bash - Shell variables set inside while loop not visible outside of it

Question

I am trying to find the pathname with the most characters in it. There might be better ways to do this. But I would like to know why this problem occurs.

LONGEST_CNT=0
find samples/ | while read line
do
    line_length=$(echo $line | wc -m)
    if [[ $line_length -gt $LONGEST_CNT ]] 
    then
        LONGEST_CNT=$line_length
        LONGEST_STR=$line
    fi
done

echo $LONGEST_CNT : $LONGEST_STR

It somehow always returns:

0 :

If I print the results for debugging inside the while loop the values are correct. So why bash does not make these variables global?

Answer 1

When you pipe into a while loop in Bash, it creates a subshell. When the subshell exits, all variables return to their previous values (which may be null or unset). This can be prevented by using process substitution.

LONGEST_CNT=0
while read -r line
do
    line_length=${#line}
    if (( line_length > LONGEST_CNT ))
    then
        LONGEST_CNT=$line_length
        LONGEST_STR=$line
    fi
done < <(find samples/ )    # process substitution

echo $LONGEST_CNT : $LONGEST_STR