Chmod 555 Explained
Chmod 555: everyone can read and execute but nobody can write. Immutable executables and protected directories.
Permission
555
r-xr-xr-x
chmod 555 filename
Permission Breakdown
| Role | Read (4) | Write (2) | Execute (1) | Octal | Meaning |
|---|---|---|---|---|---|
| Owner | r | - | x | 5 | read, execute |
| Group | r | - | x | 5 | read, execute |
| Others | r | - | x | 5 | read, execute |
Visual Permission Grid
Detailed Explanation
The permission 555 grants read and execute access to everyone while denying write access to all, including the owner.
Octal breakdown:
- 5 (Owner): read (4) + execute (1) = read and execute
- 5 (Group): read (4) + execute (1) = read and execute
- 5 (Others): read (4) + execute (1) = read and execute
In symbolic notation this is r-xr-xr-x. Everyone can read and execute the file, but nobody can modify it. Even the owner must change permissions before editing.
This permission is used for system directories and files that should be publicly accessible but never modified. It provides an extra layer of protection against accidental changes. Some system package managers install files with 555 to prevent local modifications that would be overwritten by updates. For directories, this means users can list contents and enter the directory but cannot create or delete files.
Use Case
Used for system-installed executables and read-only directories where modification should be prevented. Common in package-managed system files and protected CD-ROM mounts.