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 

Answer 2

The "correct" reply is given by Dennis. However, I find the process substitution trick extremely unreadable if the loop contains more than a few lines. When reading a script, I want to see what goes into the pipe before I see how it is processed.

So I usually prefer this trick of encapsulating the while loop in "{}".

LONGEST_CNT=0 find /usr/share/zoneinfo | \ { while read -r line     do         line_length=${#line}         if (( line_length > LONGEST_CNT ))         then             LONGEST_CNT=$line_length             LONGEST_STR=$line         fi     done     echo $LONGEST_CNT : $LONGEST_STR }