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 DEBIAN
luseradd
luseradd(8) System Manager's Manual luseradd(8)
NAME
luseradd - Add an user
SYNOPSIS
luseradd [OPTION]... user
DESCRIPTION
Adds an user with name user.
OPTIONS
-c, --gecos=gecos
Set the GECOS field to gecos. The GECOS field is traditionally used to store user's real name and other information.
-d, --directory=directory
Set user's home directory to directory. If this option is not present, a default specified by libuser configuration is used.
-g, --gid=group
Set user's primary group to group. group can either be a group ID of an existing group, or a group name; if group is a group name
and the group does not exist, it is created. If the --gid option is not specified, the default group name is user.
-i, --interactive
Ask all questions when connecting to the user database, even if default answers are set up in libuser configuration.
-k, --skeleton=directory
Populate the newly created user's home directory with a copy of directory. If this option is not present, a default directory spec-
ified by libuser configuration, or /etc/skel if libuser configuration does not specify a default, is used.
-M, --nocreatehome
Don't create a home directory and a mail spool for the newly created user.
-n, --nocreategroup
If a group is not specified using -g, use group name "users" instead of user. Note that despite the name of the option, the group
will still be created if it does not exist.
-P, --plainpassword=password
Set user's password to password. Note that the password can be viewed while running luseradd using tools such as ps(1).
-p, --password=encrypted
Set user's password to the password represented by the hash encrypted. Note that the hash can be viewed while running luseradd
using tools such as ps(1).
-r, --reserved
The user is a system user. Users that are not marked as system user usually have automatically selected user IDs above a certain
value (500 by default). This information is also passed to the libuser backends. This option implies the -M option.
-s, --shell=shell
Set user's login shell to shell. If this option is not present, a default specified by libuser configuration is used.
-u, --uid=uid
Use user ID uid for the newly created user. An user ID is selected automatically if this option is not present.
EXIT STATUS
The exit status is 0 on success, nonzero on error.
libuser Jan 12 2005 luseradd(8)