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Dennis Groß (he/him)
Dennis Groß (he/him)

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How to use Terraform variables

#terraform #cloud #cloudnative #cloudskills

Variables are used to remove hardcoded config values from your Terraform scripts and can be placed in the same file as the script that you are executing or in a dedicated [variables.tf](http://variables.tf) file (best practice).

You can provide default values to the variables and override them through the CLI or environment variables if you placed the [variables.tf](http://variables.tf) in the root module.

variables.tf

You can declare a variable with the variable block inside a *.tf script or in a dedicated [variables.tf](http://variables.tf) file.

variable "vpc_cidr" {
  type = string
  default = "172.31.0.0/16"
}
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And then use the variable in another file within the same module.

resource "aws_vpc" "main" {
  cidr_block = var.vpc_cidr
  instance_tenancy = "default"
}
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Calling the terraform plan command will use the default value of the vpc_cidr variable.

variables terraform plan

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # aws_vpc.main will be created
  + resource "aws_vpc" "main" {
      + arn                                  = (known after apply)
      + cidr_block                           = "172.31.0.0/16"
      + default_network_acl_id               = (known after apply)
      + default_route_table_id               = (known after apply)
      + default_security_group_id            = (known after apply)
      + dhcp_options_id                      = (known after apply)
      + enable_classiclink                   = (known after apply)
      + enable_classiclink_dns_support       = (known after apply)
      + enable_dns_hostnames                 = (known after apply)
      + enable_dns_support                   = true
      + enable_network_address_usage_metrics = (known after apply)
      + id                                   = (known after apply)
      + instance_tenancy                     = "default"
      + ipv6_association_id                  = (known after apply)
      + ipv6_cidr_block                      = (known after apply)
      + ipv6_cidr_block_network_border_group = (known after apply)
      + main_route_table_id                  = (known after apply)
      + owner_id                             = (known after apply)
      + tags_all                             = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.
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Overriding defaults

The value of the variable var.vpc_cidr can be overwritten with the environment or through the CLI.

Override with environment variables

All environment variable overrides need to match the pattern TF_VAR_<var_name> to override a variable.

export TF_VAR_vpc_cidr="0.0.0.0/16"

terraform plan       

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # aws_vpc.main will be created
  + resource "aws_vpc" "main" {
      + arn                                  = (known after apply)
      + cidr_block                           = "0.0.0.0/16"
      + default_network_acl_id               = (known after apply)
      + default_route_table_id               = (known after apply)
      + default_security_group_id            = (known after apply)
      + dhcp_options_id                      = (known after apply)
      + enable_classiclink                   = (known after apply)
      + enable_classiclink_dns_support       = (known after apply)
      + enable_dns_hostnames                 = (known after apply)
      + enable_dns_support                   = true
      + enable_network_address_usage_metrics = (known after apply)
      + id                                   = (known after apply)
      + instance_tenancy                     = "default"
      + ipv6_association_id                  = (known after apply)
      + ipv6_cidr_block                      = (known after apply)
      + ipv6_cidr_block_network_border_group = (known after apply)
      + main_route_table_id                  = (known after apply)
      + owner_id                             = (known after apply)
      + tags_all                             = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.
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Override with the CLI

Use the -var <var_name>="<value>" parameter to override a variable with the CLI.

terraform plan -var vpc_cidr="0.0.0.0/16"

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # aws_vpc.main will be created
  + resource "aws_vpc" "main" {
      + arn                                  = (known after apply)
      + cidr_block                           = "0.0.0.0/16"
      + default_network_acl_id               = (known after apply)
      + default_route_table_id               = (known after apply)
      + default_security_group_id            = (known after apply)
      + dhcp_options_id                      = (known after apply)
      + enable_classiclink                   = (known after apply)
      + enable_classiclink_dns_support       = (known after apply)
      + enable_dns_hostnames                 = (known after apply)
      + enable_dns_support                   = true
      + enable_network_address_usage_metrics = (known after apply)
      + id                                   = (known after apply)
      + instance_tenancy                     = "default"
      + ipv6_association_id                  = (known after apply)
      + ipv6_cidr_block                      = (known after apply)
      + ipv6_cidr_block_network_border_group = (known after apply)
      + main_route_table_id                  = (known after apply)
      + owner_id                             = (known after apply)
      + tags_all                             = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.
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Top comments (2)

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jareechang profile image
Jerry •

This one trips me up when I was learning terraform.

Now I know to add TF_VAR_ to the variables :D

Great series, Dennis!

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gdenn profile image
Dennis Groß (he/him) •

Hey Jerry, happy that it was helpful for you. I also use the same TF_VAR_PROFILE every time and I just make a reverse search on the shell to export the value. That's really neat. :)

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