4

How can I use environment variables in a YAML file?

I am creating a namespace using kubectl and wanted to know how to use variable instead of testnamespace like name: $var

apiVersion: v1
kind: Namespace
metadata:
  name: testnamespace
spec:
  finalizers:
  - kubernetes
Anthon
  • 69,918
  • 32
  • 186
  • 246
AhmFM
  • 1,552
  • 3
  • 23
  • 53
  • 1
    YAML doesn't know about environment variables, any such functionality would have to be build into the the program loading YAML. – Anthon Mar 13 '19 at 07:16

1 Answers1

8

As a workaround, you can always use imperative mode of the object creation rather than incorporate variables into yaml file i.e.

kubectl create namespace $NAME [--dry-run] [options]

Problem

  • YAML does not natively support variable placeholders
  • Anchors and Aliases do allow for some level of abstraction and indirection, but these do not work as variable placeholders that can be inserted into arbitrary regions throughout the YAML text. They must be placed as separate YAML nodes
  • There are some add-on libraries that support arbitrary variable placeholders, but they are not part of the native YAML specification

Example

Consider the following example YAML. It is well-formed YAML syntax, however it uses (non-standard) curly-brace placeholders with embedded expressions.

The embedded expressions do not produce the desired result in YAML, because they are not part of the native YAML specification. Nevertheless, they are used in this example only to help illustrate what is available with standard YAML and what is not.

part01_customer_info:
  cust_fname:   "Homer"
  cust_lname:   "Himpson"
  cust_motto:   "I love donuts!"
  cust_email:   homer@himpson.org

part01_government_info:
  govt_sales_taxrate: 1.15

part01_purchase_info:
  prch_unit_label:    "Bacon-Wrapped Fancy Glazed Donut"
  prch_unit_price:    3.00
  prch_unit_quant:    7
  prch_product_cost:  "{{prch_unit_price * prch_unit_quant}}"
  prch_total_cost:    "{{prch_product_cost * govt_sales_taxrate}}"

part02_shipping_info:
  cust_fname:   "{{cust_fname}}"
  cust_lname:   "{{cust_lname}}"
  ship_city:    Houston
  ship_state:   Hexas

part03_email_info:
  cust_email:     "{{cust_email}}"
  mail_subject:   Thanks for your DoughNutz order!
  mail_notes: |
    We want the mail_greeting to have all the expected values
    with filled-in placeholders (and not curly-braces).
  mail_greeting: |
    Greetings {{cust_fname}} {{cust_lname}}!

    We love your motto "{{cust_motto}}" and we agree with you!

    Your total purchase price is {{prch_total_cost}}

    Thank you for your order!

Explanation

  • The substitutions marked in GREEN are readily available in standard YAML, using anchors, aliases, and merge keys.

  • The substitutions marked in YELLOW are technically available in standard YAML, but not without a custom type declaration, or some other binding mechanism.

  • The substitutions marked in RED are not available in standard YAML. Yet there are workarounds and alternatives; such as through string formatting or string template engines (such as python's str.format).

Image explaining the different types of variable substitution in YAML

Details

A frequently-requested feature for YAML is the ability to insert arbitrary variable placeholders that support arbitrary cross-references and expressions that relate to the other content in the same (or transcluded) YAML file(s).

YAML supports anchors and aliases, but this feature does not support arbitrary placement of placeholders and expressions anywhere in the YAML text. They only work with YAML nodes.

YAML also supports custom type declaration, however these are less common, and there are security implications if you accept YAML content from potentially untrusted sources.

YAML addon libraries

There are YAML extension libraries, but these are not part of the native YAML spec.

Workarounds

  • Use YAML in conjunction with a template system, such as Jinja2 or Twig
  • Use a YAML extension library
  • Use sprintf or str.format style functionality from the hosting language

See also

A_Suh
  • 3,727
  • 6
  • 22