+- TalkativeTurtles (https://talkativeturtles.club)
+-- Forum: Technology (https://talkativeturtles.club/forumdisplay.php?fid=2)
+--- Forum: Operating Systems & Linux (https://talkativeturtles.club/forumdisplay.php?fid=13)
+--- Thread: Linux permissions explained - chmod, chown, and when things go wrong (/showthread.php?tid=102)
Linux permissions explained - chmod, chown, and when things go wrong - Zero Two - 06-22-2026
File permissions trip up everyone at some point. Here's a solid mental model.
The permission model
Every file has three permission sets: owner, group, others. Each set has three bits: read (4), write (2), execute (1).
-rwxr-xr-- 1 alice developers 4096 Jun 22 file.sh
- rwx = owner (alice) can read, write, execute
- r-x = group (developers) can read and execute, not write
- r-- = others can only read
chmod - changing permissions
Code:
chmod 755 file.sh # rwxr-xr-x (owner all, group+others read+execute)
chmod 644 file.txt # rw-r--r-- (owner read+write, others read only)
chmod 600 secret.key # rw------- (owner only, common for SSH keys)
chmod +x script.sh # add execute for everyone
chmod -w file.txt # remove write for everyone
chmod -R 755 /var/www # recursivechown - changing ownership
Code:
chown alice file.txt # change owner
chown alice:developers file.txt # change owner and group
chown -R www-data:www-data /var/www # recursive (common for web servers)The execute bit on directories
For directories, execute means "can enter" (traverse). A directory with r-- but no x lets you list contents but not access files inside. Usually you want r-x together on directories.
Common permission mistakes:
- chmod 777 on web-served files - never. World-writable is a security hole.
- Wrong owner on web root (nexusclade instead of www-data) - PHP-FPM can't write uploads
- SSH private key too permissive - SSH refuses keys that aren't 600
- Forgetting -R on a directory change and wondering why subdirectory access still fails