Linux File Permissions Explained: chmod, chown, and ACLs for System Admins

File permissions are the foundation of Linux security. Every file and directory has an owner, a group, and a set of permissions.

Permission Notation

ls -la /var/www/
drwxr-xr-x  5 www-data www-data 4096 Mar 18 html
-rw-r--r--  1 www-data www-data  612 Mar 18 index.html

chmod

chmod 755 script.sh
chmod 644 config.ini
chmod 600 id_rsa
chmod u+x script.sh
chmod g+w shared-file.txt

chown

sudo chown www-data:www-data /var/www/html/ -R
sudo chgrp developers /opt/project/ -R

Special Permissions

chmod 4755 /usr/bin/passwd   # SUID
chmod 2775 /opt/shared/      # SGID
chmod 1777 /tmp/             # Sticky bit
find / -perm -4000 -type f 2>/dev/null

POSIX ACLs

setfacl -m u:john:rwx /opt/project/
getfacl /opt/project/
setfacl -d -m g:developers:rwx /opt/project/

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