Terraform provides a set of built-in functions that transform and combine values within Terraform configurations. The Terraform function documentation contains a complete list. You can also use your editor autocompletion on the Fn object to find available options.
Functions can handle normal and token values and will return either tokenized values or IResolvable values.
Use Terraform functions when you need to calculate new values based on runtime values that are unknown before Terraform applies a configuration. For example, instance IDs that cloud providers assign on creation.
When inputs are available before synthesizing your code (e.g. local files), we recommend transforming the values with your preferred programming language.
The following example uses a Data Source from the AWS Provider to fetch the Availability Zones of the given region. As this data is unknown until Terraform applies the configuration, this CDKTF application uses both Terraform Outputs and the Terraform element function.
The element function gets the first element from the list of Availability Zone names.
import { TerraformStack, TerraformVariable } from "cdktf";
import { Construct } from "constructs";
import { AwsProvider } from "@cdktf/provider-aws/lib/aws-provider";
import { Fn, TerraformOutput } from "cdktf";
import { DataAwsAvailabilityZones } from "@cdktf/provider-aws/lib/data-aws-availability-zones";
export class FunctionsStack extends TerraformStack {
constructor(scope: Construct, id: string) {
super(scope, id);
new AwsProvider(this, "aws", {
region: "us-west-2",
});
const zones = new DataAwsAvailabilityZones(this, "zones", {
state: "available",
});
new TerraformOutput(this, "first-zone", {
value: Fn.element(zones.names, 0),
});
}
}
import com.hashicorp.cdktf.Fn;
import com.hashicorp.cdktf.TerraformVariable;
import com.hashicorp.cdktf.TerraformVariableConfig;
import com.hashicorp.cdktf.TerraformOutput;
import com.hashicorp.cdktf.TerraformOutputConfig;
import imports.aws.data_aws_availability_zones.DataAwsAvailabilityZones;
import imports.aws.data_aws_availability_zones.DataAwsAvailabilityZonesConfig;
DataAwsAvailabilityZones zones = new DataAwsAvailabilityZones(this, "zones",
DataAwsAvailabilityZonesConfig.builder()
.state("available")
.build());
new TerraformOutput(this, "first-zone", TerraformOutputConfig.builder()
.value(Fn.element(zones.getNames(), 0))
.build());
from cdktf import Fn, TerraformOutput
from imports.aws.provider import AwsProvider
from imports.aws.data_aws_availability_zones import DataAwsAvailabilityZones
zones = DataAwsAvailabilityZones(self, 'zones',
state="available",
)
TerraformOutput(self, 'first-zone',
value=Fn.element(zones.names, 0)
)
using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using Constructs;
using HashiCorp.Cdktf;
using aws.Provider;
using aws.DataAwsAvailabilityZones;
namespace Examples
{
class FunctionStack : TerraformStack
{
public FunctionStack(Construct scope, string name) : base(scope, name)
{
new AwsProvider(this, "aws", new AwsProviderConfig
{
Region = "eu-central-1"
});
DataAwsAvailabilityZones zones = new DataAwsAvailabilityZones(this, "zones", new DataAwsAvailabilityZonesConfig
{
State = "available"
});
new TerraformOutput(this, "first-zone", new TerraformOutputConfig
{
Value = Fn.Element(zones.Names, 0)
});
}
}
}
import (
"github.com/aws/constructs-go/constructs/v10"
"github.com/aws/jsii-runtime-go"
"github.com/hashicorp/terraform-cdk-go/cdktf"
"github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/hashicorp/aws/dataawsavailabilityzones"
aws "github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/hashicorp/aws/provider"
)
func NewFunctionsStack(scope constructs.Construct, name string) cdktf.TerraformStack {
stack := cdktf.NewTerraformStack(scope, &name)
aws.NewAwsProvider(stack, jsii.String("aws"), &aws.AwsProviderConfig{
Region: jsii.String("use-west-2"),
})
zones := dataawsavailabilityzones.NewDataAwsAvailabilityZones(stack, jsii.String("zones"), &dataawsavailabilityzones.DataAwsAvailabilityZonesConfig{
State: jsii.String("available"),
})
cdktf.NewTerraformOutput(stack, jsii.String("first-zone"), &cdktf.TerraformOutputConfig{
Value: cdktf.Fn_Element(cdktf.Token_AsAny(zones.Names()), jsii.Number(0)),
})
return stack
}
To access nested properties from untyped objects or other datasources that return a dynamic datatype, use the Terraform function lookup or, for nested access, the function "Fn.lookupNested()" which is a function offered by CDKTF that allows to avoid nesting Fn.lookup calls.
