Users, Groups and Permissions in Linux

By Diwanshu Shekhar

October 25, 2015 - 2 minutes read - 332 words

Users

Add a user:
```
sudo adduser [username]
```
Delete a user:
```
sudo userdel [username]
```

All user information is stored in a file /etc/passwd

Change a password:
```
sudo passwd [username]
```

Groups

Groups are usually used to change permissions for a group of users as changing permissions individually would be a tedious task.

Add a group:
```
sudo groupadd [groupname]
```
Delete a group:
```
sudo groupdel [groupname]
```
Add a user to a group:
```
sudo adduser [username] [groupname]
```
Delete a user from a group:
```
sudo deluser [username] [groupname]
```

All group information is stored in a file /etc/group

Permissions

Change permission:
```
sudo chmod [three digit number] [file/folder] -R (for recursive in case of folder)
```
Now, how to decide the right three digit number:

4 = read

2 = write

1 = execute

Each digit in the three digit number is the combination of 4, 2 and 1.

Each digit in the three digit number is applicable to user, group and the world respectively.

For example, the three digit number 751 will give: read, write and execute permission to the user; read and execute permission to the group; and just execute permission to the world

Note: You can only traverse a directory if you have execute permission on it. For example: to read or write a file at dir/subdir/fileName, you need to have execute permission on the parent directories dir and subdir.

Ownerships

Change user ownership:

sudo chown -R(for recursive) [username]  [file or folder name]

Change group ownership

sudo chgrp -R(for recursive) [username] [file or folder name]

Add an Existing User To A Group
```
usermod -a -G examplegroup exampleusername
```
For example: to add a user named “user1” to the sudo group -
```
usermod -a -G sudo user1
```

In RedHat Linux, to give sudo access to a user, the best practice is to add the user to the wheel group which has the following setting in the /etc/sudoers file which you can open using sudo visudo:

%wheel  ALL=(ALL)       ALL
View User's Group Information
id username

View User’s Group Information

id username

Miscellaneous