WG15 Defect Report Ref: 9945-2-104
Topic: expr | operator

This is an approved interpretation of 9945-2:1993.


Last update: 1997-05-20



	Topic:                  expr | operator
	Relevant Sections:      4.22.7

Defect Report:

	Date: Tue, 4 Apr 1995 17:39:13 -0700
	From: Chuck.Hickey@Eng.Sun.COM (Chuck Hickey [CONTRACTOR])

Dear Standards Board,
	I would like to request a formal interpretation on the following
issue concerning the expr utility in POSIX.2.

Standard:		ISO/IEC 9945-2:1993
Topic:                  expr | operator
Relevant Sections:      4.22.7

	In ISO/IEC 9945-2:1993 section 4.22.7 (Extended Description of
	the expr utility), P277, L4159-4160, the description of the |
	and & operators is:
		expr1 | expr2	Returns the evaluation of expr1 if it
				is neither null nor zero; otherwise,
				it returns the evaluation of expr2.
		expr1 & expr2	Returns the evaluation of expr1 if
				neither expression evaluates to null
				or zero; otherwise, returns zero.

	This requires that the command:
		expr "" \| ""
	produce an empty line as its output.

	In both BSD and System V historic practice, the | operator
	acted as a boolean arithmetic operator like the & operator.  If
	the expressions evaluated to null strings, the result was "0\n",
	not a null.

	The rationale in section E.4.22, P904, L6209-6213 acknowledges
	that expr could be replaced by other shell constructs in the
	POSIX.2 shell, but says the utility was kept because of the
	many historical shell scripts that use it.  The rationale also
	mentions that other changes in early drafts of the standard
	were backed out because they weren't historic practice
	(see L6229-6230).  It seems strange that this change to
	historical practice was not documented if it was intentional.

	To match historic BSD and System V implementations, the
	description of the | operator on P277, L4159-4169 should have
	been something like:
	expr1 | expr2	Returns the evaluation of expr1 if it is neither
			null nor zero; otherwise, the evaluation of
			expr2 if it is not null; otherwise, returns

	Was this change to historic practice intentional?

Interpretation response
The standard states the behavior for the | operator for the expr cmd, and
conforming implementations must conform to this.  However, concerns
have been raised about this which are being referred to the sponsor.


Forwarded to Interpretations group: Apr 5 1995
Proposed resolution circulated: May 16th
Comments due: June 15th
Finalised: June 16th 1995