chmod 644 Explained — Owner Read-Write, Others Read-Only

What Does chmod 644 Mean?

Permission 644 is the default for most regular files on Linux systems. It provides a sensible balance between usability and security:

Role	Octal	Symbolic	Permissions
Owner	6	rw-	Read + Write
Group	4	r--	Read only
Others	4	r--	Read only

Breaking Down the Digits

• 6 (Owner): Read (4) + Write (2) = 6. The owner can view and modify the file.
• 4 (Group): Read (4) only. Group members can view but not change the file.
• 4 (Others): Read (4) only. Everyone else can view the file.

Where Is 644 Used?

# Regular files
chmod 644 index.html
chmod 644 styles.css
chmod 644 config.json

# System configuration (non-sensitive)
ls -l /etc/hostname
# -rw-r--r-- 1 root root ... /etc/hostname

# Web content files
chmod 644 /var/www/html/*.html

Why Not 666?

Permission 666 (rw-rw-rw-) allows everyone to write to the file, which is almost never desired. Using 644 ensures that only the owner can make modifications while the file remains readable by scripts, web servers, and other users.

644 and umask

On most Linux systems, the default umask is 022, which means newly created files get 666 & ~022 = 644. This is why 644 appears so frequently — it is the system default for new files.