Every security incident eventually touches a file: a configuration, a credential store, a log, a payload. Who could read it, who could change it, and who could run it are not academic questions. They are the difference between a contained mistake and a full compromise.
This lesson explains the permission model you will meet on Linux, macOS and, in a different form, Windows.
Paths: absolute and relative
An absolute path starts at the root of the file system:
- Linux/macOS:
/home/alice/notes.txt - Windows:
C:\Users\alice\notes.txt
A relative path starts from where you currently are: notes.txt or ../backup/notes.txt. The symbols . and .. mean "this directory" and "parent directory".
Users, groups and root
A user account has a username, a numeric user ID (UID) and a home directory. A group bundles users so permissions can be granted once.
On Linux and macOS, the superuser is root with UID 0. Root bypasses permission checks. sudo lets a permitted user run a single command with elevated privileges, and it logs that action.
Windows uses Security Identifiers (SIDs) and groups such as Administrators. Elevation is handled through UAC and access tokens rather than a single root account.
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