Changing and diagnosing permissions

Articles

• bash — the command shell — A command interpreter for interactive input, builtins, and shell scripts.
• 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.
• namei — permissions at every step of a path — The namei command walks a path part by part and shows the type, mode and owner of each directory down to the file.
• 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.
• Processes — Viewing processes, signals and priorities; finding the processes that hog CPU or memory.
• 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.
• systemd — System and service manager that manages units and their dependencies.
• tee — output to a file and the screen — Splits a stream: writes to a file AND shows it on screen; sudo tee to write into protected files.