Master Linux Users and Groups: Permissions, Files, and Management Commands

1. Users

A user is a collection of permissions to access system resources.

Linux user categories

a. Administrator (root): has all system permissions, UID 0.

b. Regular user: limited permissions, UID 500‑60000.

c. System user: for system operation, no password login, UID 1‑499.

Files related to users

/etc/passwd

Format: account:password:UID:GID:GECOS:directory:shell

account – username

password – placeholder

UID – user ID

GID – primary group ID

GECOS – detailed info (name, age, phone, etc.)

directory – home directory

shell – login shell

/etc/shadow

Format: account:password:last_change:min:max:warn:inactive:expire:reserved

4. User groups

Group categories:

Regular group: multiple users can join.

System group: typically contains system users.

Private (primary) group: created with a user if no group specified; name matches username; can become regular group when other users are added.

5. Groups as permission containers

Users inherit permissions of the groups they belong to.

6. Files related to groups

/etc/group

Format: group_name:password:GID:user_list

group_name – name of the group

password – group password

GID – group ID

user_list – list of users belonging to the group

7. Commands to manage users and groups

useradd, usermod, groupadd, userdel

Add user

useradd [options] username

Options:

-u UID

-g GID

-d home_directory (default /home/username)

-s shell

-G supplementary_groups

Example:

# useradd -u 1888 -d /home/oracle -s /bin/sh wendy

Modify user

usermod [options] username

Options similar to useradd; -m with -b moves home directory contents.

Example:

# usermod -u 1000 -d /oracle -s /bin/bash -m wendy

Add group

groupadd [options] groupname

Option:

-g GID

Example:

# groupadd -g 1001 grp

Delete user

userdel [options] username

Option:

-r remove home directory

Example:

# userdel -r wendy