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substr(str, offset, length) returns the portion of str starting at offset with at most length characters.

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Signature

substr(str string, offset number, length number) string
offset is zero-based. A negative offset counts from the end of the string. Pass -1 for length to return everything from offset to the end of the string.

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Example

Use the first 8 characters of a full commit SHA as a short identifier in Docker image tags: 
variable "commit" {
  description = "Full git commit SHA"
  type        = string
}

computed "short_sha" {
  description = "Short commit identifier for image tagging"
  expression  = substr(var.commit, 0, 8)
}

task "build" {
  description = "Build and tag the image with the commit SHA"
  commands = [
    "docker build -t myapp:${computed.short_sha} .",
    "docker push myapp:${computed.short_sha}",
  ]
}
a3f9e21b4d6c8f12... becomes a3f9e21b.

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Notes

  • offset and length count grapheme clusters, not bytes or code points.
  • If offset is beyond the end of the string, the result is an empty string.
  • If length exceeds the available characters from offset, the result is truncated to the string end.
  • Use length = -1 to extract from a position to the end: substr(str, 2, -1).