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Re: standards/42828: Almquist shell always evaluates the contents of ${ENV} even if non-interactive

David Laight wrote:
Actually, for non-special builtins, posix requires that the builtin version only be used if the one that would be executed when $PATH is searched would be the systems 'standard' version.

I'm not 100% sure that's true... I stared at the "Command Search and Execution" section [1] for a while (it's not easy to grok!) and came away with the impression that if the system had echo as a regular built-in, the built-in would be invoked instead of ~/bin/echo even if ~/bin was first in PATH.

Here's how I interpret the logic:
  1.  'echo' doesn't contain any slashes, so rule 1. applies
  2.  echo is not a special built-in, so rule 1.a. doesn't apply
  3.  there is no function named echo, so rule 1.b. doesn't apply
  4.  echo is not in the exception list, so 1.c. doesn't apply
  5.  rule 1.d. applies and the PATH is searched for echo
  6.  echo is in the PATH (in ~/bin), so rule 1.d.i. applies
7. echo is implemented as a regular built-in, so rule 1.d.i.a. applies and the built-in version is invoked

I hope I'm misunderstanding POSIX because I'd prefer the algorithm you described.

On a related note: It's unclear what POSIX says should happen if a user defines a function whose name matches the name of a regular built-in (e.g., 'cd' or 'echo'). Specifically, rule 1.b. is awkward: Either the shell must be able to distinguish implementation-provided functions from user-defined functions (my interpretation) or it's undefined which one wins (rule 1.d.i.a. doesn't break ties between regular built-ins and shell functions).

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