Linux File Permissions & Chmod Guide

Octal permissions calculation, permission bits (rwx), sticky bits, and chown recipes

Permission Bits & Octal Values

Permissions are calculated by adding user, group, and other octal numbers

Read (r)Permission to open and read file contents or list directory

4 (Binary: 100)

Write (w)Permission to modify file or add/remove directory entries

2 (Binary: 010)

Execute (x)Permission to run script/binary or cd into directory

1 (Binary: 001)

No Permission (-)Access completely denied

0 (Binary: 000)

777 (rwxrwxrwx)Full access to everyone (Security Risk — avoid in production)

chmod 777 file

755 (rwxr-xr-x)Owner full; group and others can read & execute (Standard for web/executables)

chmod 755 script.sh

644 (rw-r--r--)Owner read & write; group and others read-only (Standard for files)

chmod 644 index.html

600 (rw-------)Owner read & write only (Sensitive config files, database credentials)

chmod 600 config.json

400 (r--------)Owner read-only (Required for SSH private keys)

chmod 400 id_rsa

Make script executableAdds execute bit for all users

chmod +x deploy.sh

Fix web directory permissions recursivelySets 755 on folders and 644 on files

find . -type d -exec chmod 755 {} +
find . -type f -exec chmod 644 {} +

Change file owner and groupRecursively change user and group ownership

chown www-data:www-data -R /var/www/html

Set Sticky Bit on directoryUsers can only delete files they own

chmod +t /tmp/shared_dir