Problem with "read from pipe"

[sbix]

Hi everybody,
I am having a strange problem.
If I write something like:
cat file_of_data | while read x
do
BLAH BLAH BLA
done
echo "x=$x"

I have this output "x="

If I write, WITHOUT CHANGING ANYTHING IN file_of_data:
while read x
do
BLAH BLAH BLAH
done < file_of_data
echo "x=$x"

The output is "x=last_value_read".
Looks like a sudden death of the process writing in the pipe whipes out in the variables changed in the do... done block
Any idea?

 

[Annihilannic]

Yes, this is an annoying problem which occurs in some shells but not others, and it is the reason why I usually advocate using the second syntax.

When the shell in question creates a pipe it actually forks 2 processes, a reader and a writer. So when those processes finish, any variables that were set while they were running are lost.

No extra processes are forked to handle a redirection, hence the variables you set are retained.

Annihilannic.

 

[sbix]

Annihilannnic, I see what you say... the do.. BLAH BLAH.. done process is ran in a different process in a proper space when reading from a pipe...
It sounds... but it means also that legions of scripts wont run with this impementation of shell...
Thanks a lot