Allow printing and/or saving output vars

[radeksimko]

It would be nice if terraform could be moving a bit towards the concept of "stacks" and sharing variables between these stacks. Inspiration here.

Module is a nice concept for code reuse, putting aside the fact that dependencies don't work well yet, it's generally not always the right solution for all problems anyway.

I'm not sure how the right API & wrapper should look like, but here's an idea:

.
├── networking-stack
│   ├── template.tf
│   ├── terraform.tfout
│   └── networking-input.tfvars
├── computing-stack
│   ├── template.tf
│   ├── terraform.tfout
│   ├── networking-input.tfvars (symlink to ../networking-stack/terraform.tfout)
│   └── computing-input.tfvars
...

Which already expects #1084 to be solved.

I believe it should have a special extension (tfout or any similar) so that if it gets generated into the same directory, it is not picked up in next run as an input.
I believe it should be generating this output by default to any default location (terraform.tfout ?) so that user doesn't have to think about passing extra option for each stack. More importantly the only way I can get to the output now is to open a (possibly giant) *.tfstate and look for outputs which isn't really nice experience.

This could also be helpful for #610

[radeksimko]

I'm just looking at terraform output and thinking that it could be the right place for such functionality...

1. We could make the argument NAME optional and if it's not provided, it will simply print all the variables in format key=value (obviously escaped strings if necessary).
2. We could also add a new argument -out=path which would write the output to that file instead of stdout. Default may still be /dev/stdout.

Usage: terraform output [options] NAME

  Reads an output variable from a Terraform state file and prints
  the value.

Options:

  -state=path      Path to the state file to read. Defaults to
                   "terraform.tfstate".

[mitchellh]

So, tfvars format is just JSON, so I think we can add a -format=json flag to terraform output to do this. I like it.

[ketzacoatl]

+1

[migibert]

+1

[donovanmuller]

+1

[chiriaev]

+1

[grebois]

+1

[mitchellh]

Going to centralize on Radek's mega issue since this is fairly simple (#2460)

[ghost]

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