If you've added some runcmd lines to your cloud-init config, and the commands don't seem to be executing, here's a few things you should know:

WTF is going on?

1. runcmd only has code to "shellify" your strings or array of strings (i.e: turns them into a shell script)
2. runcmd will only write a shell script into the file /var/lib/cloud/instance/scripts/runcmd
3. runcmd is part of the cloud_config_modules boot stage
4. You need to execute that runcmd shell script using the scripts-user module
5. scripts-user is part of the cloud_final_modules boot stage

Solution

Here's an example config that WORKS: (as of v19.3)

cloud_config_modules:
  - runcmd

cloud_final_modules:
  - scripts-user

runcmd:
  - [ touch, /tmp/myfile ]

You can test this by manually running cloud-init modules --mode config; cloud-init modules --mode final.

Yes, I agree, the docs aren't very good ;)