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User Accounts: Who Can Access What

User accounts control access. Who can log in. What they can do. Where they can go.

Here's the thing: User management is essential. You'll create users. You'll manage them. You'll control access.

The Basic Concept​

Every process runs as a user. Every file has an owner. Users control access.

Think of it like a building: Each person has an ID. Different access levels. Their actions are tracked.

Viewing Users​

Current User​

whoami                            # Current username
id                                # Your IDs and groups

Real example:

$ whoami
ubuntu
$ id
uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu),27(sudo)

User Information​

cat /etc/passwd                   # All users
getent passwd username            # Specific user

The /etc/passwd file: User database.

Format: username:x:UID:GID:comment:home:shell

Example: ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash

What matters:

  • username = Login name
  • UID = User ID (0 = root, 1000+ = regular)
  • GID = Primary group ID
  • home = Home directory
  • shell = Default shell

My take: UID 0 is root. UID 1000+ are regular users. That's what you need to know.

Creating Users​

useradd: Add User​

sudo useradd username             # Basic (no home directory)
sudo useradd -m username         # With home directory
sudo useradd -m -s /bin/bash username  # With home and shell

Common options:

  • -m = Create home directory
  • -s = Set shell
  • -c = Comment (full name)
  • -G = Secondary groups
  • -u = User ID

Real example:

sudo useradd -m -s /bin/bash -c "John Doe" -G sudo,developers john

My take: Use -m to create home directory. Users need homes.

Setting Password​

sudo passwd username             # Set password

Real example:

sudo passwd john

My take: Set passwords after creating users. Or use SSH keys. Passwords are less secure.

Modifying Users​

usermod: Modify User​

sudo usermod -s /bin/zsh username        # Change shell
sudo usermod -G group1,group2 username   # Change groups
sudo usermod -L username                  # Lock account
sudo usermod -U username                  # Unlock account

My take: usermod changes user properties. Use it to modify accounts.

Deleting Users​

userdel: Delete User​

sudo userdel username             # Delete user (keeps home)
sudo userdel -r username          # Delete with home directory

The -r flag: Removes home directory too.

My take: Use -r to remove everything. Or keep home for backups. Your choice.

Groups​

Viewing Groups​

groups                            # Your groups
groups username                    # Another user's groups
getent group groupname            # Group details
cat /etc/group                    # All groups

Creating Groups​

sudo groupadd groupname           # Create group

My take: Groups organize users. Share permissions. Useful for shared access.

Adding Users to Groups​

sudo usermod -aG groupname username  # Add to group

The -aG flags: Append to groups (doesn't remove existing).

My take: Use -aG to add to groups. Without -a, it replaces groups.

Common Patterns​

Create User with Home​

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

Add User to sudo Group​

sudo usermod -aG sudo username

My take: Sudo group gives admin access. Add users who need it.

Lock Account​

sudo usermod -L username          # Lock
sudo usermod -U username          # Unlock

My take: Lock accounts when users leave. Unlock when they return.

Common Mistakes (I've Made These)​

  1. Forgetting -m: Users created without home directories. They can't log in properly.

  2. Not setting password: Users can't log in. Set passwords or use SSH keys.

  3. Using -G without -a: Replaces groups instead of adding. Use -aG.

  4. Deleting users without backup: User data is lost. Backup first if needed.

  5. Not checking if user exists: id username to check. Don't create duplicates.

Real-World Examples​

Create Developer User​

sudo useradd -m -s /bin/bash -c "Developer" -G developers,sudo devuser
sudo passwd devuser

Create Service User​

sudo useradd -r -s /bin/false -d /var/lib/service serviceuser

The -r flag: Creates system user (UID < 1000).

My take: Service users don't need shells. Use /bin/false. They don't log in.

Add User to Multiple Groups​

sudo usermod -aG group1,group2,group3 username

What's Next?​

Now that you can manage users, you can control access. Or learn about Security to protect your system.

Personal note: When I started, I'd create users without home directories. Then they couldn't log in. I learned to use -m. Now users work properly. Small details matter. Learn them.