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Linux is a multi-user operating system. Every file and process has an owner, and access is controlled through a robust permissions system. Understanding users, groups, and permissions is critical for security and system administration.
Every user account has:
| Attribute | Description | Stored In |
|---|---|---|
| Username | Login name (e.g., alice) | /etc/passwd |
| UID | Numeric user ID (e.g., 1000) | /etc/passwd |
| GID | Primary group ID | /etc/passwd |
| Home directory | Personal directory (e.g., /home/alice) | /etc/passwd |
| Login shell | Default shell (e.g., /bin/bash) | /etc/passwd |
| Password hash | Encrypted password | /etc/shadow |
| User | UID | Purpose |
|---|---|---|
| root | 0 | Superuser with unlimited privileges |
| nobody | 65534 | Least-privileged user for services |
| www-data | 33 | Web server user (Apache/Nginx) |
| sshd | — | SSH daemon service account |
Each line represents a user account:
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