User and group management:
A> Creating and managing users and groups using user add, userdel, groupadd and passwd.
B> Controlling Access to Files with Access Control Lists (ACL) using chmod, chgrp and chown.
A> Creating and managing users and groups using user add, userdel, groupadd and passwd.
1. What is a User in Linux?
A user is simply an account created in the system.
Every person who wants to use the computer or server gets a user account.
Each user has:
A username (like rajat, admin)
A UID (User ID) → unique number that identifies the user
A home directory (like /home/rajat)
A default shell (like /bin/bash
Why we need users?
To separate files and settings of each person.
To secure the system (one user cannot access another user’s private files unless allowed).
User (UID): your identity (e.g., rajat, UID like 1001).
Key files:
/etc/passwd (user list),
/etc/shadow (password hashes),
/etc/group (groups).
Check yourself: id, whoami, groups.
Most useful options (Debian/Ubuntu shadow-utils):
-u UID, --uid UID → custom numeric UID.
-s SHELL, --shell SHELL → login shell (e.g., /bin/bash, /usr/sbin/nologin).
-u UID, --uid UID → custom numeric UID.
2. Check All Users on the System
All user accounts are stored in the file
/etc/passwd.
cat /etc/passwd
Check If a Specific User Exists
id {username}
3) useradd — Create a user
Purpose: Create a local user account (entries in /etc/passwd, /etc/shadow, /etc/group if needed).
Basic syntax:
sudo useradd [options] USERNAME
Most useful options (Debian/Ubuntu shadow-utils):
-m, --create-home → create /home/USERNAME from /etc/skel.
-d PATH, --home PATH → custom home directory.
-u UID, --uid UID → custom numeric UID.
-U, --user-group → create a private group with same name as user (Ubuntu default style).
-e DATE, --expiredate DATE → account expiry (YYYY-MM-DD).
4) passwd — Set/Change user password
Purpose: Set, change, lock/unlock, and force reset of passwords (updates /etc/shadow).
Syntax:
sudo passwd USERNAME
Useful options (admin side):
-l → lock account (disables password login).
-u → unlock.
-e → expire immediately (forces change at next login)
-d → delete password (no password set; may disable login).
-x DAYS max days, -n DAYS min days, -w DAYS warn before expiry.
Examples:
sudo passwd rajat # set/replace password
sudo passwd -l rajat # lock account
sudo passwd -u rajat # unlock account
sudo passwd -e rajat # force change on next login
Why/when: Secure accounts, rotate credentials, or temporarily block access.
4a) For New user add:(new user)
Sudu useradd username
What is a Password in Linux?
A password protects a user account.
Without the correct password, you cannot login as that user.
Passwords are stored in encrypted form in the file /etc/shadow.
4b)Pass set:for (new user)
Sudo passwd username
{set password}
# give a strong paswd
5)Change user :
Su(switch user)
su username
$ (exit for exit from current user)
SHOW USER
cd /
Ls
Cd /etc
Ls
adduser.conf hosts passwd group shadow ssh ...
cat /etc/passwd
rajat:x:1001:1001:Rajat Kapoor:/home/rajat:/bin/bash
6)For delete user
First go on Home directory
Syntax for User del :
sudo userdel {username}
7) userdel — Delete a user
Purpose: Remove the account entry; optionally remove home..
Syntax:
sudo userdel [options] USERNAME
Important options:
-r, --remove → remove home directory.
Examples:
sudo userdel rajat # remove account only
sudo userdel -r rajat # remove account + /home/rajat
8) What is a Group in Linux?
A group is a collection of users.
Groups are used to manage permissions more easily.
Why we need groups?
If 10 students need access to one folder, instead of giving permissions one by one → just create a
group (e.g., SECTION-A,B,C) and add them.
Makes management faster, cleaner, and secure
Group (GID): a set of users (e.g., students).
Syntax:
Sudo groupadd {groupname}
Check group:(in home directory)
Cat /etc/group
Check user details:
Cat /etc/passwd
9) DELETE GROUP:
sudo groupdel {groupname}
