reattach_eni.sh

#!/bin/bash 

# This script will run from ec2 user data 
# and it will deattach secondary network interface(ENI) 
# from previous instance if any old instance found 
# If ENI is not attach then it will not run deattach command
# at the last it will run eni attach command to add eni in current instance
#
# Note: Make sure to update ENI Name Tag in variable NAME_TAG

# exit if anything goes wrong
set -e 
# Configure Value of Name tag for ENI
NAME_TAG="MY_NETWORK_ENI_NAM_TAG_CHANGE_THIS"
AWS_CMD="aws --region $AWS_DEFAULT_REGION"
INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)
AWS_DEFAULT_REGION=$(curl -s http://169.254.169.254/latest/dynamic/instance-identity/document | \
                     awk 'BEGIN { FS="\""; RS="," };/region/{ print $4}')

# Get Network Interface ID
ENI_ID=$($AWS_CMD ec2 describe-network-interfaces \
          --filters Name=tag:Name,Values=$NAME_TAG \
          --query NetworkInterfaces[].NetworkInterfaceId \
          --output text)

# Attachment ID
ENI_ID=$($AWS_CMD ec2 describe-network-interfaces \
          --filters Name=tag:Name,Values=$NAME_TAG \
          --query NetworkInterfaces[].NetworkInterfaceId \
          --output text)

# Get Active Instance ID for Interface
ENI_ATTACHMENT_ID=$($AWS_CMD ec2 describe-network-interfaces \
                   --filters Name=tag:Name,Values=$NAME_TAG \
                   --query NetworkInterfaces[].Attachment[].AttachmentId \
                   --output text)

# Deattach ENI assuming that it was attach to old instance 
# in autoscaling group 
if [[ $ENI_ATTACH_INSTANCE_ID != $INSTANCE_ID ]]
then
   $AWS_CMD ec2 detach-network-interface \
     --attachment-id $ENI_ATTACHMENT_ID \
     --force
fi

# Attach ENI 
$AWS_CMD ec2 attach-network-interface \
  --instance-id $INSTANCE_ID \
  --network-interface-id $ENI_ID