One irritation in xargs's interface

March 30, 2013

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) 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:

$ 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 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.)

Written on 30 March 2013.
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Last modified: Sat Mar 30 01:44:47 2013
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