On 2011-05-28 10:14 +0200, Rick Thomas wrote:
> Can anybody explain this difference between the behavior of bash and
> ksh?
It depends on whether the shell starts a subshell for (compound)
commands in pipelines.
> When reading the man page, I would expect both of them to have the
> behavior exhibited by ksh.
> Why does bash seem to treat "return" like a single level "break" in
> this context?
>
> The "echo "$AA" | while read" is important context. If I change it to
> "for i in 0 1", return does as expected.
Actually, the pipeline is the most important. You can achieve the same
with a simpler script:
--8<---------------cut here---------------start------------->8---
strange () {
for j in 0 1 2 3
do
: | { echo 1;return; }
done
}
echo $(strange)
--8<---------------cut here---------------end--------------->8---
> I wonder if it has anything to do with "while read" causing a subshell
> to be created,
Not exactly, it's the pipeline that causes creation of the subshell.
> and bash getting confused about the "return" inside of
> a subshell.
It does not get confused; as you can see, the subshell properly returns.
> If so, it's a bug in bash that ksh gets right, so it
> ought to be fixable.
There is no wrong or right here, since the susv3 specification is rather
vague:
,----
| Additionally, each command of a multi-command pipeline is in a subshell
| environment; as an extension, however, any or all commands in a pipeline
| may be executed in the current environment.
`----
So bash implements the default, while ksh implements the extension.
Having an option in bash to control the behavior would be nice, however.
See also the following blog entry:
http://backreference.org/2010/10/23/on-pipes-subshells-and-descriptors/.
Sven
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