mount, umount \- mount and dismount file system
announces to the system that a removable file system
must be a directory (unless the root of the
mounted file system is not a directory).
It becomes the name of the newly mounted root.
The optional last argument indicates that the file system
is to be mounted read-only.
announces to the system that the removable file system previously
maintain a table of mounted devices.
If invoked without an argument,
Physically write-protected and magnetic tape file
systems must be mounted read-only
or errors will occur when access times are updated,
whether or not any explicit write is attempted.
Mounting file systems full of garbage will crash the system.
Mounting a root directory on a non-directory
makes some apparently good pathnames invalid.