Linux File Permission Numbers Explained: From drwxr-xr-x to 755

Understanding Linux File Permission Numbers

In Linux, file permissions are commonly displayed as nine characters (e.g., drwxr-xr-x), but they can also be set using a three-digit numeric code. This numeric representation is fundamental for efficient permission management.

How Permission Numbers Are Constructed

Permissions are defined for three user categories:

• Owner (User): The file's creator/owner.
• Group: Users belonging to the file's group.
• Others: All other users on the system.

Each category can have three types of access, each with a numeric value:

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1

By adding the values for the desired permissions for each user category, you get a number from 0 to 7. The three category numbers form the three-digit permission code.

Common Permission Number Examples

Here are frequently used numeric permissions and their symbolic equivalents:

444  r--r--r--  Read-only for all users.
600  rw-------  Read & write for owner only.
644  rw-r--r--  Owner: read/write; Group & Others: read-only.
666  rw-rw-rw-  Read & write for all (no execute).
700  rwx------  Full access for owner only.
744  rwxr--r--  Owner: full; Group & Others: read-only.
755  rwxr-xr-x  Owner: full; Group & Others: read & execute.
777  rwxrwxrwx  Full access (read, write, execute) for everyone.

The leading character in symbolic notation (- for a regular file, d for a directory) indicates the file type. The numeric code itself does not specify type.

Calculating Numeric Permissions

Take 755 as an example:

• The first digit 7 (4+2+1) is for the Owner: rwx (read, write, execute).
• The second digit 5 (4+0+1) is for the Group: r-x (read, execute, no write).
• The third digit 5 (4+0+1) is for Others: r-x (read, execute, no write).

Thus, the command chmod 755 filename sets these exact permissions.

Setting Permissions with chmod

The basic syntax for the chmod command is:

chmod [options] numeric_mode file_or_directory

The numeric_mode is the three-digit number. For example:

• chmod 777 file grants read, write, and execute to all users.
• chmod 644 file gives the owner read/write, and others read-only.

Special Permission Bits (The Fourth Digit)

You may encounter a four-digit code like 4755. The leading digit sets special flags:

• 4 (SUID): Set User ID. When an executable with SUID is run, it executes with the file owner's privileges, not the runner's. Warning: Use with caution due to security implications.
• 2 (SGID): Set Group ID. On a directory, new files inherit the directory's group.
• 1 (Sticky Bit): On a directory (e.g., /tmp), only the file owner or root can delete/rename files within it.

Mastering numeric permissions allows for quick and precise file and directory management with the chmod command.