svn commit: r215520 - in head: bin/sh share/man/man1 usr.bin/printf

[Jilles Tjoelker]

On Fri, Nov 19, 2010 at 03:21:27PM +0000, Alexey Dokuchaev wrote:
> On Fri, Nov 19, 2010 at 12:56:13PM +0000, Jilles Tjoelker wrote:
> > Side effect: printf is now always the builtin version (which behaves
> > identically to /usr/bin/printf) and cannot be overridden via PATH (except
> > via the undocumented %builtin mechanism).

> Are there any plans to document it?

It would have to be fixed first so it has sensible behaviour. Right now
all builtins, including ones that must execute as a builtin for full
functionality, can be overridden via %builtin, which does not seem to
make a lot of sense. POSIX (XCU 2.9.1.1 Command Search and Execution)
requires that special builtins and some other builtins such as bg, cd
and command be found without a PATH search.

For consistency with execvp(3) and perhaps also for POSIX compliance,
the special cases with % are not really good. Another way to allow
overriding builtins would be to make each overridable builtin pretend to
be in a directory (e.g. /usr/bin) and be found at that point in the PATH
search. For example, printf could pretend to be /usr/bin/printf, which
would mean PATH=/usr/bin printf would find the builtin, but
PATH=/elsewhere:/usr/bin printf would find /elsewhere/printf if it
exists. Command names containing slashes would not participate in this
and always invoke the external program. For example, /usr/bin/printf
would continue to use the external program. Some builtins in ksh93 (but
not printf) work this way.

If the echo builtin pretends to be /bin/echo, it is a bit strange
because the two utilities are rather incompatible.

If I take POSIX's list and add other utilities that must execute as a
builtin for full functionality, only very few utilities remain. Many of
these are non-standard and are not present in the filesystem.

In the specific case of printf, note that the bash, ksh93 and zsh
versions of the printf builtin support %n, writing the result into a
specified shell variable, which can only work if it is a builtin.
Although our printf does not support this and I do not plan on adding
it, it means that various shell authors think printf should always be a
builtin.

The general issue is that I do not know how exactly %builtin is or can
be used in practice.

-- 
Jilles Tjoelker