Learning Linux Users, Groups, and Permissions

This post explains how users, groups, and file permissions work in Linux. It also shows how to manage access, ownership, and security with simple commands like chmod, chown, and usermod.

1. Understanding Users in Linux

Linux has three types of users:

Root (superuser): Can do anything. Used for administration. Be careful when using it.
System users: Used by services like Nginx or MySQL. They don’t log in directly.
Regular users: People like me who use the system normally. Each one has a home directory.

To check users:

whoami     # shows my username
id         # shows UID, GID, and groups

2. User and Group Files

/etc/passwd – list of all users
/etc/shadow – stores encrypted passwords (only root can read)
/etc/group – list of all groups on the system

You can see them with:

cat /etc/passwd
sudo cat /etc/shadow
cat /etc/group

3. Creating a User

Create a user with a home directory and shell:

sudo useradd -m -s /bin/bash john
sudo passwd john

Check the user:

id john
ls /home

4. Modifying Users

Change shell:

sudo usermod -s /bin/zsh john

Add to a group:

sudo usermod -aG docker john

Remember: -aG adds the user to a group, but -G alone replaces all groups.

Lock and unlock users:

sudo usermod -L john    # lock
sudo usermod -U john    # unlock

5. Deleting Users

Remove a user and their home folder:

sudo userdel -r john

Be careful -> -r deletes all files in their home directory.

6. Understanding Groups

Groups help manage access for multiple users. Each user belongs to:

One primary group
Zero or more secondary groups

To check group info:

groups john
id john

7. Managing Groups

Create or remove groups:

sudo groupadd developers
sudo groupdel developers

Add or remove users from groups:

sudo usermod -aG developers john
sudo gpasswd -d john developers

8. Switching Users

Change user in terminal:

su - john

Go back:

exit

9. Using sudo

sudo allows a normal user to run commands as root.

Examples:

sudo apt update
sudo systemctl restart nginx

To get a root shell:

sudo -i

To see what you can run:

sudo -l

10. File Permissions (rwx)

Check file permissions:

ls -l

Example:

-rw-r--r-- 1 reza reza 1234 Oct 19 file.txt

Here:

rw- = owner can read and write
r– = group can read
r– = others can read

11. Changing Permissions with chmod

Letters method:

chmod u+x script.sh    # add execute for user
chmod g-w file.txt     # remove write for group

Numeric Permissions in Linux

We can also set file permissions using numbers instead of letters.

Each number shows what kind of access is allowed:

7 means read, write, and execute (rwx)
6 means read and write (rw-)
5 means read and execute (r-x)
4 means read only (r–)

Examples:

chmod 755 script.sh   # rwxr-xr-x
chmod 644 file.txt    # rw-r--r--
chmod 600 secret.key  # rw-------

12. Changing File Owner with chown

Change file owner and group:

sudo chown john:developers file.txt

For a whole folder:

sudo chown -R john:developers /path/to/folder

13. Common Examples

Create a New User for Development

sudo useradd -m -s /bin/bash -c "John Doe" john
sudo passwd john
sudo usermod -aG sudo john
sudo usermod -aG docker john

Set Up a Project Folder

sudo mkdir /var/www/myproject
sudo chown john:developers /var/www/myproject
sudo chmod 775 /var/www/myproject

14. Safety Tips

Never use dangerous commands like:

sudo chmod 777 /
rm -rf /

Always double-check before deleting or changing ownership.

What I Learned

Each user and group has a unique ID (UID and GID).
Use sudo instead of logging in as root.
Understand r, w, x to manage access correctly.
Use chmod for permissions and chown for ownership.
Always use usermod -aG to add groups safely.