Linux – the difference between -g and -G options in useradd

linuxpermissionsuseradd

After I look through the help. I didn't found much difference between them.

-g, –gid GROUP

The group name or number of the user's initial login group. The group
name must exist. A group number must refer to an already existing
group.

-G, –groups GROUP1[,GROUP2,…[,GROUPN]]]

A list of supplementary groups which the user is also a member of.
Each group is separated from the next by a comma, with no intervening
whitespace. The groups are subject to the same restrictions as the
group given with the -g option. The default is for the user to belong
only to the initial group.

If they are the same. Why both they exist?

Best Answer

-g sets the initial, or primary, group. This is what appears in the group field in /etc/passwd. On many distributions the primary group name is the same as the user name.

-G sets the supplementary, or additional, groups. These are the groups in /etc/group that list your user account. This might include groups such as sudo, staff, etc.

Related Solutions

How is file ownership affected across different systems

On all native unix filesystems, file ownership is stored in the form of user and group IDs. This is also the case for basic NFS operation (although there are other possibilities at least in NFSv4) and for traditional unix archive formats such as tar.

A file can in fact belong to a user or group that doesn't exist. The file belongs to a particular ID, but there's no obligation that the ID is listed in /etc/passwd or other user or group database. For example, if you store your user database on NIS or LDAP, and the database server is temporarily inaccessible, the users' files still exist.

When you carry removable media from one system to another, you should either ensure that user and group IDs match where relevant, or ignore ownership (e.g. by using a non-unix filesystem). Root is a bit of a special case because its defining characteristic is that the user ID is 0, everywhere.

The only way to control what someone in physical possession of the media will do with the data is to use an intrinsic means of protection, i.e., one that is not dependent on how they access the system, and keep something to yourself. You can use a mathematical means of protection: cryptography (encryption for confidentiality, signing for integrity; you keep the password to yourself), or a physical means of protection (e.g. a locked box).

Linux Permissions – Group Within Group File Permissions Explained

There is no such thing as a group being a member of a group. A group, by definition, has a set of user members. I've never heard of a feature that would let you specify “subgroups” where members of subgroups are automatically granted membership into the supergroup on login. If /etc/group lists group1 as a member of group2, it designates the user called group1 (if such a user exists, which is possible: user names and group names live in different name spaces).

If you want user1 to have access to file2, you have several solutions:

Make file2 world-accessible (you probably don't want this)
Make user1 the owner of file2: chown user1 file2
Add user1 to group2: adduser user1 group2
Add an ACL to file2 that grants access to either user1 or group`:
```
setfacl -m user:user1:rw file2
setfacl -m group:group1:rw file2
```
See Make all new files in a directory accessible to a group on enabling ACLs.

Best Answer

Related Solutions

How is file ownership affected across different systems

Linux Permissions – Group Within Group File Permissions Explained

Related Question