Incrementing a variable inside a Bash loop

Question

I'm trying to write a small script that will count entries in a log file, and I'm incrementing a variable (USCOUNTER) which I'm trying to use after the loop is done.

But at that moment USCOUNTER looks to be 0 instead of the actual value. Any idea what I'm doing wrong? Thanks!

FILE=$1  tail -n10 mylog > $FILE  USCOUNTER=0  cat $FILE | while read line; do   country=$(echo "$line" | cut -d' ' -f1)   if [ "US" = "$country" ]; then         USCOUNTER=`expr $USCOUNTER + 1`         echo "US counter $USCOUNTER"   fi done echo "final $USCOUNTER" 

It outputs:

US counter 1 US counter 2 US counter 3 .. final 0 

Answer 1

You are using USCOUNTER in a subshell, that's why the variable is not showing in the main shell.

Instead of cat FILE | while ..., do just a while ... done < $FILE. This way, you avoid the common problem of I set variables in a loop that's in a pipeline. Why do they disappear after the loop terminates? Or, why can't I pipe data to read?:

while read country _; do   if [ "US" = "$country" ]; then         USCOUNTER=$(expr $USCOUNTER + 1)         echo "US counter $USCOUNTER"   fi done < "$FILE" 

Note I also replaced the `` expression with a $().

I also replaced while read line; do country=$(echo "$line" | cut -d' ' -f1) with while read country _. This allows you to say while read var1 var2 ... varN where var1 contains the first word in the line, $var2 and so on, until $varN containing the remaining content.