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A comment on Most operating systems have implemented realtime signals incorrectly. Parent comment

Re:Most operating systems have implemented realtime signals ... by Borealid on Monday November 03, 2014 @03:01PM (#8553086)
Quoting from your post:

"If the command name matches the name of a special built-in utility, that special built-in utility shall be invoked."

Let echo be a "special built-in utility", and sh is compliant.

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Re:Most operating systems have implemented realtime signals ... by aidenn0 on Monday November 03, 2014 @05:52PM (#8554090)
"special built-in utilities" are specifically listed elsewhere. echo is not among them:

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3...

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Re:Most operating systems have implemented realtime signals ... by Borealid on Wednesday November 12, 2014 @05:20PM (#8598654)
A lawyerly reading still permits echo to qualify as a special built-in utility.

Two relevant excerpts: "The following "special built-in" utilities shall be supported in the shell command language ..."

"... The term "built-in" implies that the shell can execute the utility directly and does not need to search for it. An implementation may choose to make any utility a built-in; however, the special built-in utilities described here differ from regular built-in utilities ..."

(the two differences are irrelevant to this discussion as they do not mention PATH resolution behaviors)

These quotes taken together do not preclude the shell adding a built-in utility and assigning it the "special" status. The standard does not say that only the listed utilities are special; it says that the listed utilities shall be special. Adding a normal built-in utility (not "special") not on the list is explicitly condoned.

It is my belief that echo being a "special built-in utility" and thus having magic PATH behavior is POSIX compliant, although the standard is not 100% clear on this.

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