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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LEARN ABOUT NETBSD
newgrp
NEWGRP(1) BSD General Commands Manual NEWGRP(1)
NAME
newgrp -- change to a new primary group
SYNOPSIS
newgrp [-l] [group]
DESCRIPTION
The newgrp command changes a user to a new primary group (real and effective group ID) by starting a new shell. The user remains logged in
and the current directory and file creation mask remain unchanged. The user is always given a new shell even if the primary group change
fails.
The newgrp command accepts the following options:
-l The environment is changed to what would be expected if the user actually logged in again. This simulates a full login.
The group is a group name or non-negative numeric group ID from the group database. The real and effective group IDs are set to group or the
group ID associated with the group name.
If group is not specified, newgrp restores the user's real and effective group IDs to the user's primary group specified in the password
database. The user's supplementary group IDs are restored to the set specified for the user in the group database.
If the user is not a member of the specified group, and the group requires a password, the user will be prompted for the group password.
FILES
/etc/group The group database
/etc/master.passwd The user database
/etc/passwd A Version 7 format password file
EXIT STATUS
If a new shell is started the exit status is the exit status of the shell. Otherwise the exit status will be >0.
SEE ALSO
csh(1), groups(1), login(1), sh(1), su(1), umask(2), group(5), passwd(5), environ(7)
STANDARDS
The newgrp command conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
HISTORY
A newgrp command appeared in Version 6 AT&T UNIX. A newgrp command appeared in NetBSD 5.0.
BUGS
There is no convenient way to enter a password into /etc/group. The use of group passwords is strongly discouraged since they are inherently
insecure. It is not possible to stop users from obtaining the encrypted password from the group database.
BSD
June 6, 2007 BSD