UNIX for Advanced & Expert Users

(Apologies for any typos.)
Can someone help me with this one please.
I am writing a snippet that will come on here soon to work on OSX, Linux and CygWin but I hit this!

I hope this is lucid enough because I really do not understand what is going on.

Take these few lines:-

Code:

#!/bin/sh
# OR #!/usr/local/bin/dash
> /tmp/noise.raw
> /dev/null 2>&1 cat /tmp/noise.raw > /dev/dsp

And tested in ShellCheck gives this:-

Code:

$ shellcheck myscript
 
Line 3:
> /tmp/noise.raw
^-- SC2188: This redirection doesn't have a command. Move to its command (or use 'true' as no-op).

$

OK so I add a NOP:-

Code:

#!/bin/sh
# OR #!/usr/local/bin/dash
: > /tmp/noise.raw
> /dev/null 2>&1 cat /tmp/noise.raw > /dev/dsp

And tested in ShellCheck gives NO error:-

Code:

$ shellcheck myscript
No issues detected!

$

Why is a redirection to > /dev/null allowed to be a _direct_ redirection and yet : > /tmp/noise.raw requires a NOP?

Next question is this line, > /dev/null 2>&1 cat /tmp/noise.raw > /dev/dsp actually works to hide an OSX error, paraphrasing, 'I don't have permssion to access this' which is obvious as OSX does not have '/dev/dsp'.
This line also runs correctly on CygWin with > /dev/null 2>&1 at the start. So it looks like I have done something correct but don't know why?

TIA.

Barry.