Create, delete, and modify local groups and group memberships

Group administration

– Use the groupadd command to add a new group :
# groupadd [options] group_name
– Use the groupmod command to modify an existing group :
# groupmod [options] group_name
– Use groupdel to delete the group. You can remove a group even if there are users in the group. But you can not remove the primary group of an existing user. You must remove the user before removing the group.
# groupdel group_name
– Use the gpasswd command to administer the groups :
# gpasswd [options] group_name
For example : to add user test in group student –
# gpasswd -a test student

groups command

The groups command displays the group the user belongs to. For example the user oracle as shown below belongs to multiple groups which can be displayed using the groups command :
# groups oracle
oracle : oinstall dba asm asmdba oper
# grep oracle /etc/group
oinstall:x:5004:oracle
dba:x:5005:oracle
asm:x:5006:oracle
asmdba:x:5007:oracle
oper:x:5008:oracle

newgrp command

The newgroup command executes a new shell and changes a user’s real group information. For example,
Before executing newgrp command
$ id
uid=5004(oracle) gid=5004(oinstall) groups=5004(oinstall),5005(dba) ...
$ ps
   PID TTY          TIME CMD
106591 pts/0    00:00:00 bash
106672 pts/0    00:00:00 ps
After executing newgrp command
$ newgrp dba
Note the gid for the user has changed to that of the student group :
$ id
uid=5004(oracle) gid=5005(dba) groups=5005(dba),5004(oinstall) ...
Also note that a new shell has been executed.
$ ps
   PID TTY          TIME CMD
106591 pts/0    00:00:00 bash
106231 pts/0    00:00:00 bash
106672 pts/0    00:00:00 ps


Note : you can only change your real group name to a group that you are member of.

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