DevOps & Linux5 min read•Updated 2026-09-04
Linux File Permissions (chmod) Explained: Octal vs Symbolic Calculator
Master Linux read, write, and execute permissions. Understand 777, 755, 644, and 600 with interactive conversion.
MX
MultiToolX Technical Team
DevOps & Infrastructure
Key Takeaways
- Each digit in an octal permission represents the sum of Read (4), Write (2), and Execute (1).
- Standard web server permissions: 755 (rwxr-xr-x) for directories and 644 (rw-r--r--) for files.
- Never run chmod 777 in production; it grants world-writable permissions to all local users.
1. How Chmod Octal Numbers Are Calculated
The 3 octal digits correspond to three security classes:
1st digit: Owner (User)
2nd digit: Group
3rd digit: Others (World)
Each digit is formed by adding:
- Read (r) = 4
- Write (w) = 2
- Execute (x) = 1
Example: 755 = (4+2+1=7) for Owner, (4+1=5) for Group, (4+1=5) for Others.
Frequently Asked Questions
What permission should SSH private keys (.pem / id_rsa) have?
SSH private keys must be set to chmod 600 or chmod 400 (read-only for the owner), or OpenSSH will refuse the connection.
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