Why is it necessary to type "host" before the imp command when I'm using SQL*Plus to import a dmp file into Oracle? In SQL*Plus, I'm connected to an Oracle 11g database. When I type

imp help=y


It gives me

SP2-0042: unknown command "imp help=y" - rest of line ignored.


But when I type

host imp help=y


it gives mehelp with the import utility.

All the guides I've seen online for importing say to just type imp, without the host part. Why do I need to say host before using the utility?
imp is an operating system command, not an SQL*Plus command, so it should be run from the OS shell command line. If you insist on running it from SQL*Plus you need to use the SQL*Plus host command to send imp to the host OS for execution.