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

[John Vandenberg]

On 9/9/05, Andreas Gruenbacher <address@hidden> wrote:
> On Thursday 08 September 2005 23:49, John Vandenberg wrote:
> > On 9/9/05, Jean Delvare <address@hidden> wrote:
> > > Hi all,
> > >
> > > I have a half off-topic question about bash, and figured out that there
> > > were some experts here who may come to my help :)

... and some coffee-induced lunacy. :)

> > > It looks like, when using pipes in bash, the scope of variables changes.
> > > Consider the following example:
> > >
> > > --- 8< ---
> > > #!/usr/bin/bash
> > >
> > > for ((i = 0; i < 4; i++))
> > > do
> > >         n=$i
> > > done
> > >
> > > echo "n=$n i=$i"
> > > --- 8< ---
> > >
> > > This prints "n=3 i=4" as one would expect. However, if I change it to:
> > >
> > > --- 8< ---
> > > #!/usr/bin/bash
>         ^ bash commonly is in /bin
> > >
> > > for ((i = 0; i < 4; i++))
> > > do
> > >         n=$i
> > > done \
> > >
> > > | cat
> > >
> > > echo "n=$n i=$i"
> > > --- 8< ---
> > >
> > > This does print "n= i=", that is, the n and i variables are no more
> > > defined past the loop. Can someone explain why? Is there a way to change
> > > that behavior? I am in a case where I badly need the value of one inner
> > > variable after such a construct.
> >
> > The ( ) syntax creates a sub-shell.  Variables can be exported into
> > the sub-shell, but cant come back out.
> 
> The example doesn't use ( ) though; the for loop doesn't use a subshell. The
> issue here is the pipe, where each pipe command is run in its own sub-shell.
> This can be worked around with redirection though. The cat in the example
> doesn't make a lot of sense (it's useless). Think about this:

Oh.  At a quick glance, I thought using two round brackets for the for
loop may have been induced bash to create a subshell.

> #! /bin/bash
> 
> set -x
> 
> i=1
> echo $i
> 
> i=2 | i=3
> echo $i
> 
> i=4 < <(i=5)
> echo $i
> 
> i=6 <<EOF
> $(i=7)
> EOF
> echo $i

Nice example.  Thanks.
--
John