1. Use the groups command to view the group information of the current or specified user, and directly enter groups or groups john; 2. Use the id command to display the user uid, gid and its own group, execute the id username or use the option -Gn/-gn to display the group name only; 3. By viewing the /etc/group file, you can understand all group members, use the cat /etc/group command and find the corresponding user group according to the line format group_name:x:group_id:user1,user2,.... These three methods are suitable for different scenarios. Groups are the most intuitive and id is more detailed. /etc/group is suitable for global viewing of group members.

In Linux system, the method is actually quite straightforward. There are only two or three commonly used commands. The key is to know when to use which one is more appropriate.

1. Use groups command to view the group information of the current user or the specified user

This is the easiest and most straightforward way. If you just want to see which groups the currently logged-in user belongs to, enter directly:

 groups

If you want to check for other users, such as the user name is john , you can do this:

 groups john

The output result will list all groups to which the user belongs, and the order generally does not require special requirements.

Tip: If users belong to many groups on some systems, the difference between the main group and the additional group may occur, but the groups command usually lists them all.

2. Use the id command to view more detailed user identity information

In addition to group information, id can also display user ID (uid), main group ID (gid), etc. To view all relevant information about a user, you can use:

 id username

For example:

 id john

The output might look like this:

 uid=1001(john) gid=1001(john) groups=1001(john),4(adm),27(sudo)

Here you can see that john 's main group is john (gid=1001), which also belongs to adm and sudo groups.

• If you only want to see the group name, you can add option -Gn :

   id -Gn john

• If you only look at the main group name, you can use -gn :

   id -Gn john

3. View /etc/group file to learn about all group members

If you want to see which users are in a group, or manually find which groups a user belongs to, you can directly view the /etc/group file:

 cat /etc/group

The format of each line is roughly like this:

 group_name:x:group_id:user1,user2,...

for example:

 sudo:x:27:john,anna

This means that both john and anna belong to the sudo group.

Note: This file only lists users in the attached group, and the main group will not be reflected here. That is to say, if a user creates a main group with the same name by default, this will not appear in the user list in /etc/group .

Basically these are the methods. It is not complicated, but sometimes it is easy to ignore the difference between the main group and the additional group, or mistakenly think that a user is not in a certain group, but it is actually just not fully displayed. By mastering these commands, you can basically deal with most situations.

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