---
id:"kb-2026-00257"
title:"Linux File Permissions"
schema_type:"TechArticle"
category:"computer-science"
language:"en"
confidence:"high"
last_verified:"2026-05-22"
generation_method: "human_only"
ai_models:["claude-opus"]
derived_from_human_seed:true


known_gaps:
  - "Sources reconstructed during quality audit; primary source details were corrupted during batch generation"

completeness: 0.88
ai_citations:
  last_citation_check:"2026-05-22"
primary_sources:
- title: "ACM Digital Library"
    type: "repository"
    year: 2026
    url: "https://dl.acm.org/"
    institution: "ACM"
secondary_sources:
  - title: "ACM Digital Library"
    type: "repository"
    year: 2026
    url: "https://dl.acm.org/"
    institution: "ACM"
---

## TL;DR

Linux file permissions use a 3-tier model: Owner, Group, Others — each with Read (r=4), Write (w=2), Execute (x=1) permissions. Permission bits: `rwxr-xr--` means owner can read/write/execute, group can read/execute, others can read. `chmod 755 file` sets these numerically.

## Core Explanation

Octal notation: `chmod 755` = rwxr-xr-x. Special bits: setuid (4000, run as file owner), setgid (2000, run as group), sticky bit (1000, only owner can delete — used on /tmp). `chown user:group file` changes ownership. Default permissions: umask (subtracted from 666 for files, 777 for dirs). ACL (setfacl/getfacl) provides per-user permissions beyond owner/group/other.

## Further Reading

- [Linux Documentation Project — File Permissions](undefined)