User Management

Overview

Linux is a multi-user operating system, and user management is one of the core tasks of system administration. This chapter introduces the concepts, creation, modification, and deletion of users and groups.

User and Group Concepts

User Types

Type	UID Range	Description
root	0	Superuser, has all permissions
System users	1-999	Services and daemons
Regular users	1000+	Daily use user accounts

File	Description
`/etc/passwd`	User account information
`/etc/shadow`	Encrypted password information
`/etc/group`	Group information
`/etc/gshadow`	Group password information

/etc/passwd Format

username:x:UID:GID:comment:home_dir:shell
maxwell:x:1000:1000:Maxwell:/home/maxwell:/bin/bash

Field	Description
username	Username
x	Password placeholder (actual password is in shadow)
UID	User ID
GID	Primary group ID
comment	User description
home_dir	Home directory
shell	Login shell

/etc/shadow Format

username:$6$salt$hash:lastchg:min:max:warn:inactive:expire:reserved:

Field	Description
username	Username
password	Encrypted password
lastchg	Days since password changed (from 1970-01-01)
min	Minimum password age in days
max	Maximum password age in days
warn	Days before password expiration warning
inactive	Days after expiration when account is disabled
expire	Account expiration date

/etc/group Format

groupname:x:GID:members:
developers:x:1001:alice,bob

Viewing User Information

id - User ID Information

# Current user
$ id
uid=1000(maxwell) gid=1000(maxwell) groups=1000(maxwell),27(sudo),docker

# Specified user
$ id alice

# Show only UID
$ id -u maxwell

# Show only GID
$ id -g maxwell

# Show all groups
$ id -G maxwell

whoami - Current Username

$ whoami
maxwell

who - Logged-in Users

$ who
maxwell  pts/0        2025-01-09 10:00 (192.168.1.100)
alice    pts/1        2025-01-09 11:00 (192.168.1.101)

# More information
$ who -a

w - User Activity

$ w
 10:30:00 up 5 days,  3:00,  2 users, load average: 0.00, 0.01, 0.05
 USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
maxwell  pts/0    192.168.1.100    10:00    0.00s  0.01s  0.00s w
alice    pts/1    192.168.1.101    11:00    1:00m  0.00s 0.00s bash

$ last
maxwell  pts/0        192.168.1.100    Thu Jan 9 10:00   still logged in
alice    pts/1        192.168.1.101    Thu Jan 9 09:00 - 09:30  (00:30)

# Last 10 entries
$ last -10

# Specified user
$ last maxwell

groups - User's Groups

$ groups
maxwell sudo docker

$ groups alice
alice : alice developers

Creating Users

useradd - Add User

# Basic creation
$ sudo useradd username

# Create with home directory
$ sudo useradd -m username

# Specify home directory
$ sudo useradd -m -d /home/custom username

# Specify shell
$ sudo useradd -m -s /bin/bash username

# Specify primary group
$ sudo useradd -m -g groupname username

# Specify additional groups
$ sudo useradd -m -G sudo,docker username

# Specify UID
$ sudo useradd -m -u 1500 username

# Add description
$ sudo useradd -m -c "Full Name" username

# Set account expiration date
$ sudo useradd -m -e 2025-12-31 username

# Complete example
$ sudo useradd -m -s /bin/bash -c "Alice Smith" -G sudo,developers alice

adduser - Interactive Creation (Debian Family)

$ sudo adduser username
Adding user `username' ...
Adding new group `username' (1001) ...
Adding new user `username' (1001) with group `username' ...
Creating home directory `/home/username' ...
Copying files from `/etc/skel/' ...
New password:
Retype new password:
Full Name []:
Room Number []:
Work Phone []:
Home Phone []:
Other []:
Is the information correct? [Y/n]

Setting Password

# Set password
$ sudo passwd username
New password:
Retype new password:
passwd: password updated successfully

# Change your own password
$ passwd

Modifying Users

usermod - Modify User

# Change username
$ sudo usermod -l newname oldname

# Change home directory
$ sudo usermod -d /home/newdir -m username

# Change shell
$ sudo usermod -s /bin/zsh username

# Change description
$ sudo usermod -c "New Description" username

# Add to additional groups
$ sudo usermod -aG sudo username
$ sudo usermod -aG docker,developers username

# Change primary group
$ sudo usermod -g newgroup username

# Lock account
$ sudo usermod -L username

# Unlock account
$ sudo usermod -U username

# Set account expiration date
$ sudo usermod -e 2025-12-31 username

chsh - Change Shell

# Change your own shell
$ chsh -s /bin/zsh

# Change another user's shell
$ sudo chsh -s /bin/zsh username

# View available shells
$ cat /etc/shells

chfn - Modify User Information

$ sudo chfn username
Changing finger information for username.
Full Name []:
Room Number []:
Work Phone []:
Home Phone []:
Other []:

Deleting Users

userdel - Delete User

# Delete user only
$ sudo userdel username

# Delete user and home directory
$ sudo userdel -r username

# Force delete (even if user is logged in)
$ sudo userdel -f username

deluser (Debian Family)

