RHCSA Rapid Track

groupadd creates groups

• groupadd groupname without options uses the next available GID from the range specified in the /etc/login.defs file.

• The -g GID option is used to specify a specific GID.

  [student@serverX ~]$ sudo groupadd -g 5000 ateam

  Note

  Given the automatic creation of user private groups (GID 1000+), it is generally recommended to set aside a range of GID numbers to be used for supplementary groups. A higher range will avoid a collision with a system group (GID 0-999).

• The -r option will create a system group using a GID from the range of valid system GID numbers listed in the /etc/login.defs file.

  [student@serverX ~]$ sudo groupadd -r appusers

groupmod modifies existing groups

• The groupmod command is used to change a group name to a GID mapping. The -n option is used to specify a new name.

  [student@serverX ~]$ sudo groupmod -n javaapp appusers

• The -g option is used to specify a new GID.

  [student@serverX ~]$ sudo groupmod -g 6000 ateam

groupdel deletes a group

• The groupdel command will remove a group.

  [student@serverX ~]$ sudo groupdel javaapp

• A group may not be removed if it is the primary group of any existing user. As with userdel, check all file systems to ensure that no files remain owned by the group.

usermod alters group membership

• The membership of a group is controlled with user management. Change a user's primary group with usermod -g groupname.

  [student@serverX ~]$ sudo usermod -g student student

• Add a user to a supplementary group with usermod -aG groupname username.

  [student@serverX ~]$ sudo usermod -aG wheel elvis

  Important

  The use of the -a option makes usermod function in "append" mode. Without it, the user would be removed from all other supplementary groups.