Diagnose and correct file permission problems

6. Create and Configure File Systems

📘Red Hat Certified System Administrator (RHCSA – EX200)

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In IT systems, file permissions are rules that control who can read, write, or execute a file or folder. Understanding and fixing permission problems is essential to ensure that users and applications can access files safely without causing security issues.

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1. Understanding File Permissions in Linux

Linux (and Unix-based systems) uses three main permission types for three different groups:

Permission types:

• Read (r): Allows viewing the file content.
• Write (w): Allows modifying the file content.
• Execute (x): Allows running the file as a program or script.

Groups:

1. Owner: The user who owns the file.
2. Group: Other users in the same group as the file.
3. Others: All other users on the system.

Example:

-rwxrw-r--

• rwx → Owner can read, write, and execute.
• rw- → Group can read and write.
• r-- → Others can only read.

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2. Checking Permissions

To diagnose permission issues, first check who owns the file and its current permissions:

ls -l /path/to/file

Output example:

-rw-r--r-- 1 alice developers 1024 Mar 12 10:30 report.txt

• Owner: alice
• Group: developers
• Permissions: rw- for owner, r-- for group and others.

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3. Common Permission Problems

1. User cannot read a file
   • Cause: Read permission missing.
   • Example: cat report.txt If denied, check permissions.
2. User cannot write to a file
   • Cause: Write permission missing or file is owned by another user.
   • Example: echo “Update” >> report.txt
3. User cannot execute a script
   • Cause: Execute permission missing.
   • Example: ./backup.sh
4. Access denied even with correct permissions
   • Cause: File may be on a mounted filesystem with restricted permissions (like NFS) or ACL issues.

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4. Correcting Permissions

You can change file permissions using the chmod command.

Syntax:

chmod [options] permissions file

Two ways to specify permissions:

1. Symbolic: Using letters chmod u+x script.sh # Adds execute for owner
   chmod g-w report.txt # Removes write for group
   chmod o+r file.txt # Adds read for others
   • u → owner (user)
   • g → group
   • o → others
   • + → add permission
   • - → remove permission
   • = → set exact permission
2. Numeric (Octal):
   Each permission has a number:
   • Read = 4
   • Write = 2
   • Execute = 1
     Add them up for each group:
   chmod 755 script.sh
   • Owner: 7 (4+2+1 → rwx)
   • Group: 5 (4+0+1 → r-x)
   • Others: 5 (r-x)

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5. Correcting Ownership

Sometimes, permission issues occur because the wrong user or group owns the file.

Use the chown command:

chown owner:group /path/to/file

Example:

chown alice:developers report.txt

• Changes the owner to alice
• Changes the group to developers

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6. Advanced Permissions: ACLs

In modern IT environments, Access Control Lists (ACLs) allow more detailed permissions.

Check ACLs:

getfacl /path/to/file

Set ACLs:

setfacl -m u:bob:rw file.txt  # Give user bob read/write

ACLs are useful when multiple users need different levels of access that the standard owner/group/others model cannot handle.

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7. Diagnosing Permission Problems

Step-by-step approach:

1. Check current permissions: ls -l file.txt
2. Check ownership: ls -l file.txt
   • Verify owner and group.
3. Check ACLs (if normal permissions are correct but access is still denied): getfacl file.txt
4. Check mount options (for network drives or external storage):
   • Some mounts can be read-only:
   mount | grep /mnt/share
5. Correct permissions or ownership:
   • Use chmod and chown as needed.

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8. IT Environment Examples

1. Web server files
   • Apache or Nginx must read website files.
   • Commands: chown www-data:www-data /var/www/html/index.html
     chmod 644 /var/www/html/index.html
2. Scripts for automated tasks
   • Must be executable by the automation user.
   • Command: chmod +x /usr/local/bin/backup.sh
3. Shared project folder
   • Multiple developers need read/write.
   • Commands: chown -R alice:developers /srv/projects
     chmod -R 770 /srv/projects

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9. Key Commands to Remember for the Exam

Task	Command Example
View permissions	ls -l filename
Change permissions	chmod 755 script.sh
Change owner	chown alice file.txt
Check ACL	getfacl file.txt
Set ACL	setfacl -m u:bob:rw file.txt
Diagnose mount permissions	`mount

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10. Exam Tips

• Always check owner, group, and permissions before making changes.
• Remember the order: diagnose → fix permissions → fix ownership → check ACLs.
• Numeric (chmod 755) and symbolic (chmod u+x) are both acceptable on the exam.
• Understand practical scenarios like web servers, scripts, shared folders.

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This covers everything necessary for Sub-topic 6.5 for your exam in a clear, IT-focused way.