Managing Users in Linux: A Practical Guide for Beginners
Learn how to add, delete, and manage users using simple Linux commands
📜 Brief History
Origin: The useradd, usermod, and userdel commands originate from UNIX System V (SVR4) and were later adopted into Linux distributions via the Shadow Suite (
shadow-utils).Linux History:
First widely seen in early Linux distributions like Slackware (1993) and Red Hat Linux (1995).
Became a standard part of Linux account management tools, particularly when the /etc/shadow system (for storing encrypted passwords securely) became popular.
Maintainer: Now maintained as part of the shadow-utils package, used in most Debian, Ubuntu, Red Hat, CentOS, Fedora, etc.
👤 Add User with useradd
The useradd command is a low-level command-line utility used to create new user accounts on Unix and Linux systems.
✅ What useradd Does:
Requires Root Privileges: You must have root access (or use
sudo) to runuseradd, as creating or modifying user accounts is a privileged operation.Updates System Files: Adds entries to /etc/passwd, /etc/shadow, /etc/group, and /etc/gshadow.
Creates a Home Directory: Automatically creates the user’s home directory (usually /home/username) if specified or enabled by default (with the -m flag).
Creates a User Private Group: By default, also creates a group with the same name as the user.
Sets UID, Shell, and Defaults: Assigns a unique user ID (UID), default shell (e.g., /bin/bash), and applies system-wide defaults from /etc/default/useradd.
Handles Group Memberships: Can assign the user to one or more additional groups using the
-Goption.
💻 Usage
sudo useradd -m -s /bin/bash johnExplanation:
-m: Create the user's home directory (e.g., /home/john)
-s /bin/bash: Set the default shell to Bash
🛠️ Modify Users with usermod
The usermod command is a low-level command-line utility used to modify user accounts on Unix/Linux systems.
✅ What usermod Does:
Root Privileges: You must have root privileges (or use sudo) to run usermod. Modifying user accounts is a privileged operation.
⚠️ Caution: Incorrect use of
usermodcan lock users out of their accounts or introduce security issues. Always double-check your changes before executing the command.Home Directory: Changing the home directory with
usermoddoes not move existing user data unless used with the-mflag. Be cautious, as this can overwrite the new target directory if it already exists.Impact on Running Processes: Changing the UID (User ID) of a user while they have running processes can lead to unpredictable behavior. It's generally best to ensure the user is logged out before making such changes.
Updates user-related entries in /etc/passwd, /etc/shadow, /etc/group, and /etc/gshadow.
💻 Usage
✏️ Rename the User john to johnny
sudo usermod -l johnny johnExplanation:
-l, --login NEW_LOGIN: Changes the login name (username) fromjohntojohnny.Note: This does not rename the home directory (you’ll need to move that manually if needed).
✏️ Optional: Rename the Home Directory Too
If you also want to rename the home directory and move contents:
sudo usermod -d /home/johnny -m johnny-d: New home directory path-m: Move the content from the old home to the new path
👥 ✏️Change the Primary Group of the User
sudo usermod -g marketing johnnyExplanation:
-g, --gid GROUP: Sets the primary group of the userjohnnytomarketing.
👥 ✏️Add a User to Multiple Groups
sudo usermod -G developers,testers johnnyExplanation:
-G, --groups GROUP1[,GROUP2...]: Sets the complete list of supplementary groups the user belongs to. Any groups the user was previously a member of that are not listed will be removed. Separate group names with commas.
👥 ✏️Add a User to a Supplementary Group (without removing existing ones)
sudo usermod -aG webmaster johnnyExplanation:
-aG,--append: Adds the user to the specified supplementary group(s) without removing them from any existing ones.The group(s) must already exist.
This is commonly used to grant additional access (e.g., to
webmaster,developers, testers, etc.).
❌ Delete Users with userdel
The userdel command is a low-level utility used to delete existing user accounts on Unix/Linux systems.
✅ What userdel Does:
Removes user entries from
/etc/passwd,/etc/shadow,/etc/group, and/etc/gshadow.Optionally deletes the user's home directory (e.g.,
/home/alice) and mail spool (e.g.,/var/mail/alice) when used with the-rflag.Does not remove manually created files owned by the user elsewhere on the system (e.g., files in
/var/www,/tmp, etc.).Does not delete the user's primary group (the User Private Group), even if no other users are part of it.
Fails if the user is currently logged in or has active processes (unless used with
-fon some systems like Red Hat-based distros).
💻 Usage
sudo userdel -r johnnyExplanation:
Delete the user
johnnyRemove his home directory and mail spool (if they exist)
🧾 Command Summary
Command Purpose Notes
useradd Add a new user Use -m, -s, -G for full setup
usermod Modify existing user accounts Rename, change groups/home directory
userdel Delete a user account Use -r to remove home and mail spool
🔍 Verify Changes
After making changes, you can verify them using:
You can verify changes with commands like id johnny, groups johnny, and getent passwd johnny.
id johnny
groups johnny
getent passwd johnny