const v = new TerraformVariable(this, "complex_object", {
type: "object({users: list(object({name: string}))})",
});
new TerraformOutput(this, "users", { value: Fn.lookup(v.value, "users") });
new TerraformOutput(this, "first_user_name", {
value: Fn.lookupNested(v.value, ["users", 0, "name"]),
});
v = TerraformVariable(self, "complex-object",
type = 'object({users: list(object({name: string}))})',
)
TerraformOutput(self, 'users',
value=Fn.lookup(v.string_value, "users")
)
TerraformOutput(self, 'first_user_name',
value=Fn.lookup_nested(v.string_value, ["users", 0, "name"])
)
TerraformVariable v = new TerraformVariable(this, "complex_object", TerraformVariableConfig.builder()
.type("object({users: list(object({name: string}))})")
.build());
new TerraformOutput(this, "users", TerraformOutputConfig.builder()
.value(Fn.lookup(v.getValue(), "users"))
.build());
new TerraformOutput(this, "first-user-name", TerraformOutputConfig.builder()
.value(Fn.lookupNested(v.getValue(), Arrays.asList("users", "0", "name")))
.build());
TerraformVariable v = new TerraformVariable(this, "complex_object", new TerraformVariableConfig
{
Type = "object({users: list(object({name: string}))})",
});
new TerraformOutput(this, "users", new TerraformOutputConfig
{
Value = Fn.Lookup(v.Value, "users")
});
new TerraformOutput(this, "first-user-name", new TerraformOutputConfig
{
Value = Fn.LookupNested(v.Value, new[] { "users", "0", "name" })
});
v := cdktf.NewTerraformVariable(stack, jsii.String("complex-object"), &cdktf.TerraformVariableConfig{
Type: jsii.String("object({users: list(object({name: string}))})"),
})
cdktf.NewTerraformOutput(stack, jsii.String("users"), &cdktf.TerraformOutputConfig{
Value: cdktf.Fn_Lookup(v.Value(), jsii.String("users"), nil),
})
cdktf.NewTerraformOutput(stack, jsii.String("first-user-name"), &cdktf.TerraformOutputConfig{
Value: cdktf.Fn_LookupNested(v.Value(), &[]interface{}{"users", 0, "name"}),
})
Another helper function offered by CDKTF is Fn.rawString which can be used to escape raw strings that contain characters that CDKTF or Terraform would try to interpret otherwise.
new TerraformOutput(this, "quotes", {
value: Fn.rawString(`"b"`),
});
new TerraformOutput(this, "template", {
value: Fn.rawString("${TEMPLATE}"),
});
TerraformOutput(self, 'quotes',
value=Fn.raw_string('"b"')
)
TerraformOutput(self, 'template',
value=Fn.raw_string('${TEMPLATE}')
)
new TerraformOutput(this, "quotes", TerraformOutputConfig.builder()
.value(Fn.rawString("\"b\""))
.build());
new TerraformOutput(this, "template", TerraformOutputConfig.builder()
.value(Fn.rawString("${TEMPLATE}"))
.build());
new TerraformOutput(this, "quotes", new TerraformOutputConfig
{
Value = Fn.RawString("\"b\"")
});
new TerraformOutput(this, "template", new TerraformOutputConfig
{
Value = Fn.RawString("${TEMPLATE}")
});
cdktf.NewTerraformOutput(stack, jsii.String("quotes"), &cdktf.TerraformOutputConfig{
Value: cdktf.Fn_RawString(jsii.String("\"b\"")),
})
cdktf.NewTerraformOutput(stack, jsii.String("template"), &cdktf.TerraformOutputConfig{
Value: cdktf.Fn_RawString(jsii.String("${TEMPLATE}")),
})
Use the Op object to include operators like !, +, and -.
