How to check the Linux group list and about /etc/group file

What kind of group is in this Linux OS?

You may want to see a list of groups. This can be resolved by checking the /etc/group file.

Last Update : December 18, 2018

How to check the Linux group list and about /etc/group file Contents

How to read the /etc/group file
Log in to a group
/etc/group and /etc/gshadow
Output group list from /etc/group file

1. How to read the /etc/group file

Group information of Linux OS can be checked in the /etc/group file. In the /etc/group file, group name, group password, group ID, and account list are described. The contents of /etc/group are like this.

[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

bar_group:x:501:foo2,foo3

bar_group	group name
x	The letter "x" or the encrypted password. "x" means shadow password is used.
501	Group ID ( GID )
foo2,foo3	A list of user accounts that belong to "bar_group" as subgroups. Comma separated.

Notes

Users can belong to more than one group.

The basic group of the user is called the primary group or the initial group, and the group belonging to others is called the sub group.

When a user creates a file, the user's primary group is set as a group of files.

2. Log in to a group

It is possible to log in to the specified group by using the newgrp command.

Usage

newgrp　group_name

When you log in to a group, the primary group of the logged in account is changed to the logged in group.

If you create a file, directory, etc., it will be that of the logged-in group.

[foo@localhost ~]$ id -a ← check the id
uid=500(foo) gid=501(foo) groups=501(foo),503(subgroup)
↑ Primary group is "foo"

[foo@localhost ~]$ newgrp subgroup ← Login to 'subgroup' group
[foo@localhost ~]$ id -a
uid=500(foo) gid=503(subgroup) groups=501(foo),503(subgroup)
↑ Primary group changed to "subgroup".

[foo@localhost ~]$ touch test.txt
[foo@localhost ~]$ ls -l test.txt
-rw-r--r--  1 foo subgroup 0 Oct 27 00:53 test.txt
↑ The group of the created file becomes "subgroup".

[foo@localhost ~]$ exit ← Log out from "subgroup" group
exit
[foo@kazmax ~]$ id -a
uid=500(foo) gid=501(foo) groups=501(foo),503(subgroup)
↑ Primary group returns to "foo" group.

Notes

To set the group password, use the gpasswd command.

Usage

gpasswd group_name

3. /etc/group and /etc/gshadow

It is the same as the relationship between /etc/passwd and /etc/shadow.

The /etc/group file is readable by all users. For this reason, encrypted passwords can also be seen by ordinary users, so it seems that they can analyze passwords.

In environments where shadow passwords are used, the group password is encrypted and stored in /etc/gshadow. This file is a file that can only be read by root. Security is improved because general users can not read.

[foo@localhost ~]$ ls -l /etc/gshadow
-r--------  1 root root 606  Dec  1 00:23 /etc/gshadow← Only root can read

4. Output group list from /etc/group file

Output the user list from /etc/group file.

[foo@localhost ~]$ cut -d: -f1 /etc/group
root
bin
daemon
sys
adm
tty

Notes

It is convenient to register with alias.

Add the following to ".bashrc" in the home directory so that it can be used when logging in.

[foo@localhost ~]$ cd
[foo@localhost ~]$ vi .bashrc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
alias listgroups="cut -d: -f1 /etc/group" ← Append

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[foo@localhost ~]$ source .bashrc
[foo@localhost ~]$ listgroups
root
bin
daemon
sys
adm
tty

Notes

You can also output a list of groups even if you specify the option "group" for the "getent" command. Also, it will display the group registered in ldap.

# getent 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