Mastering Linux User Accounts, Groups, and File Permissions

Proper user and permission management is a crucial part of Linux system security and organization. This is the second part of the Linux series. We'll explore user accounts, groups, and file permissions in a detailed way. You'll learn how to create and manage users, assign groups, and control file access with permissions.

Summary Table

Command	Description
`useradd <user>`	"Create a new user account."
`passwd <user>`	Set or change a user's password
`getent passwd`	Display all user entries.
`cat /etc/passwd`	Show all user entries (file view)
`id <user>`	Show UID, GID, and groups for a user
`su - <user>`	Switch to another user (login shell)
`sudo <command>`	Run command as root (if configured)
`userdel <user>`	Delete a user account
`groupadd <group>`	Create a new group
`getent group`	Show all groups
`cat /etc/group`	Show groups (file view)
`groupdel <group>`	Delete a group
`usermod -aG <group> <user>`	Add a user to a group
`chown <user> <file>`	Change file owner
`chown <user> <file1> <file2>`	Change owner of multiple files
`chown <user> *`	Change owner of all files in directory
`chgrp <group> <file>`	Change group ownership of file
`chgrp <group> *`	Change group for all files
`chown <user>:<group> <file>`	Change owner and group for file
`chown <user>:<group> *`	Change owner and group for all files
`chown -R <user>:<group> <folder>`	Recursively change owner/group of folder and contents
`chmod 777 <file>`	Give read/write/execute to all
`chmod 751 <file>`	Set specific permissions (rwxr-x--x)
`chmod -R 777 <folder>`	Recursively set all permissions for folder

Linux User, Group, and Permission Commands

1. User Management Commands

useradd <user>

# Create a new user
sudo useradd john

# Create user with home directory and default shell
sudo useradd -m -s /bin/bash john

Creates a new user account.

-m: Create home directory
-s: Specify login shell

passwd <user>
```
sudo passwd john
```
Sets or changes the password of a user. The root user can change any user's password.

Sample output:
```
New password:
Retype new password:
passwd: password updated successfully
    
```
getent passwd / cat /etc/passwd
```
getent passwd
```
Shows all user account entries from the system’s account database.

Sample output:
```
root:x:0:0:root:/root:/bin/bash
alice:x:1001:1001:Alice:/home/alice:/bin/bash
    
```
id <user>
```
id john
```
Shows the UID, GID, and group memberships of the specified user.

Sample output:
```
uid=1001(john) gid=1001(john) groups=1001(john),27(sudo)
    
```
su - <user>
```
su - john
```
Switches to another user account and loads that user's environment.

sudo <command>

sudo apt update

Runs a command with elevated privileges if the current user has sudo access.

Sample output:

Hit:1 http://archive.ubuntu.com/ubuntu focal InRelease
Reading package lists... Done

userdel <user>
```
sudo userdel john
```
Deletes a user account from the system.

2. Group Management Commands

groupadd <group>
```
sudo groupadd developers
```
Creates a new group on the system.
getent group / cat /etc/group
```
getent group
```
Displays all groups in the system’s account database.

Sample output:
```
root:x:0:
developers:x:1002:alice
    
```
groupdel <group>
```
sudo groupdel developers
```
Deletes a group from the system.
usermod -aG <group> <user>
```
sudo usermod -aG developers john
```
Adds a user to a supplementary group without removing them from other groups.
- -a: Append to existing groups
- -G: Specify supplementary groups

3. File Ownership Commands

chown <user> <file>
```
sudo chown john file.txt
```
Changes the ownership of a file to the specified user.
chown <user> <file1> <file2>
```
sudo chown john file1.txt file2.txt
```
Changes the ownership of multiple files at once.
chown <user> *
```
sudo chown john *
```
Changes the ownership of all files in the current directory to the specified user.
chgrp <group> <file>
```
sudo chgrp developers file.txt
```
Changes the group ownership of a file.
chgrp <group> *
```
sudo chgrp developers *
```
Changes the group ownership of all files in the current directory.
chown <user>:<group> <file>
```
sudo chown john:developers file.txt
```
Changes both the owner and group of a file.
chown <user>:<group> *
```
sudo chown john:developers *
```
Changes both the owner and group for all files in the current directory.
chown -R <user>:<group> <folder>
```
sudo chown -R john:developers myfolder
```
Recursively changes the owner and group of a folder and all its contents.
- -R: Apply changes recursively

4. File Permission Commands

chmod 777 <file>
```
chmod 777 file.txt
```
Gives read, write, and execute permissions to all users for the file.
chmod 751 <file>
```
chmod 751 file.txt
```
Sets the file permissions to:
- rwx for owner (7)
- r-x for group (5)
- --x for others (1)
chmod -R 777 <folder>
```
chmod -R 777 myfolder
```
Recursively sets full read, write, and execute permissions for all files and subdirectories inside a folder.
- -R: Apply changes recursively