Terraform contains() vs strcontains(): Which One, and the Gotchas
By Vladimir Mikhalev · Docker Captain · CNCF Ambassador · IBM Champion
Terraform is declarative. Which is a polite way of saying you don’t get if-statements like a normal programmer. You still need to validate inputs. You still need to gate deployments and steer behavior. So you reach for the logic tools the language actually hands you.
Two of them look trivial and turn out to be ridiculously useful: contains() and strcontains().
If a bad variable, a missing VM size, or a misnamed AZ has ever cost you a Saturday, read on.
Here’s how both work. No fluff.
contains(): is the value in there, or are you debugging all weekend
contains() checks whether a specific value sits inside a list or a set.
contains(list, value) => boolReturns true if the list has the value. false if it doesn’t. That’s the whole API.
Basic? Yes. But once your infra logic starts leaning on user input, region capabilities, or feature flags, this little function turns into the bouncer at the door of your Terraform nightclub.
Real example: Azure VM sizes
Picture someone on the team requesting a beefy VM in a tiny region that can’t host it. You want to catch that before the apply blows up.
variable "region" { default = "uksouth"}
variable "vm_size" { default = "Standard_DS2_v2"}
data "azurerm_virtual_machine_sizes" "example" { location = var.region}
output "is_supported" { value = contains(data.azurerm_virtual_machine_sizes.example.sizes, var.vm_size)}If vm_size isn’t offered in that region, you get false. Feed that into a count, a for_each, or a validation block. Whatever you pick, it beats watching Terraform barf mid-deploy.
strcontains(): when you’re parsing strings like it’s bash again
Now strcontains(). This is Terraform answering one question: does this string have that other string inside it?
strcontains(string, substr) => boolReach for it when there’s no list, just a single string. An AZ name, a tag, a label. You want to match a pattern in it.
Example: is this AZ optimized
strcontains("us-east-1b-optimal", "optimal") // returns trueIt earns its keep when providers or modules hand back strings with metadata baked in and you need to route logic off that.
Say you only want to deploy when the target zone is labeled “optimal”:
locals { is_optimal_zone = strcontains(var.availability_zone, "optimal")}Now local.is_optimal_zone is your switch. Use it to create resources, set tags, add taints, whatever the condition gates.
Gotchas that bit me, so they don’t bite you
contains()wants the exact value. Case-sensitive. No fuzzy matching.strcontains()doesn’t do regex or wildcards. Pure substring, nothing more.- Maps don’t behave with
contains()the way you’d hope. Lists and sets only. Trycontains({ key = "val" }, "key")and you’ll be disappointed. - Nulls can wreck your results. Validate your variables, or wrap them in
coalesce()when it matters.
Bonus: validate with style
Want to enforce rules on inputs? Drop these functions into validation blocks.
variable "az" { type = string default = "us-east-1b-optimal"
validation { condition = strcontains(var.az, "optimal") error_message = "Only 'optimal' AZs are allowed for this deployment." }}Now Terraform fails early, with a message a human can read, instead of crashing halfway through the plan.
TL;DR
- Reach for
contains()with lists or sets. - Reach for
strcontains()for substring checks on a single string. - Pair them with
validation,count, orfor_eachfor logic that stays clean and safe inside your modules. - Don’t guess. Test.