Support Terraform's --target flag

[so0k]

I haven't tried this out yet, but is it possible to specify resource targets with --target? We use this heavily when making infrastructure updates

[lkysow]

@so0k you can append flags to any terraform command (ex. plan, apply) using the extra_arguments configuration key in an atlantis.yaml config file living in your project (see https://github.com/hootsuite/atlantis#project-specific-customization).

So you could have

# atlantis.yaml
extra_arguments:
- command: plan
  arguments:
  - "-target=TARGET"

But does it make sense to set this against an entire project or on different plan/apply runs do you need to set a target? In which case you'd like -target to be an option to the atlantis plan and atlantis apply commands?

[nwalke]

Reading -target in from a PR comment would be ideal for us.

[so0k]

Same as nwalke, we often just want to plan and apply just one module (and each PR can be a different module)

[anubhavmishra]

We might try out by having an terraform plan/apply argument passed down from the comment during plan or apply. For example atlantis plan -target=something or atlantis plan -refresh=true