On Sep 11, 2013, at 6:06 AM, Weijun Wang <[email protected]> wrote:

> It seems there needs a pair of " around `. Don't why. Maybe it's about the 
> pipes?

This command
  test `cat /dev/null` != 1

expands after command substitution to a command with two arguments:
  test != 1

That gets a complaint from test.

If you add double quotes around the backquotes, then the following two commands 
are equivalent:
  test "`cat /dev/null`" != 1
  test "" != 1

...and they give the desired "true" result, because there are three arguments, 
the first of which is a null string.

— John

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