As Ravinder has already said it is because of the pipeline. Basically in classical Bourne Shell (and its descendants, bash - Bourne Again Shell - among them) code like this:

Code:

process1 | process2

is executed by running the process1 in the main shell and process2 in its own subshell. Therefore the while-loop you use runs in its own subshell, with its own variables and inside this shell your variables work - which is why your counter is increased - but when the subshell is left (at the end of the loop) all these variables are deleted. This is why your counter variable is zero at the end.

Notice that the other descendant of the Bourne shell - the Korn Shell, ksh - does things differently. In ksh your code would work as you obviously expect because it executes the last element of a pipeline in the main shell. You can force bash (since, IIRC version 4) to do the same but you must set this option explicitly:

Code:

shopt -s lastpipe

will set the behavior of bash to exactly the same as ksh always had. So your options are: switch to ksh (most of the code is the same but there are subtle differences between bash and ksh) or set the lastpipe option.

By the way: notice that pipelines and redirections word differently in bash, regardless of the lastpipe-setting! This simple line-counter:

Code:

(( lines = 0 ))
cat /path/to/file | while read line ; do
     (( lines++ ))
done
echo $lines

will not work (for the same reasons as your code) but:

Code:

(( lines = 0 ))
while read line ; do
     (( lines++ ))
done < /path/to/file
echo $lines

will.

I hope this helps.

bakunin

/PS: only now saw that peasant has also addressed the ksh<->bash difference. He is absolutely right.