| Numeric | Symbolic | Usage |
|---|---|---|
| 755 | rwxr-xr-x | Standard directories, executables |
| 644 | rw-r--r-- | Standard files, web pages |
| 600 | rw------- | SSH keys, private files |
| 777 | rwxrwxrwx | Full access (not recommended) |
| 400 | r-------- | Read-only for owner |
Unix file permissions control who can read, write, and execute files and directories. Each file has three permission groups: owner, group, and others. Each group can have read (4), write (2), and execute (1) permissions. The numeric notation sums these values — 755 means owner has full access (7=4+2+1), while group and others can read and execute (5=4+1). The chmod command changes these permissions. Setting correct permissions is critical for server security — misconfigured permissions are one of the most common causes of security vulnerabilities in web hosting.