[Bug 1494] doesn't look on path for $SHELL

[bugzilla-daemon at bugzilla.mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=1494





--- Comment #3 from Vincent Lefevre <vincent at vinc17.org>  2009-01-21 22:19:06 ---
(In reply to comment #2)
> My copy of Single Unix Specification states:

I'd rather use POSIX.1-2008.

> > SHELL
> >     This variable shall represent a pathname of the user's
> >     preferred command language interpreter. [...]

POSIX.1-2008 says the same thing (note: just "pathname", not "absolute
pathname").

> Its definition of "pathname":
> 
> > A character string that is used to identify a file. 
> > [...]
> > Note:
> >     Pathname Resolution is defined in detail in _Pathname Resolution_.

POSIX.1-2008 says more:

3.266 Pathname
  A character string that is used to identify a file. In the context
  of POSIX.1-2008, a pathname may be limited to {PATH_MAX} bytes,
  including the terminating null byte. It has an optional beginning
  <slash>, followed by zero or more filenames separated by <slash>
  characters. [...]

  Note: Pathname Resolution is defined in detail in Section 4.12
  (on page 111).

It has also the notion of absolute pathname (when the pathname begins
with a slash) and relative pathname (when the pathname does not begin
with a slash).

> I'd say that a non-absolute pathname for $SHELL clearly does not
> "identify a file"

According to POSIX.1-2008, relative pathnames identify a file,
otherwise there would be only one kind of pathnames: absolute ones.
POSIX uses the term "pathname" also for execvp, for instance.

> unless one happens to be in the same directory as the
> shell itself and the "Pathname Resolution" section doesn't specify that
> pathnames be resolved using $PATH.

Because it is context-dependent. For instance, relative pathnames are
sometimes resolved against the current working directory, not by using
$PATH.

IMHO, if POSIX wanted to disallow relative pathnames for $SHELL, it
would have said "absolute pathname". It does not say to use $PATH, but
this is implicit: in the PATH definition: "This variable shall
represent the sequence of path prefixes that certain functions and
utilities apply in searching for an executable file known only by a
filename." For $SHELL, we are also in the case of an executable.

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