Permissions

Every file has read/write/execute permissions for its owner, its group, and everyone else. ls -l shows them as a string like -rwxr--r--.

Example: making a script executable

$ chmod 755 deploy.sh
$ chown ada:ada deploy.sh
$ ls -l deploy.sh

chmod 755 sets permissions using three digits, owner (7 = read+write+execute), group (5 = read+execute), everyone else (5 = read+execute). chown ada:ada changes who owns the file. Misconfigured permissions are a genuine, common source of real security vulnerabilities, a config file containing secrets that's readable by "everyone" when it should only be readable by its owner is a classic, easily-overlooked mistake, and exactly the kind of thing a security assessment specifically checks for.