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set `ping -qc3 xxx.xxx.xxx.xxx 2>/dev/null | grep transmitted`

From: rick
Subject: set `ping -qc3 xxx.xxx.xxx.xxx 2>/dev/null | grep transmitted`
Date: Tue, 30 Oct 2001 23:35:32 -0500

I cut/paste this from the ppp-howto, the next line is my question.
if [ $4 -gt 0 ];

here's what I understand:    ping  and piping the output to grep
looking for "transmitted" with stderr going to null all inside execute
if the 4th item (should be a 3 if the interface is up) exists and has
its group id bit set(-g) and if the file descriptor is open and refers
to a terminal (-t) returns true.
my confusion:
what did I "set"? everything is inside "execute quotes"
It must be a file, because it must exsist and the fd is open , then
where is it, can I look at it?
what is the "0' for.
I don't understand this piece of code, I would like to.
Thanks for your help.
Pardon me if this is the wrong group, I did a search for bash.

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