Increment Bash Variable with += Operator Another common operator which can be used to increment a bash variable is the += operator. This operator is a short form for the sum operator. The first operand and the result variable name are the same and assigned with a single statement.
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or declare i as an integer variable and use the += operator for incrementing its value.
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
COUNTER=1
while [ Your != "done" ]
do
echo " $COUNTER "
COUNTER=$[$COUNTER +1]
done
TESTED BASH: Centos, SuSE, RH
COUNTER=$((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
Try to use
COUNTER=$((COUNTER+1))
instead of
COUNTER=$((COUNTER))
I think this single awk call is equivalent to your grep|grep|awk|awk
pipeline: please test it. Your last awk command appears to change nothing at all.
The problem with COUNTER is that the while loop is running in a subshell, so any changes to the variable vanish when the subshell exits. You need to access the value of COUNTER in that same subshell. Or take @DennisWilliamson's advice, use a process substitution, and avoid the subshell altogether.
awk '
/GET \/log_/ && /upstream timed out/ {
split($0, a, ", ")
split(a[2] FS a[4] FS $0, b)
print "http://example.com" b[5] "&ip=" b[2] "&date=" b[7] "&time=" b[8] "&end=1"
}
' | {
while read WFY_URL
do
echo $WFY_URL #Some more action
(( COUNTER++ ))
done
echo $COUNTER
}
count=0
base=1
(( count += base ))
Instead of using a temporary file, you can avoid creating a subshell around the while
loop by using process substitution.
while ...
do
...
done < <(grep ...)
By the way, you should be able to transform all that grep, grep, awk, awk, awk
into a single awk
.
Starting with Bash 4.2, there is a lastpipe
option that
runs the last command of a pipeline in the current shell context. The lastpipe option has no effect if job control is enabled.
bash -c 'echo foo | while read -r s; do c=3; done; echo "$c"'
bash -c 'shopt -s lastpipe; echo foo | while read -r s; do c=3; done; echo "$c"'
3
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