#!/bin/bash

if [ "$1" == "-h" ]; then
  cat <<END
AWS AutoScaling Termination Helper:
terminate an instance in an auto scaling group

Usage: $0 {group-name}

END
  exit
fi

INSTANCES=$(aws autoscaling describe-auto-scaling-groups --auto-scaling-group-names $1 --query 'AutoScalingGroups[0].Instances[]' --output text)

INSTANCEIDS=$(awk '{print $3}' <<<"$INSTANCES")

COUNT=$(wc -l <<<"$INSTANCES")

IFS=$'\n'
c=1
for i in $INSTANCES; do
    echo $((c++)): $i
done

read -p "Which one? " N
if [[ "$N" == "" || "$N" != [0-9]* || "$N" -gt "$COUNT" ]]; then
    echo ERROR: Invalid selection
    exit
fi

read -p "Replace? [Y/n] " R

if [[ "$R" != [nN] ]]; then
    TERMREPL=REPLACE
    DECREMENT='--no-should-decrement-desired-capacity'
else
    TERMREPL=TERMINATE
    DECREMENT='--should-decrement-desired-capacity'
fi

SELECTEDID=$(head "-$N" <<<"$INSTANCEIDS" | tail -1)
SELECTEDLINE=$(head "-$N" <<<"$INSTANCES" | tail -1)

echo About to $TERMREPL instance $N: $SELECTEDLINE
read -p "Are you sure? [y/N] " S

if [[ "$S" != [yY] ]]; then
    echo Aborting.
    exit
else
    echo Proceeding.
    aws autoscaling terminate-instance-in-auto-scaling-group $DECREMENT --instance-id $SELECTEDID
fi