Understanding Linux Permissions: The Basics

Why Permissions Matter

Permissions protect your system from accidents and mischief. Understanding them keeps your apps happy and your logs quiet.

The Three Characters

Users: owner, group, others. For each you can grant read, write, xecute.

-rwxr-x---  1 app app  4096 Feb 20  index.js

Symbolic vs Numeric Modes

Two ways to set permissions.

• Symbolic: chmod u=rw,g=r,o= file
• Numeric: chmod 640 file where 6=r+w, 4=r, 0=none

Common Recipes

• Make a script executable:
  chmod u+x deploy.sh
• Secure a private key:
  chmod 600 ~/.ssh/id_rsa
• Web directory typical perms:
  find /var/www/site -type d -exec chmod 755 {{}} and find /var/www/site -type f -exec chmod 644 {{}}

Ownership with chown and chgrp

Ownership defines who gets the keys.

chown -R app:app /var/www/site

Setuid, Setgid, and Sticky Bit

Special modes with special powers.

• setuid (4): run as file owner. Example: chmod 4755 /usr/bin/somebin
• setgid (2): run as group or inherit group on dirs. Example: chmod 2755 /shared
• sticky (1): only owner may delete in world writable dir. Example: chmod 1777 /tmp

ACLs for Fine Grained Control

When traditional bits are not enough, use ACLs.

setfacl -m u:berty:rwx /data/reports
getfacl /data/reports

Troubleshooting Checklist

• Check the mode: ls -l
• Check ownership: ls -l then chown
• Check parent directory execute bit
• Watch out for umask