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))