Linux File Permissions And Chmod Patterns

Owner: rwx (7).
Group: r-x (5).
Other: r-- (4).
Octal: 754.

rw-r--r-- — owner read+write, group read, others read.

rwxr-xr-x — typical for executables and directories.

rw------- — typical for private files like ~/.ssh/id_ed25519.

rwx------ — typical for ~/.ssh directory.

SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check.

Adds the execute bit for the file's owner only.

setgid on directory: new files inherit group of dir.
sticky bit: only owner can delete files in dir (like /tmp).

chmod u+s /usr/bin/passwd — runs as file owner (root) regardless of caller. Use with care.

4755.

find . -type d -exec chmod 755 {} \; -o -type f -exec chmod 644 {} \;

chown user:group path; add -R for recursive.