Bash's cat, while, let and pipe lead to strange scoping behavior

Question

So here's my script :

count=0
cat myfile | while read line
do
    #some stuff...
    let count++
    echo $count
done
echo $count

The last echo statement output 0 instead of the number of lines in the file, although the echo statement in the while loop prints the incrementing value.

The problem is with the piping, because, with a simple while loop, the last echo statement prints the correct value. How can I get this to work?

Answer 1

In Bash, you can use process substitution and avoid the temporary file and the variables in the while loop will be retained.

count=0
while read -r line  # you should almost always use -r
do
    #some stuff...
    (( count++ ))   # alternative form 
    echo $count
done < <(tac myfile)
echo $count

Answer 2

When using pipes, a new subshell is created and any variables in it are lost after it. Use the following code instead:

count=0
while read line
do
    #some stuff...
    let count++
    echo $count
done < myfile
echo $count

Example with a command: tac:

count=0
# Create a random file
tmpfile=$(mktemp)
tac myfile > $tmpfile
while read line
do
    #some stuff...
    let count++
    echo $count
done < $tmpfile
# remove temp file
rm $tmpfile
echo $count