Linux User Management & File Permissions

Create users, manage groups, assign sudo access, and master chmod.

Linux user management & file permissions

Linux is a multi-user operating system. Every file has an owner and a set of permissions that control who can read, write, or execute it. Understanding users, groups, and permissions is essential for securing your servers and managing teams on shared infrastructure.

1. Two Core Concepts

Authentication — verifying who you are. Managed by creating a user and setting a password.

Authorization — what you’re allowed to do. Managed by adding users to groups and editing the sudoers file.

root is the superuser with full access to everything. Normal users like ec2-user have limited permissions and must use sudo for admin actions.

2. User Commands

useradd username: Create a new user

passwd username: Set or change a user’s password

userdel username: Delete a user account

su — username: Switch to another user account

sudo -i: Switch to root user

cat /etc/passwd: List all users on the system

who or w: Show who is currently logged in

last: Show recent login history for all users

Example — Create a new user and set a password:

useradd john
passwd john
cat /etc/passwd # verify user was created

3. Granting sudo Access (visudo)

By default, new users cannot run admin commands. To grant sudo access, you edit the sudoers file using visudo:

Step 1 — Open the sudoers file:

visudo

Step 2 — Navigate to the root line (around line 100) and add your user below it:

root ALL=(ALL) ALL
john ALL=(ALL) ALL

Step 3 — Save with :wq, then verify:

su - john

sudo useradd pushpa # this should now work

Never edit /etc/sudoers directly. Always use visudo — it validates syntax before saving, protecting you from locking yourself out.

4. Group Commands

Groups let you manage permissions for multiple users at once. Add a user to a group to give them collective access to shared resources.

groupadd groupname: Create a new group

usermod -aG groupname username: Add a user to a group

cat /etc/group: List all groups on the system

Example — Create a ‘devops’ group and add a user to it:

groupadd devops

usermod -aG devops john

cat /etc/group # verify john is in devops group

5. Understanding File Permissions

Every file has three permission sets — for the Owner (User), the Group, and Others (World). Each set has three permission bits: r (read), w (write), x (execute).

Reading a permission string:

-rw-r--r-- 1 root root 0 Dec 22 07:59 test.txt
^^^
||| → Owner: rw- = can read and write (no execute)
   ^^^ → Group: r-- = can read only
      ^^^ → Others: r-- = can read only

Permission values: r = 4, w = 2, x = 1. Add them to get the octal number.

6. chmod — Changing Permissions

chmod 777 filename

rwx rwx rwx — Full access for everyone

chmod 775 filename

rwx rwx r-x — Owner/Group full, Others read+execute

chmod 755 filename

rwx r-x r-x — Owner full, Group/Others read+execute

chmod 664 filename

rw- rw- r — — Owner/Group read+write, Others read only

chmod 644 filename

rw- r — r — — Owner read+write, Others read only

chmod 400 filename

r — — — — — — Owner read only (very restrictive)

chmod 600 is common for SSH private key files (.pem). It means only the owner can read/write — no one else has any access.

Next up → Part 5: Linux DevOps Commands — environment variables, Git, compression, and the commands you’ll use every day in CI/CD pipelines.