2.f. verifying user access level

id & groups Verifying user access levels.

What You Will Learn

1. Understand the role of **user IDs (UIDs) and **group IDs (GIDs) in Linux.
2. Learn how to check which groups a user belongs to.
3. Use id, groups, and related commands to verify access levels.
4. Diagnose permission issues by comparing UID/GID against file ownership.
5. Apply verification to **WordPress folder ownership (/var/www).
6. Distinguish between **primary group and supplementary groups.
7. Interpret id output to troubleshoot permission denied errors.
8. Implement best practices for group management in WordPress VPS.

Prerequisites

• **Access Level: Normal user account; sudo for checking other users.
• **Software: Pre-installed Linux tools (id, groups, whoami).
• **Knowledge: Basic Linux permissions (ls -l), ownership concepts.

5W + 1H Framework

Question	Answer
What	id, groups, whoami → commands to view user and group information.
Why	To confirm whether a user has the right permissions for WordPress folders/files.
When	Before granting permissions, after user creation, during troubleshooting.
Where	/etc/passwd, /etc/group, and runtime environment of /var/www.
Who	Admins, developers, DevOps working with multi-user WordPress VPS.
How	By running id USER, groups USER, whoami, and comparing with file ownership.

Core Commands Reference

Command	Purpose	Example	Expected Output
id	Show UID, GID, and group memberships	id	uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu),27(sudo),33(www-data)
id USERNAME	Check details for another user	id www-data	uid=33(www-data) gid=33(www-data) groups=33(www-data)
groups	List groups current user belongs to	groups	ubuntu : ubuntu sudo www-data
groups USERNAME	List groups for another user	groups ubuntu	ubuntu : ubuntu sudo www-data
whoami	Show current effective username	whoami	ubuntu
getent passwd USERNAME	Get account entry from /etc/passwd	getent passwd www-data	www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
getent group GROUPNAME	Get group entry from /etc/group	getent group www-data	www-data:x:33:ubuntu

Understanding UID and GID

• **UID (User ID): Unique number identifying a user.
• Example: uid=1000(ubuntu)
• **GID (Group ID): Primary group associated with the user.
• Example: gid=1000(ubuntu)
• **Supplementary Groups: Additional groups granting extra rights.
• Example: groups=1000(ubuntu),27(sudo),33(www-data)

WordPress Example:

If wp-content/uploads/ is owned by www-data:www-data, the PHP process (www-data) needs to appear in the id or groups output for uploads to work.

Practical WordPress Use Cases

Scenario	Command	Expected Outcome
Check if ubuntu user can manage uploads	groups ubuntu	Should include www-data if ACL/group access configured.
Verify PHP-FPM user identity	`ps aux	grep php-fpm→ thenid USER`
Debug file ownership error	ls -l wp-content/uploads/ + id	Compare owner/group of files vs your groups.
Audit all WordPress users/groups	`getent passwd	grep -E "ubuntu

Command Examples with Expected Outputs

Check Current User

whoami

Output:

ubuntu

Check UID, GID, and Groups

id

Output:

uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu),27(sudo),33(www-data)

Check Groups for Specific User

groups www-data

Output:

www-data : www-data

Get Detailed Entry

getent passwd www-data

Output:

www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin

Verify Group Membership

getent group www-data

Output:

www-data:x:33:ubuntu

Meaning: ubuntu belongs to www-data group.

Benefits for WordPress Management

Action	Benefit
id / groups	Confirms user belongs to web server group (www-data).
whoami	Prevents accidental root vs normal user operations.
getent	Verifies system-level user and group database entries.
Comparing with ls -l	Direct mapping between file ownership and user access.

Implementation Steps

1. Check your identity:

   whoami
   

2. Verify your UID, GID, and groups:

   id
   

3. Confirm youre in www-data (or PHP user group):

   groups
   

4. Audit WordPress ownership vs groups:

   ls -ld /var/www/html/wp-content/uploads/
   id
   

5. If missing access, add user to group:

   sudo usermod -aG www-data ubuntu
   

Troubleshooting Matrix

Symptom	Likely Cause	Fix
“Permission denied” when uploading	User not in www-data group	sudo usermod -aG www-data <user> then re-login
Files owned by root	Commands run as sudo created files	sudo chown -R www-data:www-data /var/www/html
Web server cannot write to uploads	PHP-FPM user not in group	Check id www-data and adjust ACLs or ownership
Group changes not applied	User session not refreshed	Re-login or newgrp www-data

Quick Lab

# 1. Check current user
whoami

# 2. View groups
groups

# 3. Inspect uploads ownership
ls -ld /var/www/html/wp-content/uploads/

# 4. Verify web server identity
id www-data

# 5. Add your user to web server group (if missing)
sudo usermod -aG www-data ubuntu

Expected Results:

• You confirm your username (whoami).
• Groups include www-data.
• Uploads directory shows www-data ownership.

Cheat Sheet

Command	Purpose
whoami	Show current username
id	Show UID, GID, groups
id USER	Show another user’s details
groups	List current user’s groups
groups USER	List another user’s groups
getent passwd USER	View user entry
getent group GROUP	View group entry
usermod -aG group user	Add user to group

Mini Quiz

1. What does id show that groups doesnt?
2. Why is being part of www-data group important for WordPress admins?
3. Which command verifies the primary group of a user?
4. What must you do after usermod -aG to apply group changes?
5. How would you check if ubuntu is part of www-data group?