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In the comments to the accepted answer to this Unix & Linux StackExchange question, it is stated that $_ is "one of only some 7 special parameters defined by POSIX". I can't find that definition although I can find eight other special parameters defined by POSIX and I've seen $_ defined in the ksh and bash man pages. Can someone point me to the definition in POSIX?

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    Neither the question you link to nor its answers claim that $_ is POSIX. – terdon May 13 '14 at 22:58
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    @terdon It's a comment by mikeserv. He's wrong. – Gilles 'SO- stop being evil' May 13 '14 at 23:55
  • @terdon I've updated my question to clarify where that statement is in the question. – jrw32982 May 14 '14 at 2:20
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The only reference I can find to the special parameter $_ in POSIX is in the rationale section on Shell Variables. This excerpt implies that it was used by a variety of shells, but not in a standard way by all and was omitted intentionally:

_

(Underscore.) While underscore is historical practice, its overloaded usage in the KornShell is confusing, and it has been omitted from the Shell and Utilities volume of IEEE Std 1003.1-2001.

So it may be used by most or all POSIX-conforming shells, but it is decidedly not defined by POSIX.

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