User Management

adduser - Add users step by step¶

adduser is interactive. It asks questions and sets things up automatically.

sudo adduser alice # new user named alice

• Prompts for a password for alice (UNIX password)

• Asks for full name, room number, work phone, home phone, other (press Enter to skip)

• Confirms info with Y - Linux creates default files and UserID (UID) automatically.

ls /home
# alice  diogo  (new user's home folder appears)
ls -la /home/alice
# .bashrc  .profile  (default files)

Change password¶

Another user's password¶

sudo passwd alice

• New password for Alice
• Confirm new password

You're an admin. You don't need to know the old password to change it.

Your own password¶

passwd # no sudo required

You'll be asked for:

• Your current password
• New password
• Confirm new password

Delete a user¶

sudo userdel alice      # deletes user, keeps /home/alice
sudo userdel -r alice   # deletes user AND /home/alice

Why keep the home folder?¶

userdel without -r removes Alice's access (password, login ability) but keeps her files. You can delete the folder later with rm -rf /home/alice if you decide you don't need them.

passwd alice   # Permission Denied
sudo !!        # Works!

!! retrieves the last written command, though it is commonly used with sudo.

Where user info is stored¶

Don't edit these files directly. Use adduser, passwd, userdel instead.

/etc/passwd explained¶

View all users on the system:

cat /etc/passwd

Each line represents an account (users & system/service accounts). Example lines:

root:x:0:0:root:/root:/bin/bash
your-username:x:1000:1000:Real Name:/home/your-username:/bin/bash

Fields are separated by a : (colon)

your-username:x:1000:1000:Real Name:/home/your-username:/bin/bash
        │     │   │    │       │               │            │
        │     │   │    │       │               │            └── Shell (bash)
        │     │   │    │       │               │
        │     │   │    │       │               └── Home directory
        │     │   │    │       │
        │     │   │    │       └── Full name (GECOS)
        │     │   │    │
        │     │   │    └── Group ID (GID)
        │     │   │
        │     │   └── User ID (UID)
        │     │       (first regular user starts at 1000)
        │     │
        │     └── Password placeholder
        │         (x means in /etc/shadow)
        │
        └── Username

If you add the username, room, work phone, home phone, and other information, that information will appear here after groupID separated by , (comma)

/etc/shadow explained¶

This file stores encrypted passwords. Only root can read it.

sudo cat /etc/shadow

Each line represents an account. Example lines:

root:$6$randomhash...:19120:0:99999:7:::
your-username:$6$differenthash...:19120:0:99999:7:::
daemon:*:19120:0:99999:7:::

Fields are separated by a : (colon)

your-username:$6$differenthash...:19120:0:99999:7:::
       │      │                     │   │   │   │
       │      │                     │   │   │   └── Warn 7 days before expiration
       │      │                     │   │   │
       │      │                     │   │   └── Max: 99999 days (never expires)
       │      │                     │   │
       │      │                     │   └── Min: 0 days (can change anytime)
       │      │                     │
       │      │                     └── Last changed: 19120 days since Jan 1, 1970
       │      │
       │      └── $6$ (SHA-512) + Salt + Encrypted hash
       │
       └── Username

Service accounts have * or ! instead of a password. You can't log into them.

/etc/group explained¶

This file stores group information. Groups let you manage permissions for multiple users at once.

cat /etc/group

Each line represents one group. Example lines:

root:x:0:
sudo:x:27:alice,bob
developers:x:1001:alice
your-username:x:1000:

Fields are separated by a : (colon)

developers:x:1001:your-username,alice
     │     │  │          │
     │     │  │          └── Members (users in this group, comma-separated)
     │     │  │
     │     │  └── Group ID (GID)
     │     │
     │     └── Password placeholder (x means in /etc/gshadow)
     │
     └── Group name

Check Your Understanding¶

1. Add a new user¶

You want to create a new user named john.

What command do you type?

2. Change another user's password¶

Alice forgot her password. You need to set a new one for her.

What command do you type?

3. Change your own password¶

What command do you type?

4. Delete a user (keep files)¶

Alice left the company. She shouldn't log in anymore, but her files might be needed later.

What command do you type?

5. Delete user AND home folder¶

Bob was a temporary test account. You need the disk space back.

What command do you type?

6. Forgot sudo¶

You typed passwd alice and got "Permission denied".

What's the fastest way to re-run it with sudo?

7. Where is password stored?¶

Which file stores encrypted passwords (only readable by root)?

8. Service account indicator¶

You see daemon:*:... in /etc/shadow. What does * mean?