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https://github.com/apparentlymart/terraform-filter-vars

A small utility for filtering .tfvars files to include only definitions related to a particular Terraform module
https://github.com/apparentlymart/terraform-filter-vars

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A small utility for filtering .tfvars files to include only definitions related to a particular Terraform module

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# terraform-filter-vars



`terraform-filter-vars` is a helper tool for wrangling one or more "superset"

`.tfvars` files into a single `.tfvars` file containing only the definitions

relevant to a particular root Terraform module.



In systems that have common conventions around running Terraform such that

some variables are defined in the same way across many configurations, it can

be convenient to produce one or more "superset" variables files that contain

values for all of those variables, even though not all of the configurations

will use all of them.



As an aid to those who might've made typos in their `.tfvars` files, Terraform

generates warnings for definitions of undeclared variables, which can be

off-putting for those using "superset" variables files.



`terraform-filter-vars`, then, allows preprocessing of these superset files

to filter down to only the variables that are declared for a particular module,

so that Terraform won't generate those warnings.



There are

[pre-compiled releases of this program](https://github.com/apparentlymart/terraform-filter-vars/releases)

targeting the same platforms that Terraform itself targets.



## Example



In the `example` subdirectory of this program's repository are some `.tfvars`

files with the following contents:



```

# a.tfvars



foo = "foo from a.tfvars"



# Bar is blah blah blah

bar = "bar from a.tfvars"

```



```

# b.tfvars



# a.tfvars defines this one too, but if b.tfvars is later in the arguments list

# then it will "win" and override it.

bar = "bar from b.tfvars"



baz = "baz from b.tfvars"



# Not used by the example module, so will be filtered out

boop = "boop from b.tfvars"

```



There is also an example Terraform module in `./example/module` that has

declarations only for variable names `foo`, `bar`, and `baz`.



We can run `terraform-filter-vars` against that module and these example

`.tfvars` files to produce a merged and filtered result:



```

$ terraform-filter-vars ./example/module ./example/a.tfvars ./example/b.tfvars

# a.tfvars defines this one too, but if b.tfvars is later in the arguments list

# then it will "win" and override it.

bar = "bar from b.tfvars"

baz = "baz from b.tfvars"

foo = "foo from a.tfvars"

```



Notice that `boop` is not defined in the result at all, and that the definiton

of `bar` was taken from `b.tfvars` in preference to `a.tfvars` because it

appeared later in the command line arguments.



We can write that file to disk using the `-out` option or using shell I/O

redirection:



```

$ terraform-filter-vars -out=filtered.tfvars ./example/module ./example/a.tfvars ./example/b.tfvars

```



If creating a temporary file isn't desirable, and if we are using a shell

that supports automatic FIFO-based I/O redirection, we can use such a

redirection to pass the result directly to Terraform. For example, in `bash`

from within the `example/module` directory:



```

$ terraform plan -var-file=<(terraform-filter-vars . ../a.tfvars ../b.tfvars)

```



This `<( ... )` syntax in bash creates a pipe on a new file descriptor,

launches `terraform-filter-vars` with its stdout connected to the write end of

that pipe, and then replaces that sequences with a path that can be opened to

connect to the read end of the pipe, thus causing Terraform to read the

result as a `.tfvars` file.