode file ... DESCRIPTION
The mode of each named file is changed according to mode, which may be absolute or symbolic. An absolute mode is an octal number con- structed from the OR of the following modes: 4000 set user ID on execution 2000 set group ID on execution 1000 sticky bit, see chmod(2) 0400 read by owner 0200 write by owner 0100 execute (search in directory) by owner 0070 read, write, execute (search) by group 0007 read, write, execute (search) by others A symbolic mode has the form: [who] op permission [op permission] ... The who part is a combination of the letters u (for user's permissions), g (group) and o (other). The letter a stands for ugo. If who is omitted, the default is a but the setting of the file creation mask (see umask(2)) is taken into account. Op can be + to add permission to the file's mode, - to take away permission and = to assign permission absolutely (all other bits will be reset). Permission is any combination of the letters r (read), w (write), x (execute), s (set owner or group id) and t (save text - sticky). Let- ters u, g or o indicate that permission is to be taken from the current mode. Omitting permission is only useful with = to take away all permissions. The first example denies write permission to others, the second makes a file executable: chmod o-w file chmod +x file Multiple symbolic modes separated by commas may be given. Operations are performed in the order specified. The letter s is only useful with u or g. Only the owner of a file (or the super-user) may change its mode. SEE ALSO
ls(1), chmod(2), chown (1), stat(2), umask(2) CHMOD(1)
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