You can avoid using the Downward API by using HOSTNAME:

command:
- bash
- c
- |
  ordinal=${HOSTNAME##*-}  
  if [[ "$ordinal" = "0" ]]; then
    ...
  else
    ...
  fi 

I don't know of a non-hacky way to do it, but I know a hack that works. First, each pod in a StatefulSet gets a unique predictable name. It can discover that name via the downward API or just by calling hostname. So I have shell script as then entrypoint to my container and that script gets it's pod/hostname. From there it calls the "real" executable using command line args appropriate for the specific host.

For example, one of my scripts expects the pod name to be mapped into the environment as POD_NAME via downward api. It then does something like:

#!/bin/bash

pet_number=${POD_NAME##*-}

if [ pet_number == 0 ]
then
   # stuff here
fi

# etc.

I found one less hacky way to pass in ordinal index into container using the lifecycle hooks

  containers:
  - name: cp-kafka
    imagePullPolicy: Always
    image: confluentinc/cp-kafka:4.0.0
    resources:
      requests:
        memory: "2Gi"
        cpu: 0.5
    ports:
    - containerPort: 9093
      name: server
      protocol: TCP
    lifecycle:
      postStart:
        exec:
          command: ["/bin/sh", "-c", "export KAFKA_BROKER_ID=${HOSTNAME##*-}"]
    env:
      - name: KAFKA_ZOOKEEPER_CONNECT
        value: zk-cs.default.svc.cluster.local:2181
      - name: KAFKA_ADVERTISED_LISTENERS
        value: PLAINTEXT://localhost:9093

Recommended way, see https://kubernetes.io/docs/tasks/run-application/run-replicated-stateful-application/#statefulset

# Generate server-id from pod ordinal index.

[[ `hostname` =~ -([0-9]+)$ ]] || exit 1
ordinal=${BASH_REMATCH[1]}

# ${ordinal} now holds the replica number

server-id=$((100 + $ordinal))

# Copy appropriate conf.d files from config-map to emptyDir.
if [[ $ordinal -eq 0 ]]; then
  # do something
else
  # do something else
fi

In case you want to track the progress of this, the ticket in question for this feature is here: https://github.com/kubernetes/kubernetes/issues/30427

The proposal involves putting the ordinal as a label on the pod and then using the downward api to pull that out into an environment variable or something.