Linux File Permissions & Chmod: The Complete Octal and Security Guide

1. The Foundations of Unix & Linux Permissions

In Unix-based operating systems, every filesystem object (regular files, directories, sockets, symbolic links, and device nodes) is governed by an ownership and access control triplet: User (owner), Group, and Others.

Each of these three roles has three discrete permission flags: Read (r), Write (w), and Execute (x). When displayed in the terminal with 'ls -l', permissions appear as a 10-character string such as '-rwxr-xr-x', where the leading character represents the file type ('-' for regular files, 'd' for directories).

2. The Octal Binary Mathematics Behind Chmod

The octal system represents three binary bits (2^2, 2^1, 2^0) with numeric values 4, 2, and 1. Combining these values allows any permission set to be expressed as a single digit from 0 to 7:

• Read (r) = 4 (binary 100) • Write (w) = 2 (binary 010) • Execute (x) = 1 (binary 001)

Adding these weights produces standard modes: 7 (4+2+1 = rwx), 6 (4+2 = rw-), 5 (4+1 = r-x), and 4 (4 = r--). A mode like 755 translates directly to Owner=7 (rwx), Group=5 (r-x), Others=5 (r-x).

3. Advanced Permission Bits: SUID, SGID, and Sticky Bit

In addition to standard user/group/other bits, Linux supports three special execution bits represented by a 4-digit octal prefix:

1. SUID (Set User ID, 4000): When set on an executable file, the process executes with the privileges of the file owner rather than the calling user (e.g. /usr/bin/passwd).

2. SGID (Set Group ID, 2000): In directories, files created within inherit the group ownership of the directory rather than the primary group of the creating user. Critical for collaborative folders.

3. Sticky Bit (1000): Indicated by a trailing 't' in symbolic mode (e.g. 1777 on /tmp). Prevents non-root users from deleting or renaming files owned by other users within shared directories.