usermod — Change account attributes

💬 "usermod command"

usermod changes the attributes of an existing account. The most common use is adding supplementary groups.

If you use -G alone instead of -aG, all existing supplementary groups are removed, so be careful.

• Concept: Changes account attributes such as group, shell, and home directory.
• -aG: Adds groups while keeping existing groups.
• -s: Changes the login shell.
• -L/-U: Locks/unlocks the account.

Commonly Used Options

3 Essential Command Combinations

• usermod -aG www-data myapp — Add to the web server group
• usermod -aG docker deploy — Grant permission to run Docker
• id deploy — Verify the group change result

💬 "Please give an example of a procedure to check before accidentally deleting"

This is a step-by-step summary of situations commonly encountered on live production servers.

Step 1 · Check Current Groups

Check the current state before making changes.

id deploy

Step 2 · Add a Group

Use -aG to add a group while keeping existing groups.

usermod -aG www-data deploy

Step 3 · Log In Again and Verify

Group changes take effect from a new session.

id deploy

How to Use This in Practice

1. Grant a Service Group to a Deployment User

Situation — The deployment account needs to read and write to the web root.

usermod -aG www-data deploy && id deploy

Result — www-data is added to the supplementary groups.

Point — Using -G alone is prohibited because it removes existing groups.

2. Lock an Account

Situation — You need to immediately block a former employee's account.

usermod -L -s /usr/sbin/nologin olduser

Result — Logins are blocked and the shell is disabled.

Point — To block key authentication as well, clean up authorized_keys.

Caution

• -G alone overwrites supplementary groups. Always use -aG.
• Group changes do not apply immediately to the current login session.
• Moving the home directory (-m) can take a long time for large data and may leave the location inconsistent if it fails.

Actual Execution Screen

This is the result after adding a supplementary group and verifying the outcome.

!usermod command execution result: group addition confirmed

Live production server

Source (HTML): https://games.aidebatehub.com/programs/linux/security/usermod.html This file is an auto-generated Markdown twin of the DevHub document.