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:

alice:x:1000:1000:Alice Smith:/home/alice:/bin/bash
│     │ │    │    │            │           │
│     │ │    │    │            │           └── Login shell
│     │ │    │    │            └── Home directory
│     │ │    │    └── GECOS (full name / comment)
│     │ │    └── Primary GID
│     │ └── UID
│     └── Password placeholder (actual hash in /etc/shadow)
└── Username

Managing Users

sudo useradd -m -s /bin/bash bob     # create user bob with home dir and bash shell
sudo passwd bob                       # set password for bob
sudo usermod -aG docker bob          # add bob to the docker group
sudo userdel -r bob                   # delete bob and their home directory
id alice                              # show UID, GID, and groups for alice
whoami                                # show current username

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Groups

Groups allow you to manage permissions for multiple users at once.

/etc/group

developers:x:1001:alice,bob,charlie
│          │ │    │
│          │ │    └── Group members
│          │ └── GID
│          └── Password placeholder
└── Group name

Managing Groups

sudo groupadd developers            # create a new group
sudo groupdel developers            # delete a group
sudo usermod -aG developers alice   # add alice to developers group
groups alice                         # list groups for alice

Tip: Always use -aG (append to groups) when adding users to groups. Using -G without -a replaces all secondary groups.

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File Permissions

Every file and directory has three sets of permissions for three categories of users:

Permission Categories

Category	Symbol	Description
Owner	u	The user who owns the file
Group	g	Members of the file's group
Others	o	Everyone else

Permission Types

Permission	Symbol	On File	On Directory
Read	r (4)	View file contents	List directory contents
Write	w (2)	Modify file contents	Create/delete files in directory
Execute	x (1)	Run as a program	Enter (cd into) the directory

Reading Permission Strings