Permissions and ownership

Running ls -l reveals a file's full permission and ownership information:

Breaking down -rwxr-xr--: the leading character is the file type (- for a regular file, d for a directory); the next nine characters are three groups of three — permissions for the owner (alice), the group (developers), and everyone else, in that order.

chmod sets permissions, either symbolically or numerically. The numeric form represents each permission group as a single digit (read=4, write=2, execute=1, summed):

chmod 777 grants full read, write, and execute permission to literally everyone — a "fix" often reached for when something fails with "Permission denied" and the actual cause hasn't been diagnosed. It works, in the narrow sense that the error goes away — but it also means any user or process on the system (and on a shared or compromised server, potentially an attacker) can modify or execute that file freely. The better instinct: figure out the actual required permission (often much narrower — the specific user or group that legitimately needs access) rather than reaching for "everyone, everything" as a shortcut.

chown changes who owns a file — relevant when a file was created by one user (or process, like a web server running as its own dedicated user) but needs to be owned by another:

sudo ("superuser do") runs a single command with elevated (administrator) permissions, rather than requiring you to be permanently logged in as an all-powerful root user for your entire session:

Using sudo per-command rather than working as root all the time is a deliberate safety practice — it means a mistake in an ordinary command doesn't have unrestricted system-wide permission to cause damage, only the specific command you explicitly elevated does.