Users and groups
Table of contents
- The concept of Linux users
- Root vs standard user
- Information commands
- Creating and managing users
- Managing groups
- The /etc/passwd and /etc/group files
1 - The concept of Linux users
Linux is a multi-user system. Each user has:
- A unique identifier (UID)
- A personal directory
- Specific permissions
Types of users
| Type | UID | Usage |
|---|---|---|
| root | 0 | Super-administrator |
| System | 1-999 | Services and daemons |
| Humans | 1000+ | Real users |
Why several users?
| Reason | Example |
|---|---|
| Security | Isolate access |
| Audit | Trace actions |
| Permissions | Control who does what |
| Services | One user per service |
info
Each service (nginx, mysql, etc.) generally has its own user to limit the damage in case of compromise.
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2 - Root vs standard user
The root user
| Characteristic | Root | Standard user |
|---|---|---|
| UID | 0 | 1000+ |
| Home | /root | /home/username |
| Prompt | # | $ |
| Power | Unlimited | Limited |
The dangers of root
danger
Working as root is dangerous!
# Une erreur de frappe peut être catastrophique
rm -rf / home/user # Espace en trop = système détruit !
Best practices
| Do | Avoid |
|---|---|
Use sudo occasionally | Logging in as root |
| Create dedicated users | Doing everything as root |
| Limit privileges | Sharing the root password |
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3 - Information commands
whoami - Who am I?
$ whoami
john
id - Detailed information
$ id
uid=1000(john) gid=1000(john) groups=1000(john),27(sudo),docker(999)
$ id john
uid=1000(john) gid=1000(john) groups=1000(john),27(sudo)
$ id -u # Juste l'UID
1000
$ id -g # Juste le GID principal
1000
$ id -G # Tous les GIDs
1000 27 999
who - Connected users
$ who
john pts/0 2024-01-15 10:30 (192.168.1.100)
alice pts/1 2024-01-15 11:00 (192.168.1.101)
$ who -a # Informations détaillées
w - User activity
$ w
10:45:00 up 5 days, 2:30, 2 users, load average: 0.15, 0.10, 0.05
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
john pts/0 192.168.1.100 10:30 0.00s 0.05s 0.00s w
alice pts/1 192.168.1.101 11:00 5:00 0.02s 0.02s vim file.txt
last - Login history
$ last
john pts/0 192.168.1.100 Mon Jan 15 10:30 still logged in
alice pts/1 192.168.1.101 Mon Jan 15 11:00 still logged in
john pts/0 192.168.1.100 Sun Jan 14 09:00 - 18:00 (09:00)
$ last -n 5 # 5 dernières connexions
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4 - Creating and managing users
useradd - Create a user
# Création basique
sudo useradd alice
# Création complète (recommandé)
sudo useradd -m -s /bin/bash -c "Alice Martin" alice
| Option | Effect |
|---|---|
-m | Create the home directory |
-s | Login shell |
-c | Comment (full name) |
-g | Primary group |
-G | Additional groups |
-d | Custom home path |
-e | Expiration date |
adduser - Interactive alternative (Debian/Ubuntu)
sudo adduser bob
# Assistant interactif qui demande :
# - Mot de passe
# - Nom complet
# - Autres informations
tip
adduser is friendlier than useradd on Debian/Ubuntu. It automatically creates the home and asks for the password.
passwd - Set the password
# Définir le mot de passe d'un utilisateur
sudo passwd alice
# Changer son propre mot de passe
passwd
# Verrouiller un compte
sudo passwd -l alice
# Déverrouiller un compte
sudo passwd -u alice
# Forcer le changement à la prochaine connexion
sudo passwd -e alice
usermod - Modify a user
# Changer le shell
sudo usermod -s /bin/zsh alice
# Ajouter à un groupe (sans retirer des autres)
sudo usermod -aG docker alice
# Changer le home
sudo usermod -d /home/new_alice -m alice
# Renommer
sudo usermod -l alice_new alice
warning
The -aG (append Groups) option is crucial! Without the a, the user is removed from all their existing groups.
userdel - Delete a user
# Supprimer l'utilisateur (garde le home)
sudo userdel alice
# Supprimer l'utilisateur ET son home
sudo userdel -r alice
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5 - Managing groups
The concept of groups
groupadd - Create a group
# Créer un groupe
sudo groupadd developers
# Créer avec un GID spécifique
sudo groupadd -g 2000 devops
groupmod - Modify a group
# Renommer un groupe
sudo groupmod -n dev developers
# Changer le GID
sudo groupmod -g 2001 devops
groupdel - Delete a group
sudo groupdel developers
gpasswd - Manage members
# Ajouter un utilisateur au groupe
sudo gpasswd -a alice developers
# Retirer un utilisateur du groupe
sudo gpasswd -d alice developers
# Définir les administrateurs du groupe
sudo gpasswd -A alice developers
groups - View a user's groups
$ groups
john sudo docker
$ groups alice
alice : alice developers docker
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6 - The /etc/passwd and /etc/group files
/etc/passwd - The user database
$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
john:x:1000:1000:John Doe:/home/john:/bin/bash
Structure:
john:x:1000:1000:John Doe:/home/john:/bin/bash
│ │ │ │ │ │ │
│ │ │ │ │ │ └── Shell
│ │ │ │ │ └── Home
│ │ │ │ └── Commentaire (GECOS)
│ │ │ └── GID principal
│ │ └── UID
│ └── Mot de passe (x = dans /etc/shadow)
└── Nom d'utilisateur
/etc/shadow - Passwords (encrypted)
$ sudo cat /etc/shadow
john:$6$xyz...:19000:0:99999:7:::
| Field | Meaning |
|---|---|
| 1 | Username |
| 2 | Encrypted password |
| 3 | Last modification |
| 4 | Minimum age |
| 5 | Maximum age |
| 6 | Warning period |
| 7 | Inactivity period |
| 8 | Expiration date |
/etc/group - The group database
$ cat /etc/group
sudo:x:27:john,alice
docker:x:999:john,bob
developers:x:1001:alice,bob,carol
Structure:
developers:x:1001:alice,bob,carol
│ │ │ │
│ │ │ └── Membres
│ │ └── GID
│ └── Mot de passe (rarement utilisé)
└── Nom du groupe
Useful commands
# Voir un utilisateur spécifique
grep john /etc/passwd
# Compter les utilisateurs humains (UID >= 1000)
awk -F: '$3 >= 1000 {print $1}' /etc/passwd
# Voir les groupes d'un utilisateur
grep john /etc/group
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Key takeaways
- Linux distinguishes root (UID 0), system (1-999), and humans (1000+)
- Never work as root directly → use
sudo useradd -m -s /bin/bashoradduserto createusermod -aG group userto add to groups (watch out for the-a!)/etc/passwd= user info,/etc/shadow= passwords- A user has a primary group and can have secondary groups