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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CHGRP(1) BSD General Commands Manual CHGRP(1)
NAME
chgrp -- change group
SYNOPSIS
chgrp [-fhv] [-R [-H | -L | -P]] group file ...
DESCRIPTION
The chgrp utility sets the group ID of the file named by each file operand to the group ID specified by the group operand.
The following options are available:
-f The force option ignores errors, except for usage errors and doesn't query about strange modes (unless the user does not have proper
permissions).
-H If the -R option is specified, symbolic links on the command line are followed. (Symbolic links encountered in the tree traversal
are not followed).
-h If the file is a symbolic link, the group ID of the link itself is changed rather than the file that is pointed to.
-L If the -R option is specified, all symbolic links are followed.
-P If the -R option is specified, no symbolic links are followed. This is the default. Use -h to change the group ID of a symbolic
link.
-R Change the group ID for the file hierarchies rooted in the files instead of just the files themselves.
-v Cause chgrp to be verbose, showing files as the group is modified.
The -H, -L and -P options are ignored unless the -R option is specified. In addition, these options override each other and the command's
actions are determined by the last one specified.
The group operand can be either a group name from the group database, or a numeric group ID. If a group name is also a numeric group ID, the
operand is used as a group name.
The user invoking chgrp must belong to the specified group and be the owner of the file, or be the super-user.
DIAGNOSTICS
The chgrp utility exits 0 on success, and >0 if an error occurs.
COMPATIBILITY
In previous versions of this system, symbolic links did not have groups.
The -v option is non-standard and its use in scripts is not recommended.
FILES
/etc/group group ID file
SEE ALSO
chown(2), fts(3), group(5), passwd(5), symlink(7), chown(8)
STANDARDS
The chgrp utility is expected to be IEEE Std 1003.2 (``POSIX.2'') compatible.
BSD
March 31, 1994 BSD