A shared directory: setgid, sticky, and umask

Articles

• ls — list directory contents — The ls command lists files and directories with permissions, owner, size and modification time, hidden ones too.
• stat — file metadata — Full file information — size, permissions, owner, timestamps (atime/mtime/ctime), inode.
• File permissions — The Unix permission model — read/write/execute, owner/group/other, special bits.
• Users and groups — Linux accounts and groups; usermod -G without -a replaces all of a user's supplementary groups instead of adding one.
• chmod / chown — permissions and ownership — Changing permissions (chmod) and owner/group (chown) — numeric and symbolic syntax, umask, chgrp.
• umask — the mask for new files and directories — A process's mask for new files: its bits are removed from 666 for a file and 777 for a directory. With 022 a file gets 644 and a directory 755; with 002 the group keeps write access.
• Special permission bits: setuid, setgid, sticky — Special permission bits: setuid and setgid run a program as its owner; setgid and sticky shape shared directories.
• sudo — running commands as another user — Grants temporary root rights via sudoers; edit the file only through visudo, or a mistake locks everyone out of sudo.