Users, Groups and Permissions

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.

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Users

User Accounts

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

Special Users

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

/etc/passwd

Each line represents a user account: