whichami

#!/bin/bash

# Detect the scale index and total count of this container service.
# Thanks! https://stackoverflow.com/a/64799824

# Usage
#   whichami   return the index number of this container.
#   whichami --count   return the total number of containers.

# get the container IP
IP=`ifconfig eth0 | grep 'inet ' | awk '{print $2}'`

# get the service name you specified in the docker-compose.yml 
# by a reverse DNS lookup on the IP
SERVICE=`dig -x $IP +short | cut -d'_' -f2`

# the number of replicas is equal to the A records 
# associated with the service name
COUNT=`dig $SERVICE +short | wc -l`

# extract the replica number from the same PTR entry
INDEX=`dig -x $IP +short | sed 's/.*_\([0-9]*\)\..*/\1/'`

# Echo index or count.
if [[ "${*}" == "--count" ]]; then
  echo $COUNT
else
  echo $INDEX
fi