Added validate command

[sorenmat]

Added verify command to validate .tf files.

This can be usefull if you want to verify your terraform files on a CI server or as a webhook for
verifying commits to a repository.
It's a simpler verfication than running a terraform plan.
It will verify:

• Missing or undefined variables
• HCL structure error i.e. wrongly named/missing references

[sorenmat]

@radeksimko Any decision on this one ?

[radeksimko]

Hey @sorenmat ,
I really like the idea. I was myself looking for something like this in our CI, I just didn't have time to review the code thoroughly.

Also it would help the reviewer & users if you attached docs describing the purpose of that command - i.e. what are the typical errors/mistakes that will be detected and won't be detected.

e.g. circular dependencies, offline validation (ValidateFunc in schema), invalid references, invalid HCL syntax, ... ?

---

It's not quite solving #3190 yet, is it? (ain't saying it should)

[sorenmat]

I think it's pretty close to solving #3190 from how I read it. Atleast that is how we use it.
We run it on our CI servers to validate the .tf files for syntax errors.
So it kinda works like plan in offline mode.

[josephholsten]

@sorenmat this seems like a no-brainer. Got time to update this onto master? If not, I'll gladly do it.

@phinze, @mitchellh this could use your 👀. Has tests, needs doc & rebase.

[josephholsten]

lol, the only merge conflict was in .gitignore. rebased up in #5004

[sorenmat]

@josephholsten I rebased an added some documentation to website/source/docs/commands
If someone could proof read it, that would be awesome.

[josephholsten]

looks great!

@phinze this is fully ready for your 👀, has tests & doc.

[radeksimko]

Functionally this looks great.

There's a few things I'd like to discuss though:

• the term verify is often used in the context of formal (e.g. by certification authority) verification which ends with getting a certificate. I'd say that validate is more precise term for what we're doing here. After all most of the underlying method names are also using this term. Would you mind renaming the command?
• The test case you included is great, however would you mind also adding a few more which would cover other failures? It doesn't have to be covering all the cases, although it would be awesome 🏆 , but 2-3 more would be nice. See my list below. ⬇️
• I think we should mention some examples in the documentation (i.e. what is considered a failure) ⬇️
• Can you link the new docs page from website/source/layouts/docs.erb so people can actually get to it from the menu?

 * invalid [HCL](https://github.com/hashicorp/hcl) syntax (e.g. missing trailing quote or equal sign)
 * invalid HCL references (e.g. variable name or attribute which doesn't exist)
 * same `provider` declared multiple times
 * same `module` declared multiple times
 * same `resource` declared multiple times
 * same `output` declared multiple times
 * invalid `module` name
 * interpolation used in places where it's unsupported

[radeksimko]

Can we reword this a little bit to not mention format and structure? What we're really checking is syntax mostly.

Formatting will be checked by fmt per #4955 eventually.

I would in fact even explicitly mentioned in a separate paragraph that this command does not check the format of your code. We may expand that paragraph later on when we merge fmt as linked above.

---

It's pretty much like go vet VS gofmt whereas go vet is what we do.

[radeksimko]

Besides the things mentioned, this looks really good, I'm looking forward to see it in master! 😃
Thanks @sorenmat

[sorenmat]

@radeksimko glad to be able to help, and thanks for an awesome product !

I've renamed the command to validate and updated the documentation as requested.
I've also added a few more test to cover the checks defined in config.go.
Please note: There is no check in config.Validate that checks for multiple output's with the same name.
Is this an error / missing feature ?

If it is I will be glad to do another PR with that fix :)

[radeksimko]

There is no check in config.Validate that checks for multiple output's with the same name.

Good catch! I just opened a new issue for this: #5052
If you want to send another PR to fix that, that's fine, I believe this is out of scope for this PR though.

I think this is good to go! 🚢

[ghost]

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