Re: why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

[Jilles Tjoelker <jilles@stack.nl>]

On Tue, Jan 31, 2017 at 06:27:00PM -0800, Parke wrote:
> On Tue, Jan 31, 2017 at 12:00 PM, Mark Galeck <mark_galeck@pacbell.net> wrote:
> > This is strange. Why save, dup and dup again to restore, descriptors
> > in the parent, when it would be much simpler to just dup in the
> > child, and not have to save and restore. This is simpler and I
> > checked it works the same:

> > I am sure there must be a good reason and I am not understanding
> > something deeper. What is it?

> I am not a dash developer, but one reason to make system calls in the
> parent is that it is much simpler to handle errors in the parent.

> In your example:

> > if (!fork()) {
> >     fd = open64("foobar.txt", O_WRONLY|O_CREAT);
> >     dup2(fd, 1);
> >     execl("/bin/date", "date", (char *)NULL);
> > }

> What happens if open64 fails?  How should the child inform the parent
> of this specific error?

In general, you are right, but in the shell's case there is no
difficulty. The error is reported via an error message to stderr and a
non-zero exit status of the command, and the child process can easily do
those things.

-- 
Jilles Tjoelker