sh(1): POSIX "Command Search and Execution"

[<tlaronde%kergis.com@localhost>]

For some reason[*], I looked at sh(1) "Command Search and Execution"
in POSIX (issue 7 2018 and then issue 8 2024).

Starting with the 7th issue (2018), and concerning a command without
any slash, the behavior is:


---8<---i7 2018

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

b) If the command name matches the name of a utility listed in the
following table [edited; but the list is defined, no ellipsis], the
results are unspecified.

c) If the command name matches the name of a function known to this
shell, the function shall be invoked as described in Function
Definition Command. If the implementation has provided a standard
utility in the form of a function, it shall not be recognized at this
point. It shall be invoked in conjunction with the path search in step
1e.

d) If the command name matches the name [XSI] [Option Start]  of the type
or ulimit utility, or [Option End]  of a utility listed in the
following table [edited, but no ellipsis either], that utility shall be
invoked.

e) Otherwise, the command shall be searched for using the PATH
environment variable as described in XBD Environment Variables :

	i) If the search is successful:

		a) If the system has implemented the utility as a regular built-in or
		as a shell function, it shall be invoked at this point in the path
		search.

--->8---i7 2018

From the specification above, I'm puzzled about two things: regular
built-ins and PATH search:

In issue 7, built-ins are segregated in two groups: "special
built-ins" and "regular built-ins", the latter being the complement of
the former (a built-in that is not "special").

But in the spec, a regular built-in can only be invoked in e), that is
the corresponding name file has to be accessible via the PATH. If it
is not, one can not invoke a regular built-in? This may have sense for
an utility required by POSIX (that _has_ to be provided), but there
may be a regular built-in that POSIX doesn't speak about...

And what does "a successful search" mean? From the referenced
paragraph "XBD Environment Variables":

---8<---issue 7 2018
The list shall be searched from beginning to end, applying the
filename to each prefix, until an executable file with the specified
name and appropriate execution permissions is found. 
--->8---issue 7 2018

But this contradicts the use of the shell in the paragraph I'm talking
about, since if the permissions can be stat'ed, the "executable" nature
of the file can not be ascertained without exec'ing---and this test
may flag the pathname as invalid, and will void the further
requirement that the shell should try to execute a not executable
file (supposed to be a script chunk without the shebang).

So what does "found" mean, in this case? A regular file, accessible
with the correct permissions, so an exception from the "PATH"
paragraph rules?

Furthermore, the spec says in "2.14. Special Built-In Utilities":

---8<---issue 7 2018
The term "built-in" implies that the shell can execute the utility
directly and does not need to search for it. 
--->8---issue 7 2018

The proposition is for all built-ins. And this contradicts the
paragraph where the built-in has to be searched for previously...

It seems to me that POSIX has focused on not special built-ins
implementing a required utility and that _these_ were the target of
the quoted portions, forgetting that there may be other built-ins,
that, in a POSIX compliant shell, could then not be used unless they
appear in the PATH (this implicit linked to the POSIX utilities seems
to me to be this: 

"The special built-in utilities in this section need not be provided
in a manner accessible via the exec family of functions defined in
the System Interfaces volume of POSIX.1-2017."

i.e. not special built-ins have to be provided in a manner accessible
via the exec family of functions.)


The issue 8 2024 adds to the fun...

The d) has been simplified with the specification of "intrinsic
utilities"---but this is still not "regular built-ins" in general.

The variation is in e), the i.a)

---8<---issue 8 2024
if the system has implemented the utility as a built-in or as a shell
function, and the built-in or function is associated with the
directory that was most recently tested during the successful PATH
search, that built-in or function shall be invoked.
--->8---issue 8 2024

What does:

"the built-in or function is associated with the 
directory that was most recently tested during the successful
PATH search"

mean? How is a directory "associated" to a built-in or a function,
unless by the very finding of a corresponding filename (what type? What
permissions?  What content?) in a directory in the PATH search?

Note: in the NetBSD implementation---I didn't look in the CSRG
archives to see if these are in fact here from long ago---there are
prefixes in the path: "%builtins" and "%func";
perhaps are these an attempt to this association? These are builtins
or funcs if the prefix is specified as the preceding "dir" in PATH?

Could somebody explain this in an "international" english, that is
something a not english native speaker with an average english
vocabulary could parse?

TIA
 
[*]: The reason why I looked at the spec is that, under Plan9, there is
a feature that I find quite neat and consistent: utilities can be
organized in subfolders and one can invoke from the shell (rc(1)) an
utility like this: "ip/ipconfig ...". This organized the utilities in
groups, instead of putting everything flat in a directory. 
I thus wanted to see how I could add this (it is not POSIX compliant)
by setting an option, without disturbing much the POSIX behavior
or introducing security problems that the POSIX spec had tried to
address...

-- 
        Thierry Laronde <tlaronde +AT+ kergis +dot+ com>
                     http://www.kergis.com/
                    http://kertex.kergis.com/
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