Now here's an absolutely amazing idea for a shell feature

[Guy Harris]

>Don't copy the file!  Keep track of the length and truncate (BSD has
>a syscall, AT&T has a ioctl/fctnl,

More properly, System V Release 3.1 and later have an F_FREESP "fcntl"
(no "ioctl") that is undocumented.  Earlier S5 releases do not have
this.

It may be documented in S5R4, although it's not in the Third Edition of
the SVID.  There may be an BSD-compatible "ftruncate()" wrapper provided
as well.