== One irritation in _xargs_'s interface

Xargs is generally a nice command that more or less works right.
Some people could criticize Unix for needing it so much (which is
mostly [[a product of command line length limitations
ExecEnvironmentIssue]]) and the need for _-0_ is a bit annoying,
but on the whole it's good. But _xargs_ has one little corner case
that is really annoying; as a bonus, it's even non-portable in an
irritating way.

Here it is, presented in illustrated form:
.pn prewrap on
>  $ xargs echo does run </dev/null

Now the question: will this produce any output? In other words, does
_xargs_ run the command once *even if there are no (extra) arguments
to give to it*?
The answer is that it does in some but not all versions of _xargs_:

* Solaris 10 runs _echo_ once and has no option to disable this.
* GNU findutils _xargs_ (commonly used on Linux) normally runs _echo_
  once but can turn this off with _-r_ aka _--no-run-if-empty_.
* FreeBSD doesn't run _echo_ and has no option to change this.
  Recent versions accept _-r_ for compatibility with GNU _xargs_;
  old versions don't.
* OpenBSD runs _echo_ once but can turn this behavior off with _-r_.
* Mac OS X doesn't run _echo_ and has no _-r_ argument.

Based on the current manpage, NetBSD _xargs_ behaves the same as
FreeBSD _xargs_ (including accepting a do-nothing _-r_ argument).

[[The Single Unix Specification for _xargs_
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/xargs.html]] is
rather ambiguous about what behavior is allowed or required; it
certainly never definitely states things either way (and it has no
_-r_ argument). My close reading leads me to believe that SuS
probably requires _xargs_ to run _echo_ once, but only by implication.
This would match what I believe is historical behavior (as suggested
by Solaris, which is very historical). I assume that at some point
FreeBSD decided that this historical behavior was a bad idea and
changed it.

My view is that (historical) _xargs_ behavior is stupid and is a bear
trap waiting to bite you in unusual situations. You almost never
want to run the _xargs_ command even if there is nothing for it to
operate on. In many situations and usages you'll get odd results if
there is nothing to operate on; in extreme cases you may get dangerous
explosions. This is an easy issue to overlook because everyone almost
always uses _xargs_ in situations that do generate arguments list
(especially when you're testing your command lines or scripts). In fact
I suspect that many people using _xargs_ on Linux, Solaris, and OpenBSD
machines don't even know about this potential gotcha, which sort of
proves my point.

(This entry is yet another illustration of how a simple entry idea can
turn out much more interesting than I expected when I started writing
it.  Before I started actually checking systems I would have confidently
told you that all versions of _xargs_ would run _echo_ once; I had no
idea how tangled the actual situation was.)