Implementation of effective user groups and initial user groups in Linux

First check the /etc/group file:

[root@localhost /]# cat /etc/group
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
tty:x:5:
disk:x:6:root
lp:x:7:daemon,lp
mem:x:8:
kmem:x:9:
wheel:x:10:root
...

Each line of this file represents a user group, with colons separating the fields. There are 4 fields in total, namely:

• ==User Group Name==
• ==User Group Password==: Usually no setting is required. This setting is usually used for "User Group Administrators".
• ==GID==: User group ID
• ==Account names supported by this user group==: An account can join multiple user groups. If a user wants to join a user group, just add the user's username into this field.

Initial User Group

The fourth field (GID) of the line corresponding to the user in /etc/passwd is the initial user group. When a user logs into the system, he or she immediately has the relevant permissions of this initial user group.

[root@localhost /]# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
...

Valid User Groups

The effective user group indicates the user group that the user belongs to at this moment. Before the effective user group is switched, the effective user group is the initial user group.

If you want to know the user's current effective user group and supported user groups (the fourth field in the /etc/group file contains the user's user group), use groups.

[root@localhost /]# groups
root bin daemon sys adm disk wheel

In this output information, the first user group output is the current effective user group (but some systems do not do this).

If I create a new file with touch, the owner of the file is root, and the user group is root (the effective user group).

Switch effective user group: newgrp

Use the newgrp command to switch the effective user group. Note that you can only switch the user groups supported by the current account.

Assume that the current user supports the two user groups "test" and "foo". Then newgrp test and newgrp foo are valid. Everything else is invalid.

The principle behind this conversion is to create a new shell. If you want to restore to the previous user group, you need to use exit or Ctrl+D to log out of the current shell.

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