chmod & chown on Linux | Permissions Guide

Permission errors look like “Permission denied” on a script, a web server that cannot read a site root, or an SSH client refusing a private key.
The fix is almost always the triplet user / group / other plus who owns the file.

This guide ties into SSH keys (private keys should be 600) and
Nginx document roots (often 755 dirs, 644 files).

Read ls -l

The first column is the mode string; the third and fourth columns are owner and group. Directories need the execute bit to be entered.

bashinspect

ls -l /var/www/html/index.html
ls -ld /var/www/html

Symbolic chmod

Add or remove bits for user (u), group (g), others (o), or all (a). +x adds execute; -w removes write.

bashscripts and keys

chmod u+x deploy.sh
chmod 600 ~/.ssh/id_ed25519

Octal Mode (755, 644, 600)

Three digits are owner, group, other. Each digit sums read (4), write (2), execute (1). This is what tutorials mean by “chmod 755”.

bashcommon modes

chmod 755 /usr/local/bin/myapp
chmod 644 /var/www/html/*.html

Note755 on a directory means you can cd into it; without x on a dir, listing may work but entering fails.

Recursive chmod

Web trees often need find plus two chmod passes: directories 755, files 644. Avoid chmod -R 777 except in disposable lab VMs.

bashweb root pattern

find /var/www/mysite -type d -exec chmod 755 {} +
find /var/www/mysite -type f -exec chmod 644 {} +

chown: Change Owner and Group

Only root can give away files to another user. Use user:group syntax; -R walks the tree.

bashownership

sudo chown www-data:www-data -R /var/www/mysite

Special Bits (Brief)

setuid, setgid, and sticky appear as extra letters in ls -l (e.g. drwxrwxrwt on /tmp). Change these only when you understand the security impact.

bashspot sticky

ls -ld /tmp

Verify the Full Path

namei -l shows permissions on every component of a path — useful when a parent directory blocks access even if the file mode looks correct.

bashpath check

namei -l /var/www/mysite/index.html
stat -c '%a %U:%G %n' /var/www/mysite/index.html

Quick Reference

Dirs often 755; files 644; SSH private keys 600
chown user:group for app ownership
namei -l when parent perms block access

chmod and chown on Linux: File Permissions, Octal Mode and Ownership