Technical Deep-Dive: Chmod 755 (-rwxr-xr-x)
Bitwise binary arithmetic, POSIX DAC rules, and security guidelines for production servers.
1. Binary Calculation Mechanics
POSIX file systems represent permissions in triplets of binary bits where Read is 4 (2^2), Write is 2 (2^1), and Execute is 1 (2^0). In chmod 755, each digit computes as:
- • Owner: 7 (Read Write Exec)
- • Group: 5 (Read Exec)
- • Others: 5 (Read Exec)
2. Practical CLI Execution
Apply this configuration to files or directories with the standard Linux terminal commands:
Frequently Asked Questions
Why is chmod 755 necessary for Linux web directories (/var/www)?
In Linux, reading the contents of a directory requires the Read (r) bit, but entering or traversing into a directory requires the Execute (x) bit. With chmod 755 (rwxr-xr-x), web server daemons like Nginx or Apache (running as www-data) can traverse directories and serve files without needing write access, preventing web visitors from modifying your server files.
chmod 755 vs 644: What is the difference?
Chmod 755 includes the execute bit (x) for Owner, Group, and Others, making it suitable for directories and executable binaries/scripts. Chmod 644 (rw-r--r--) omits the execute bit entirely, making it the secure standard for static content like HTML, CSS, JavaScript, images, and config files where script execution must be prevented.
How do I recursively apply 755 to directories only?
Run: find /path/to/folder -type d -exec chmod 755 {} +. This safely updates directories without erroneously marking text or media files as executables.