I want to run commands manually inside the instance built by packer for investigation when packer build fails.
Packer remove the instance by default when the build fails.
We can change the behavior by -on-error=abort option.

$ AWS_ACCESS_KEY=*** AWS_SECRET_KEY=*** packer build -on-error=abort app.json

Then you can ssh to the instance after packer build exited with error.

ssh into EC2 instance built by packer

This time I use AMI Builder (EBS backed) as builder.
Packer allocates ssh key randomly by default and I don’t know where it is.
So I can’t ssh into the instance.
To avoid this, we can specify ssh keys by ssh_keypair_name and ssh_private_key_file in the packer config file.
https://www.packer.io/docs/builders/amazon-ebs.html#ssh_keypair_name

{
  "variables": {
    "aws_access_key": "",
    "aws_secret_key": ""
  },
  "builders": [{
    "type": "amazon-ebs",
    "access_key": "{{user `aws_access_key`}}",
    "secret_key": "{{user `aws_secret_key`}}",
    "region": "us-east-1",
    "vpc_id": "***",
    "subnet_id": "***",
    "source_ami": "ami-427a392a",
    "instance_type": "c3.large",
    "ssh_username": "ubuntu",
    "ssh_keypair_name": "salt",
    "ssh_private_key_file": "/Users/me/.ssh/salt.pem",
    "ami_name": "image-app {{timestamp}}"
  }],
  "provisioners": [
    ...
    {
      "type": "shell",
      "inline": [
        "sudo curl -o bootstrap-salt.sh -L https://bootstrap.saltstack.com",
        "sudo sh bootstrap-salt.sh -P git v2016.11.6"
      ]
    },
    {
      "type": "salt-masterless",
      "skip_bootstrap": true,
      "local_state_tree": "../file_roots",
      "local_pillar_roots": "../pillar_roots",
      "minion_config": "minion-app"
    },
    ...
  ]
}

Then I can ssh into the instance with the key I specified.