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Docker & Kubernetes · Helm – The Package Manager for Kubernetes · Lesson 4 of 5

Template Syntax and Values

Helm templates are Go templates — actions in {{ }}, built-in objects such as .Values and .Release, pipelines, if/range, include helpers; values are overridden with -f or --set.

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# templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-config
  labels:
    {{- include "mychart.labels" . | nindent 4 }}
data:
  food: {{ .Values.favorite.food | upper | quote }}
  {{- if .Values.debug }}
  debug: "true"
  {{- end }}
Part Example
Built-in objects .Values, .Release.Name, .Release.Namespace, .Chart.Version, .Files, .Capabilities
Pipelines, functions {{ .Values.food \| default "rice" \| upper \| quote }}
Flow control if/else, range (loop over list/dict), with; variables {{- $name := .Release.Name }}
Helpers define in _helpers.tpl, called with include "mychart.labels" . then nindent
{{- / -}} Trim whitespace/newlines on the left/right 
flowchart LR
  D["values.yaml (defaults)"] --> M["Final values"]
  F["-f prod-values.yaml"] --> M
  S["--set replicaCount=2"] --> M
  M --> T["Template"] --> R["Manifest"]

Precedence: values.yaml < -f files (a later file beats an earlier one) < --set.

Hooks (helm.sh/hook: pre-install, post-upgrade…) run Jobs/Pods at points of the release lifecycle; helm test runs the test pods in templates/tests/.

Traps

Command Meaning
helm upgrade web bitnami/nginx -n web -f values.yaml --set replicaCount=2 Upgrade with new values
helm history web -n web List revisions
helm rollback web 1 -n web Go back to revision 1

Functions you will use often

Function Example Purpose
toYaml {{- toYaml .Values.resources \| nindent 12 }} Insert a whole block of values
required {{ required "image.repository is required" .Values.image.repository }} Fail rendering with a clear message
default {{ .Values.port \| default 8080 }} Fallback value
tpl {{ tpl .Values.extraConfig . }} Render a template string stored in values
sha256sum {{ include (print $.Template.BasePath "/configmap.yaml") . \| sha256sum }} Checksum annotation that restarts pods when config changes
lookup {{ lookup "v1" "Secret" .Release.Namespace "db" }} Read an existing object from the cluster during install

lookup returns nothing under helm template, because there is no cluster connection.

Loops and scope

env:
{{- range $key, $value := .Values.env }}
  - name: {{ $key }}
    value: {{ $value | quote }}
{{- end }}

Inside range and with, the dot . changes to the current item; use $ to reach the top-level context, for example {{ $.Release.Name }}.

Reading files

.Files.Get "config/app.conf" reads a file packaged in the chart, and (.Files.Glob "dashboards/*.json").AsConfig turns a set of files into ConfigMap data. Files listed in .helmignore are not available.

Debugging templates

Command Shows
helm template web ./mychart --debug Rendered output plus the error location
helm install web ./mychart --dry-run=server Validates against the live cluster without changing it
helm get manifest web -n web What a deployed release actually contains
helm diff upgrade web ./mychart -n web Changes an upgrade would make (helm-diff plugin)

Hook details

Hooks accept helm.sh/hook-weight to order several hooks, and helm.sh/hook-delete-policy (before-hook-creation, hook-succeeded, hook-failed) to clean up the hook Jobs. A pre-upgrade hook is a common place to run database migrations before new pods start.

Practice interactively →