How does #!/bin/sh work ? Why does it sometimes not ?

[Ian Watson]

I've seen loads of shell scripts start with 
#!/bin/sh
as the first line.

I understand that the C shell sees this and knows to execute it as a Bourne
shell script.  I've tried this on both HP-UX 7.0 (which works as I expect) and
SCO Unix V.3.2 (which doesn't).  I can't seem to get SCO's csh to run the
script through the Bourne shell.  Is this feature of the C shell standard
Unix, and am I doing something wrong ?

Why can't I find this feature mentioned in TFM for either HP-UX or SCO Unix ?
Am I just not looking hard enough / in the right place ?

Also, on the HP-UX, I notice that

$ csh script
and
$ csh < script
run the script in the C shell, but
$ csh
followed by
% script
does what I want (runs script through Bourne shell).  Can anyone explain under
what circumstances the #!/bin/sh line is or is not honoured by the C shell ?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ian Watson, HP Pinewood Information Systems Division, England.
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