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Full Discussion: Redirection? (I do find some of the strangest things.)
Top Forums UNIX for Advanced & Expert Users Redirection? (I do find some of the strangest things.) Post 302989215 by wisecracker on Tuesday 10th of January 2017 04:41:43 AM
Old 01-10-2017
Redirection? (I do find some of the strangest things.)
(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.
 

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LEARN ABOUT NETBSD

paths

PATHS(3)						   BSD Library Functions Manual 						  PATHS(3)

NAME

     paths -- default system paths

SYNOPSIS

     #include <paths.h>

DESCRIPTION

     The <paths.h> header defines some default paths used in NetBSD.  All defined constants are prefixed with _PATH.  The constants include:

	   o   The default user search path set by login(1) and associated utilities such as rshd(8).  This is defined by _PATH_DEFPATH and
	       includes directories such as /usr/bin and /usr/local/bin.

	   o   Default paths for some utilities and device nodes.  Examples include such paths as /dev/null (_PATH_DEVNULL), /dev/mem (_PATH_MEM),
	       and /etc/nologin (_PATH_NOLOGIN), among others.

	   o   Paths for some default directories such as /dev (_PATH_DEV) and /tmp (_PATH_TMP) as well as paths for some miscellaneous utilities
	       such as csh(1), /bin/csh (_PATH_CSHELL).

SEE ALSO

     whereis(1), sysexits(3), types(3), hier(7)

CAVEATS

     The <paths.h> header is specific to NetBSD.

BSD
								   April 8, 2011							       BSD
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