Re: [Quilt-dev] [OT] bash question

[Andreas Gruenbacher]

On Friday 09 September 2005 10:07, Jean Delvare wrote:
> Hi Andreas,
>
> > > > for ((i = 0; i < 4; i++))
> > > > do
> > > >         n=$i
> > > > done \
> > > >
> > > > | cat
> >
> > (...)
> > The issue here is the pipe, where each pipe command is run in its own
> > sub-shell.
>
> That's what I had finally figured out, yeah.
>
> > The cat in the example doesn't make a lot of sense (it's useless).
>
> Yup, I know, it was simply for the benefit of the example. My real code
> does sense and is useful :)
>
> > Think about this:
> >
> > #! /bin/bash
> >
> > set -x
> >
> > i=1
> > echo $i
> >
> > i=2 | i=3
> > echo $i
>
> At this point I was surprised. Firstly because I'd have expected that at
> least one half of the pipe would be run by the current process (and as
> such would succeed in setting the value of i), and that's not the case.
> Secondly because there's only one '+' in the trace log in front of
> "i=2" and "i=3". I thought that subshells were identified by '++'
> instead?

I was wondering about that, too. No idea.

> > i=4 < <(i=5)
> > echo $i
>
> I think I remember that older versions of bash don't support this syntax
> though. John was wanting to replace them, right?
>
> > i=6 <<EOF
> > $(i=7)
> > EOF
> > echo $i
>
> Nice, thanks for the lesson. However it doesn't seem to address my
> immediate problem. In both cases, the part which is able to set the
> value of variables is the part receiving the data flow, not the part
> emitting it. Is there a way to somehow revert these constructs so that
> emitting part is run by the current process instead?

I never needed such a contruct yet, but it could be implemented like this. On 
the other hand, you might better want to avoid things like this; I wouldn't 
be surprised if bash has some bugs here.

#! /bin/bash

i=1
echo i=$((i++)) > >(sed -e 's:^:| :')
# Q: how to wait for the subshell?
echo i=$i

j=1
exec 3> >(sed -e 's:^:| :')
echo j=$((j++)) >&3
exec 3>&-
# Q: how to wait for the subshell?
echo j=$j

k=1
dir=$(mktemp -dt ${0##*/}.XXXXXX)
trap "rm -rf $dir" EXIT
mkfifo $dir/fifo
sed -e 's:^:| :' < $dir/fifo &
echo k=$((k++)) > $dir/fifo
wait
echo k=$k

-- Andreas.