Terraform: Add function to truncate string

[joshuajabbour]

It would be helpful to be able to truncate strings, because some providers limit the length of identifiers and such. A new built-in function like truncate(string, length) would help users solve this problem.

[billf]

Agree this would be useful.

if anyone finds themselves on this issue stuck until it is added, you can use the replace() function to work around it, e.g. ${replace(var.foo, "/(.{0,10})(.*)/", "$1")}

[joshuajabbour]

Ah smart; hadn't noticed replace and regex...

[spanktar]

this seems like an oversight in the interpolations toolbox. Yes we can hack around it, but it should really be there.

[nathanielks]

More concisely than replace + regex is the substr function:

${substr(var.some_string, 0, 10)}"

[steveh]

Looks like that was added March this year: https://github.com/hashicorp/terraform/pull/12870

So, can probably close this now.

[joshuajabbour]

Excellent. The new substr function solves this.

[yermulnik]

Apologies for posting to closed issue, but I've faced that substr() don't accept input if it's shorter than offset+length: substr: 'offset + length' cannot be larger than the length of the string in. So I had to google around and found solution in this post: ${format("%.14s", var.project_name)}

[davidarmstronglewis]

Ran into this myself using aws_elasticache_cluster, where the cluster_id has to be 20 characters or under: programmatically shortening some variables using substr will work, unless the variables being interpolated are under the desired length. Wrapping the substr in a ternary also doesn't work as apparently both sides of the ternary get evaluating, causing an error.

Consider the example:

locals {
    ClusterId  = "${length(var.project) > 11 ? substr(var.project, 0, 12) : var.project}-${length(var.environment) > 4 ? substr(var.environment, 0, 4) : var.environment}-mem"
    // ...
}

resource "aws_elasticache_cluster" "Memcached" {
    cluster_id      = "${local.ClusterId}"
    // ...
}

If we give the variables environment and project the values of hi and u, we'll get an error:

Error: Error asking for user input: 1 error(s) occurred:

* module.memcached.local.ClusterId: local.ClusterId: substr: 'offset + length' cannot be larger than the length of the string in:

${length(var.project) > 11 ? substr(var.project, 0, 12) : var.project}-${length(var.environment) > 4 ? substr(var.environment, 0, 4) : var.environment}-mem

If the ternary branches were lazily evaluated, this technique would work. Fixing substr to understand truncating strings shorted than the max length would be a better solution. I'm sure there's other reasons to not evaluate both branches of a ternary, however...

[threemachines]

We ran into the ElastiCache case as part of a module, where the expected name length was unpredictable, but we found a different solution: we substr the entire string if it's not too long. I suppose it's a matter of opinion if that's better or hackier than a hypothetical ternary 😅

replication_group_id = "${substr("${var.cluster_name}", 0, min(length("${var.cluster_name}"), 20))}"

Just in case it helps anybody else.

[hashibot[bot]]

I'm going to lock this issue because it has been closed for _30 days_ ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.