groups

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

Users can be listed in groups. Groups allow you to set permissions on the group level instead of having to set permissions for every individual user.

Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users are advised to use this graphical tool. More experienced users can use command line tools to manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant files directly with vi or vigr.

groupadd

Groups can be created with the groupadd command. The example below shows the creation of five (empty) groups.

root@linux:~# groupadd tennis
root@linux:~# groupadd football
root@linux:~# groupadd snooker
root@linux:~# groupadd formula1
root@linux:~# groupadd salsa

group file

Users can be a member of several groups. Group membership is defined by the /etc/group file.

root@linux:~# tail -5 /etc/group
tennis:x:1006:
football:x:1007:
snooker:x:1008:
formula1:x:1009:
salsa:x:1010:
root@linux:~#

The first field is the group's name. The second field is the group's (encrypted) password (can be empty). The third field is the group identification or GID. The fourth field is the list of members, these groups have no members.

groups

A user can type the groups command to see a list of groups where the user belongs to.

[harry@linux ~]$ groups
harry sports
[harry@linux ~]$

usermod

Group membership can be modified with the useradd or usermod command.

root@linux:~# usermod -a -G tennis inge
root@linux:~# usermod -a -G tennis katrien
root@linux:~# usermod -a -G salsa katrien
root@linux:~# usermod -a -G snooker sandra
root@linux:~# usermod -a -G formula1 annelies
root@linux:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
snooker:x:1008:sandra
formula1:x:1009:annelies
salsa:x:1010:katrien
root@linux:~#

Be careful when using usermod to add users to groups. By default, the usermod command will remove the user from every group of which he is a member if the group is not listed in the command! Using the -a (append) switch prevents this behaviour.

groupmod

You can change the group name with the groupmod command.

root@linux:~# groupmod -n darts snooker 
root@linux:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
formula1:x:1009:annelies
salsa:x:1010:katrien
darts:x:1008:sandra

groupdel

You can permanently remove a group with the groupdel command.

root@linux:~# groupdel tennis
root@linux:~#

gpasswd

You can delegate control of group membership to another user with the gpasswd command. In the example below we delegate permissions to add and remove group members to serena for the sports group. Then we su to serena and add harry to the sports group.

[root@linux ~]# gpasswd -A serena sports
[root@linux ~]# su - serena
[serena@linux ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry)
[serena@linux ~]$ gpasswd -a harry sports
Adding user harry to group sports
[serena@linux ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry),522(sports)
[serena@linux ~]$ tail -1 /etc/group
sports:x:522:serena,venus,harry
[serena@linux ~]$

Group administrators do not have to be a member of the group. They can remove themselves from a group, but this does not influence their ability to add or remove members.

[serena@linux ~]$ gpasswd -d serena sports
Removing user serena from group sports
[serena@linux ~]$ exit

Information about group administrators is kept in the /etc/gshadow file.

[root@linux ~]# tail -1 /etc/gshadow
sports:!:serena:venus,harry
[root@linux ~]#

To remove all group administrators from a group, use the gpasswd command to set an empty administrators list.

[root@linux ~]# gpasswd -A "" sports

newgrp

You can start a child shell with a new temporary primary group using the newgrp command.

root@linux:~# mkdir prigroup
root@linux:~# cd prigroup/
root@linux:~/prigroup# touch standard.txt
root@linux:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt
root@linux:~/prigroup# echo $SHLVL
1
root@linux:~/prigroup# newgrp tennis
root@linux:~/prigroup# echo $SHLVL
2
root@linux:~/prigroup# touch newgrp.txt
root@linux:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root tennis 0 Apr 13 17:49 newgrp.txt
-rw-r--r--. 1 root root   0 Apr 13 17:49 standard.txt
root@linux:~/prigroup# exit
exit
root@linux:~/prigroup#

vigr

Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since it will do proper locking of the file. Only experienced senior administrators should use vi or vigr to manage groups.

practice: groups

1. Create the groups tennis, football and sports.

2. In one command, make venus a member of tennis and sports.

3. Rename the football group to foot.

4. Use vi to add serena to the tennis group.

5. Use the id command to verify that serena is a member of tennis.

6. Make someone responsible for managing group membership of foot and sports. Test that it works.

solution: groups