Why does my Bash counter reset after while loop

Question

I have a Bash script where I want to count how many things were done when looping through a file. The count seems to work within the loop but after it the variable seems reset.

nKeys=0
cat afile | while read -r line
do
  #...do stuff
  let nKeys=nKeys+1
  # this will print 1,2,..., etc as expected
  echo Done entry $nKeys
done
# PROBLEM: this always prints "... 0 keys"
echo Finished writing $destFile, $nKeys keys

The output of the above is something alone the lines of:

Done entry 1
Done entry 2
Finished writing /blah, 0 keys

The output I want is:

Done entry 1
Done entry 2
Finished writing /blah, 2 keys

I am not quite sure why nKeys is 0 after the loop :( I assume it's something basic but damned if I can spot it despite looking at http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-7.html and other resources.

Fingers crossed someone else can look at it and go "well duh! You have to ..."!

Answer 1

In the just-released Bash 4.2, you can do this to prevent creating a subshell:

shopt -s lastpipe

Also, as you'll probably see at the link Ignacio provided, you have a Useless Use of cat.

while read -r line
do
    ...
done < afile