import { Fn, TerraformOutput } from "cdktf";
import { Op } from "cdktf";
const zones = new DataAwsAvailabilityZones(this, "zones", {
state: "available",
});
// ...
new TerraformOutput(this, "half-of-the-zone", {
value: Op.div(Fn.lengthOf(zones.names), 2),
});
DataAwsAvailabilityZones zones = new DataAwsAvailabilityZones(this, "zones", new DataAwsAvailabilityZonesConfig {
State = "available"
});
new TerraformOutput(this, "half-of-the-zone", new TerraformOutputConfig {
Value = Op.Div(Fn.LengthOf(Token.AsList(zones.Names)), 2)
});
import (
"fmt"
"github.com/aws/constructs-go/constructs/v10"
"github.com/aws/jsii-runtime-go"
"github.com/hashicorp/terraform-cdk-go/cdktf"
"github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/hashicorp/aws/dataawsavailabilityzones"
aws "github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/hashicorp/aws/provider"
)
zones := dataawsavailabilityzones.NewDataAwsAvailabilityZones(stack, jsii.String("zones"),
&dataawsavailabilityzones.DataAwsAvailabilityZonesConfig{
State: jsii.String("available"),
},
)
cdktf.NewTerraformOutput(stack, jsii.String("half-of-the-zone"), &cdktf.TerraformOutputConfig{
Value: cdktf.Op_Div(
cdktf.Fn_LengthOf(cdktf.Token_AsAny(zones.Names())),
jsii.Number(2),
),
})
It is also possible to use all built-in Terraform functions without using CDKTF's Fn.* functions described above. To write Terraform built-in functions the same as you would in HCL, simply wrap the desired string within the HCL ${ and } syntax. Note: CDKTF doesn't do any further processing within the escaped syntax (${ and }), and thus is unable to handle nested escape syntaxes yet.
import { Fn, TerraformOutput } from "cdktf";
import { Op } from "cdktf";
import { DataAwsAvailabilityZones } from "@cdktf/provider-aws/lib/data-aws-availability-zones";
const zones = new DataAwsAvailabilityZones(this, "zones", {
state: "available",
});
// ...
new TerraformOutput(this, "half-of-the-zone-raw", {
value: `\${length(${zones.fqn}.names) / 2}`,
});
DataAwsAvailabilityZones zones = new DataAwsAvailabilityZones(this, "zones", new DataAwsAvailabilityZonesConfig
{
State = "available"
});
new TerraformOutput(this, "half-of-the-zone", new TerraformOutputConfig
{
Value = $"${{length({zones.Fqn}.names) / 2}}"
});
import (
"fmt"
"github.com/aws/constructs-go/constructs/v10"
"github.com/aws/jsii-runtime-go"
"github.com/hashicorp/terraform-cdk-go/cdktf"
"github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/hashicorp/aws/dataawsavailabilityzones"
aws "github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/hashicorp/aws/provider"
)
zones := dataawsavailabilityzones.NewDataAwsAvailabilityZones(stack, jsii.String("zones"),
&dataawsavailabilityzones.DataAwsAvailabilityZonesConfig{
State: jsii.String("available"),
},
)
cdktf.NewTerraformOutput(stack, jsii.String("half-of-the-zone-raw"), &cdktf.TerraformOutputConfig{
Value: jsii.String(
fmt.Sprintf("${length(%s.names) / 2}",
*cdktf.Token_AsString(zones.Names(), &cdktf.EncodingOptions{}),
),
),
})
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