INSTANCE_ID not available from "runcmd"

[Eric Dahl]

The AWS-related environment variable `INSTANCE_ID` is not populated within the context of a "runcmd", like it is for a "bootcmd". In "bootcmd", the value is set to the AWS EC2 instance id, as obtained from the metadata endpoint.

I reproduced this with a minimal cloud-init file:

```
#cloud-config

# bootcmd executes during the first boot only
bootcmd:
  - echo "INSTANCE_ID=$INSTANCE_ID" >> /tmp/bootcmd

# runcmd executes these lines during the initial boot
runcmd:
  - echo "INSTANCE_ID=$INSTANCE_ID" >> /tmp/runcmd
```

[Ryan Harper (raharper)]

Thank you for filing a bug. Cloud-init does not provide the INSTANCE_ID env variable for runcmd as you say. You may be interested in using the instance-data.json in a jinja template. Please take a look:

https://cloudinit.readthedocs.io/en/latest/topics/instancedata.html#using-instance-data

[James Falcon (falcojr)]

instance-data is now the supported way to achieve this sort of behavior. This can be done with:

## template: jinja
#cloud-config

# bootcmd executes during the first boot only
bootcmd:
  - echo "INSTANCE_ID={{v1.instance_id}}" >> /tmp/bootcmd

# runcmd executes these lines during the initial boot
runcmd:
  - echo "INSTANCE_ID={{v1.instance_id}}" >> /tmp/runcmd

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3147