while question

[David A. Bandel]

On Tue, Nov 18, 2008 at 10:50 AM, idan72 <ishai at cisco.com> wrote:
>
> Hi,
>
> I try to run the following code below that contain a while statement.
> As you can see in the output it read each line in the file and echo it.
> My question is why when I exit the while loop the value of the vars yyy and
> f is undefined.

you need to gain a fundamental understanding of the command execution
environment.  The parent process feeding the while loop, cannot gain
variables or their values from child processes.  The while loop execs
a new child each time through.  Move the "done" do after the 'echo
f=$f' and you'll get values for the l-yyy and f=$f (but the value
through each loop and they will be gone after you exit that loop).

This is kinda like global vs local variables in a program.  Once you
exit a routine/subroutine, the variables and their values are no
longer available.  Most progarmming languages, however, give you a way
to "return" a value from a subroutine to the calling routine.

What you appear to be trying to do, however, is export a value to a
parent process (not possible).  Best you could hope for is to write
the values you need the parent to have to a file and source that file
once out of the loop (if I am understanding what you're trying to do).

>
> Thanks
>
> cat junk.txt | while read yyy; do  echo $yyy ;export f=1 ; done ; echo
> l-$yyy; echo f=$f ;
> 444
> 555
> 333
> l-
> f=
> [ishai at mip-devtest ishai]$ cat junk.txt
> 444
> 555
> 333


Ciao,

David A. Bandel
-- 
Focus on the dream, not the competition.
            - Nemesis Air Racing Team motto