Re: [bash-bug] A note for read builtin

[Dr. Werner Fink]

On Wed, Jun 23, 2010 at 10:00:12AM +0200, Werner Fink wrote:
> On Tue, Jun 22, 2010 at 04:50:54PM -0400, Chet Ramey wrote:
> > On 6/18/10 10:05 AM, Dr. Werner Fink wrote:
> > 
> > > as now is visible the last command in the pipe sequence done
> > > in the bash is a real sub process whereas in the ksh it is not.
> > > 
> > > The question rises: Why does the bash require a sub peocess/shell
> > > for the final command of a pipe sequence.
> > 
> > It's an implementation decision: the code as it exists today is more
> > straightforward.  Since Posix requires that all commands in a pipeline
> > be executed in a subshell environment, though it allows the ksh behavior
> > as an extension, there's been no requirement to change it.
> > 
> > If anyone would like to take a shot at changing the code to not fork when
> > executing a command with a pipe for stdin, I'd be glad to take a look at
> > the result.
> 
> OK ... after a deeper look into execute_cmd.c I'd like to suggest
> something like in the attachment.  Anyone around with a better
> solution? Maybe there is a way to emulate the first pipe commands
> as a named fifo for the last element of the pipe which may avoid
> moving stdin around.  Anyway the patch makes things like:
> 
>  bash/bash-4.1> ./bash -c 'echo a b | while read x y; do a=$x b=$y; done; 
> echo $a $b'
>  a b
>  bash/bash-4.1> ./bash -c 'echo a b | read a b ; echo $a $b'
>  a b
> 
> work.  The change of sh_coproc is simply to avoid a (correct)
> compiler warning.

Yet an other version of the patch to avoid trouble with the
coproc builtin tested out in tests/coproc.tests.  There is one
difference more with tests/redir.tests at

  exit 3 | $EXIT > $TMPDIR/null-redir-e
  echo $? -- ${PIPESTATUS[@]}
  testf $TMPDIR/null-redir-e

  exit 4 | > $TMPDIR/null-redir-f
  echo $? -- ${PIPESTATUS[@]}
  testf $TMPDIR/null-redir-f

which now reports

  0 -- 3 0
  0 -- 4 0

instead of

  0 -- 0
  0 -- 0

but seems to me ok.


   Werner

-- 
  "Having a smoking section in a restaurant is like having
          a peeing section in a swimming pool." -- Edward Burr

bash-4.1-pipe.dif
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