Users and permissions

The Unix permission model — read/write/execute, owner/group/other, special bits.

Linux accounts and groups; usermod -G without -a replaces all of a user's supplementary groups instead of adding one.

umask is a process mask: which permissions to remove from a new file (from 666) and directory (from 777).

Special permission bits: setuid and setgid run a program as its owner; setgid and sticky shape shared directories.

Flexible rights beyond rwx — getfacl/setfacl, per-user and per-group grants.

The namei command walks a path part by part and shows the type, mode and owner of each directory down to the file.

The useradd command creates a user account with a home directory, shell, groups and UID; passwd sets the password.

The passwd command changes your own password; as root it sets, locks and shows the passwords of other users.

The usermod command changes an existing account — groups, login shell, home directory, name and locking.

Changing permissions (chmod) and owner/group (chown) — numeric and symbolic syntax, umask, chgrp.

Reads and changes local password aging settings and an account expiration date.

A login-shell replacement that reports refusal and exits instead of opening an interactive shell.