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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 SUSE

mrtg-squid

MRTG-SQUID(1)							       mrtg							     MRTG-SQUID(1)

NAME

       mrtg-squid - using mrtg to monitor Squid

DESCRIPTION

       Squid 2.3 knows SNMP and you can therefore use mrtg to monitor it quite easily.

       I have made some modifications to mrtg which simplify this.  My work is based on earlier modification made by: matija.grabnar@arnes.si and
       kostas@nlanr.net.

MODIFICATIONS

       I added new code for displaying correct units to the previous patches "perminute" and "perhour" ("option" tokens), which allows other mea-
       surement in addition to "persecond".

       Then I created a new option token "dorelpercent" which allows the calculation of the percentage of IN-stream / OUT-stream on the fly and
       then displays it on a fixed scale from 0% to 100%. For my requirements, this does good work. Maybe someone wants a floating scale. It
       should not be a problem to implement it, too (but give me an option to keep my fixed scale). If IN-stream is always less than OUT-stream
       both lines (OUT-stream and relative percent) are always displayed on top of IN-stream bulk. Otherwise this option makes no sense. With this
       option you can display hitrates, errorrates (for router monitoring: rel. droprates) easily now.

       If you use this options please consider that you need a 5th colourname/value pair in your Colours statements!

       Due to some discussion on this list, I have implemented two tokens too:

       "kilo" and "kMG"

       "kilo" should contain the value of k (1000 or 1024), where 1000 is the default.

       "kMG" is a comma separated list of multiplier prefixes, used instead of "", "k", "M", "G", "T" on the MRTG display. Leave the place free,
       if you want no prefix.

       Also an incomplete list of OIDs for the new SQUID release is added.

       I hope you enjoy it.

CONFIG EXAMPLE

       You can measure responsetimes in ms and display it with MRTG correctly with:

	kMG[measure-ms]: m,,k,M,G,T
	short[measure-ms]: s

       You can display now MB/s as 1024*1024 B/s with:

	kilo[volume]: 1024

       A sample config for squid:

	Target[proxy-hit]: cacheHttpHits&cacheProtoClientHttpRequests:public@proxy
	Title[proxy-hit]: HTTP Hits
	PageTop[proxy-hit]: <H2>proxy Cache Statistics: HTTP Hits / Requests</H2>
	Suppress[proxy-hit]: y
	LegendI[proxy-hit]:  HTTP hits
	LegendO[proxy-hit]:  HTTP requests
	Legend1[proxy-hit]:  HTTP hits
	Legend2[proxy-hit]:  HTTP requests
	YLegend[proxy-hit]: perminute
	ShortLegend[proxy-hit]: req/min
	Options[proxy-hit]: nopercent, perminute, dorelpercent

	Target[proxy-srvkbinout]: cacheServerInKb&cacheServerOutKb:public@proxy
	Title[proxy-srvkbinout]: Cache Server Traffic In / Out
	PageTop[proxy-srvkbinout]: <H2>Cache Statistics: Server traffic volume (In/Out) </H2>
	Suppress[proxy-srvkbinout]: y
	LegendI[proxy-srvkbinout]:  Traffic In
	LegendO[proxy-srvkbinout]:  Traffic Out
	Legend1[proxy-srvkbinout]:  Traffic In
	Legend2[proxy-srvkbinout]:  Traffic Out
	YLegend[proxy-srvkbinout]: per minute
	ShortLegend[proxy-srvkbinout]: b/min
	kMG[proxy-srvkbinout]: k,M,G,T
	kilo[proxy-srvkbinout]: 1024
	Options[proxy-srvkbinout]: nopercent, perminute

AUTHOR

       Andreas Papst <andreas.papst@univie.ac.at> Dirk-Luder Kreie <deelkar@gmx.de>

2.16.2								    2008-05-16							     MRTG-SQUID(1)
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