HP-UX

Null is "Nothing"

Zero is actually a device....

On HP-UX 10.20, both /dev/zero and /dev/null have the same major number. That means that the same exact driver is invoked when either file is opened. That driver will look at the minor number which is how it know to behave differently. This driver is a psuedo-driver since it controls no hardware.

A read to /dev/null returns end of file.

A read to /dev/zero succeeds and returns all zeroes.

A simple way of looking at it is to say:

cat /dev/null > my_file
will make my_file zero-length, whereas

cat /dev/zero > my_file
will fill the file with NULL characters...

$ dd if=/dev/zero of=/my/home/test_file bs=1 count=10
$ xd test_file
0000000 0000 0000 0000 0000 0000
000000a
$

Be carefull not to "cat" zero zero to anywhere, lest your filesystem fill up relatively quickly...