Quoth Jonathan Nieder <[email protected]>, on 2010-11-26 01:10:47 -0600:
> > Inserting, say, a
> > debugging echo before the actual command is virtually guaranteed to
> > not exec the final command directly.
>
> How do you like the ksh93 behavior?
I wasn't previously aware of the ksh93 behavior. So they _do_ do that
in cases where the entire script is not a single simple command. (My
previous experience was always otherwise, including with Bash, pdksh,
I think zsh though that appears to exhibit ksh93-esque behavior now so
I may have misremembered, and one or two other shells.)
ksh93 appears to also handle various other tail positions, including
subshells and && and ||. That's more the sort of thing I could get
behind if I were made Dictator of Shells. :-)
Obviously my information above was out of date. I amend it to say "in
many cases will not exec the final command directly". (This means
that a program that wishes to be portable to multiple underlying
shells can rely even less on the process tree shape, so my previous
points mostly stand anyway.)
> Actually, let me take that back. If you actually want a guarantee
> that future versions of dash will have or lack this feature,
Let me be a little clearer, at the risk of restating myself. I don't
personally need a guarantee either way, especially since it's
something I'll have to deal with regardless of what dash does in the
future. I mostly wanted to ensure that people reading the bug trail
were aware of some of the subtler ramifications of a decision to do
tail execution optimization in shells, and in particular, I wanted to:
- Add a dissenting voice to some ideas from upthread that making the
last command a child of the shell is "wrong" and that relying on a
highly compact process tree shape that was not actually requested
is a sane thing to do in a POSIX or GNU/Linux environment, which
IMHO it is not.
- Point out that full tail execution optimization is not as
localized as the "anything without shell meta-stuff" that some
people think it is, and that partial tail execution optimization
can lead to subtle problems if not done carefully. Cf. a vaguely
similar case where I was attempting to use an XSI shell builtin
(which I was willing to rely on on the target system) as a single
command in a series of commands in a makefile, and GNU Make
decided that it didn't look enough like a shell thing and could be
"optimized" into an exec, which naturally hosed the command. Of
course, the case of doing exec transformation _in_ the shell is
not as bad as that.
Thanks for the attention.
---> Drake Wilson
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