When cannot find the main source file, it doesn't quit. For example, when I typed mf fred, said:
This is METAFONT ...
**fred
! I can't find file `fred.mf'.
<*> fred
Please type another input file name:
The usual program interrupts (eg, Control-C) don't work here,
and the `Please type ...' prompt does not understand
commands: it will read only the first word, and insist on
interpreting this as a file name.
Beginners faced with this often wonder how to avoid an endless loop or a reboot, or try to think of a file that they do have in 's path. In the latter case, the canonical name to use is `null', standing for the file `null.mf'.
In fact, the solution is much easier: on the systems that I have tried, a simple end of file marker (`control-Z' in MS-DOS, `control-D' in UNIX) stops in its tracks:
! Emergency stop.
<*> fred
End of file on the terminal!