# Delete user
$ sudo deluser username

# Delete user and home directory
$ sudo deluser --remove-home username

# Delete all user's files
$ sudo deluser --remove-all-files username

Group Management

Create Group

# Create group
$ sudo groupadd groupname

# Specify GID
$ sudo groupadd -g 1500 groupname

# Create system group
$ sudo groupadd -r systemgroup

Modify Group

# Rename group
$ sudo groupmod -n newname oldname

# Modify GID
$ sudo groupmod -g 1600 groupname

Delete Group

$ sudo groupdel groupname

Managing Group Members

# Add user to group
$ sudo usermod -aG groupname username
$ sudo gpasswd -a username groupname

# Remove user from group
$ sudo gpasswd -d username groupname

# Set group administrator
$ sudo gpasswd -A admin_user groupname

# View group members
$ getent group groupname

Password Management

passwd - Password Operations

# Set password
$ sudo passwd username

# Lock account
$ sudo passwd -l username

# Unlock account
$ sudo passwd -u username

# Delete password (no password login)
$ sudo passwd -d username

# Force password change on next login
$ sudo passwd -e username

# View password status
$ sudo passwd -S username
username P 01/09/2025 0 99999 7 -1

chage - Password Expiration Policy

# View password information
$ sudo chage -l username
Last password change                : Jan 09, 2025
Password expires                    : never
Password inactive                   : never
Account expires                     : never
Minimum number of days between password change   : 0
Maximum number of days between password change   : 99999
Number of days of warning before password expires: 7

# Set maximum age
$ sudo chage -M 90 username

# Set minimum age
$ sudo chage -m 7 username

# Set warning days
$ sudo chage -W 14 username

# Set expiration date
$ sudo chage -E 2025-12-31 username

# Force password change on next login
$ sudo chage -d 0 username

sudo Permissions

Configuring sudo

Edit /etc/sudoers (using visudo):

$ sudo visudo

# User permission rules
# Username Host=(Identity) Commands
maxwell ALL=(ALL:ALL) ALL

# Group permission rules
%sudo ALL=(ALL:ALL) ALL

# No password execution
maxwell ALL=(ALL) NOPASSWD: ALL

# Limit commands
bob ALL=(ALL) /usr/bin/apt, /usr/bin/systemctl

# Aliases
User_Alias ADMINS = alice, bob
Cmnd_Alias SERVICES = /usr/bin/systemctl
ADMINS ALL=(ALL) SERVICES

Adding User to sudo Group

# Method 1: Use usermod
$ sudo usermod -aG sudo username

# Method 2: Edit sudoers
$ sudo visudo
# Add: username ALL=(ALL:ALL) ALL

Using sudo

# Execute as root
$ sudo command

# Execute as different user
$ sudo -u username command

# Switch to root shell
$ sudo -i
$ sudo su -

# Keep environment variables
$ sudo -E command

# Edit file
$ sudo -e /etc/hosts
$ sudoedit /etc/hosts

Switching Users

su - Switch User

# Switch to root
$ su -
Password:

# Switch to other user
$ su - username

# Don't load user environment
$ su username

# Execute single command
$ su - username -c "command"

sudo su vs su

# Use sudo su (use current user password)
$ sudo su -

# Use su (use root password)
$ su -

Practical Tips

Batch Create Users

#!/bin/bash
# Batch create users from file
while IFS=: read -r username password; do
    sudo useradd -m -s /bin/bash "$username"
    echo "$username:$password" | chpasswd
done < users.txt

Find Users Without Passwords

$ sudo awk -F: '($2 == "" || $2 == "!") {print $1}' /etc/shadow

Find UID 0 Users

$ awk -F: '$3 == 0 {print $1}' /etc/passwd

List All Regular Users

$ awk -F: '$3 >= 1000 && $3 < 65534 {print $1}' /etc/passwd

Summary

This chapter introduced Linux user management:

User operations: useradd, usermod, userdel
Group operations: groupadd, groupmod, groupdel
Password management: passwd, chage
sudo permissions: Configuring and using sudo
User switching: su, sudo su

Good user management is foundation of system security. Follow principle of least privilege - grant only necessary permissions.

Previous chapter: Regular Expressions

Next chapter: Process Management

#User Management

#Overview

#User and Group Concepts

#User Types

#User Related Files

#/etc/passwd Format

#/etc/shadow Format

#/etc/group Format

#Viewing User Information

#id - User ID Information

#whoami - Current Username

#who - Logged-in Users

#w - User Activity

#last - Login History

#groups - User's Groups

#Creating Users

#useradd - Add User

#adduser - Interactive Creation (Debian Family)

#Setting Password

#Modifying Users

#usermod - Modify User

#chsh - Change Shell

#chfn - Modify User Information

#Deleting Users

#userdel - Delete User

#deluser (Debian Family)

#Group Management

#Create Group

#Modify Group

#Delete Group

#Managing Group Members

#Password Management

#passwd - Password Operations

#chage - Password Expiration Policy

#sudo Permissions

#Configuring sudo

#Adding User to sudo Group

#Using sudo

#Switching Users

#su - Switch User

#sudo su vs su

#Practical Tips

#Batch Create Users

#Find Users Without Passwords

#Find UID 0 Users

#List All Regular Users

#Summary