wierd while loop result

Hi, Friends,

In the following simple code, the WHILE LP doesn't give the result I
expected. Did I do it wrong?

You can create a config.txt with 3 lines (can be anything) in it. and
then run mytest.sh

The final a is 0, instead of 3. not sure why. Is the a in the while
loop a local variable which in only available inside the loop, not
outside the loop? Thanks.

script mytest.sh
#!/bin/sh
a=0
b=0
while read line
do
a=`expr $a + 1`
echo now a is $a
done < config.txt
echo "finally a is $a"
for i in x y z
do
b=`expr $b + 1`
echo now b is $b
done
echo finally b is $b

BILL wrote:
Hi, Friends,
>
In the following simple code, the WHILE LP doesn't give the result I
expected. Did I do it wrong?
>
You can create a config.txt with 3 lines (can be anything) in it. and
then run mytest.sh
>
The final a is 0, instead of 3. not sure why. Is the a in the while
loop a local variable which in only available inside the loop, not
outside the loop? Thanks.
>
script mytest.sh
#!/bin/sh
a=0
b=0
while read line
do
a=`expr $a + 1`
echo now a is $a
done < config.txt
echo "finally a is $a"
for i in x y z
do
b=`expr $b + 1`
echo now b is $b
done
echo finally b is $b


The while loop is a block.
The traditional Unix sh runs this in a sub-shell.
According to the Unix principal a child process inherits all settings
and cannot give anything back but the exit code.

My observation is that Posix shells don't do this,
unless forced by a pipe.

Unlike the while loop, the for loop is never treated as a block.


--
echo imhcea\.lophc.tcs.hmo |
sed 's2\(\)\(.\{5\}\)2\2\122;s1\(.\)\(.\)1\2\11g;1s;\. ;::;2'

Michael Tosch wrote:
The while loop is a block.

That's not the relevant point. It's about the redirection:

Redirection of a command (other than a simple-command) causes a sub-shell
to be created. Such non-simple commands are control structures like
"while", "for" and "if", but also "{ list;}",

My observation is that Posix shells don't do this,
unless forced by a pipe.

You can even narrow it down to traditional Bourne shells.

BTW: Using a pipeline is a different issue, though. And the usage of
"forced" here is misleading: PSIX doesn't determine whether (the last
element of) a pipeline is a separate environment or not. And among
the implementations you will find both ways.

Unlike the while loop, the for loop is never treated as a block.

Well, you should try it :)