Depending on your platform you might be able to set an environment variable to the name and then reference that in the config. For example, I have a nightly job that processes the prior day index. In the script that runs I set
INDEXDATE=`date --date="-1 day" '+%Y.%m.%d'`
and in the logstash configuration I have an elasticsearch input that reads
index => "logstash-${INDEXDATE}"