Question

Here is my simplified code. ls is simplified of find command. wc is simplified for another command.

n=0
ls *.cpp | while read filename ; do
  c=`wc -l $filename | awk '{print $1}'}`
  n=`expr $n + $c`
done
echo "$n"  #always be 0

In previous code failed, the following success

n=0
ls *.cpp > /tmp/list
while read filename ; do
  c=`wc -l $filename | awk '{print $1}'}`
  n=`expr $n + $c`
done < /tmp/list
echo "$n" 

How to use first form and get correct answer?
Or, how to use 2nd form but without another /tmp/list file?

Maybe my problem is how to pass variable between subshell which is generated automatically

Answer 1

In a pipe, every process except the first one is in a subshell, so the values of $c and $n are lost. If you want to keep the count, you should put the while loop first:

while read ...; do
    c=...
    n=...
done < <(find . -maxdepth 1 -name '*.cpp')
echo "$n"

But there’s a better way:

wc -l *.cpp | tail -1 | awk '{ print $1 }'