Counter increment in Bash loop not working

I have the following simple script where I am running a loop and want to maintain a COUNTER. I am unable to figure out why the counter is not updating. Is it due to subshell thats getting created? How can I potentially fix this?

#!/bin/bash

WFY_PATH=/var/log/nginx
WFY_FILE=error.log
COUNTER=0
grep 'GET /log_' $WFY_PATH/$WFY_FILE | grep 'upstream timed out' | awk -F ', ' '{print $2,$4,$0}' | awk '{print "http://domain.com"$5"&ip="$2"&date="$7"&time="$8"&end=1"}' | awk -F '&end=1' '{print $1"&end=1"}' |
(
while read WFY_URL
do
    echo $WFY_URL #Some more action
    COUNTER=$((COUNTER+1))
done
)

echo $COUNTER # output = 0

Solution 1:

First, you are not increasing the counter. Changing COUNTER=$((COUNTER)) into COUNTER=$((COUNTER + 1)) or COUNTER=$[COUNTER + 1] will increase it.

Second, it's trickier to back-propagate subshell variables to the callee as you surmise. Variables in a subshell are not available outside the subshell. These are variables local to the child process.

One way to solve it is using a temp file for storing the intermediate value:

TEMPFILE=/tmp/$$.tmp
echo 0 > $TEMPFILE

# Loop goes here
  # Fetch the value and increase it
  COUNTER=$[$(cat $TEMPFILE) + 1]

  # Store the new value
  echo $COUNTER > $TEMPFILE

# Loop done, script done, delete the file
unlink $TEMPFILE

Solution 2:

COUNTER=1
while [ Your != "done" ]
do
     echo " $COUNTER "
     COUNTER=$[$COUNTER +1]
done

TESTED BASH: Centos, SuSE, RH

Solution 3:

COUNTER=$((COUNTER+1)) 

is quite a clumsy construct in modern programming.

(( COUNTER++ ))

looks more "modern". You can also use

let COUNTER++

if you think that improves readability. Sometimes, Bash gives too many ways of doing things - Perl philosophy I suppose - when perhaps the Python "there is only one right way to do it" might be more appropriate. That's a debatable statement if ever there was one! Anyway, I would suggest the aim (in this case) is not just to increment a variable but (general rule) to also write code that someone else can understand and support. Conformity goes a long way to achieving that.

HTH

Solution 4:

Try to use

COUNTER=$((COUNTER+1))

instead of

COUNTER=$((COUNTER))