File Permissions & Ownership

Intermediate Linux Fundamentals
chmodchownchgrpumaskgetfaclsetfacl

Objective

Manage Unix file permissions using symbolic and numeric notation, set ownership, configure default permissions with umask, and use ACLs for fine-grained control.

Tools & Technologies

  • chmod
  • chown
  • chgrp
  • umask
  • getfacl
  • setfacl

Key Commands

chmod 750 script.sh
chown user:group file
umask 022
setfacl -m u:bob:rw file
getfacl file

Architecture Overview

graph TD subgraph Permission Bits P[rw-r--r--\n644] --> O[Owner\nrw-\n6] P --> G[Group\nr--\n4] P --> W[World\nr--\n4] end subgraph Calculation O2[r=4 w=2 x=1] G2[7=rwx 6=rw- 5=r-x 4=r--] end style P fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0

Step-by-Step Process

01
Read the Permission String

The ls -l output shows permissions as a 10-character string. Learn to decode it.

ls -l file.txt
# -rw-r--r-- 1 alice staff 1234 Jan 1 file.txt
# │└┬┘└┬┘└┬┘
# │ │  │  └── world: r--
# │ │  └───── group: r--
# │ └──────── owner: rw-
# └────────── type: - file, d dir, l link
02
chmod — Numeric Mode

Calculate permissions: r=4, w=2, x=1. Add digits for owner/group/world.

chmod 755 script.sh  # rwxr-xr-x
chmod 644 data.txt   # rw-r--r--
chmod 600 secret.key # rw-------
chmod 777 public/    # rwxrwxrwx (avoid!)
03
chmod — Symbolic Mode

Use symbolic notation for relative changes without knowing the current value.

chmod +x script.sh      # add execute for all
chmod u+x,g-w file      # owner +x, group -w
chmod o= file           # remove all world perms
chmod a=r file          # all can only read
04
chown & umask

Change file ownership and set the default permission mask for new files.

chown alice file.txt          # change owner
chown alice:devs file.txt     # owner and group
chgrp devs file.txt           # change group only

umask                         # show current mask
umask 027                     # new default: 750 dirs, 640 files

Challenges & Solutions

  • chmod -R on / is catastrophic — always double-check recursive commands
  • setuid/setgid on shell scripts is ignored by Linux kernel

Key Takeaways

  • Default permissions = 666 (files) or 777 (dirs) minus umask
  • ACLs override standard permissions for named users/groups