markusholtermann
Linux • • Markus Holtermann
Ansible, YAML

Reference variables in Ansible YAML

Today I found myself in a typical DRY situation when working on an Ansible role. I was defining a slightly more complex variable to later be used in an Ansible template. The variable looked a bit like this:

backups:
  - name: primary
    output: /mnt/data/extern
    includes:
      - /etc/
      - /home/
      - /var/lib/
  - name: secondary
    output: /mnt/data/intern
    includes:
      - /etc/
      - /home/
      - /var/lib/
  - name: remote
    output: "ssh://user@host:/mnt/data/backup/client1"
    includes:
      - /etc/
      - /home/
      - /var/lib/
  - name: grab-n-go
    output: /mnt/usb-drive
    includes:
      - /home/markus/documents/important

As you can easily see, the includes item always contains the same values for the first 3 items. I looked into the possibility to define the list of directories only once and use a variable in those places.

I started off with this approach:

backup_includes:
  - /etc/
  - /home/
  - /var/lib/

backups:
  - name: primary
    output: /mnt/data/extern
    includes: backup_includes
  - name: secondary
    output: /mnt/data/intern
    includes: backup_includes
  - name: remote
    output: "ssh://user@host:/mnt/data/backup/client1"
    includes: backup_includes
  - name: grab-n-go
    output: /mnt/usb-drive
    includes:
      - /home/markus/documents/important

Turns out, that does not work. Instead, when iterating over backups.X.includes Jinja2 is iterating over the string backup_includes. That’s of course not what I want.

But YAML does provide a solution to reference other variables, which I found on StackOverflow. It’s also shown on Wikipedia. Using & to define an anchor and * to dereference it.

backup_includes: &backup_includes
  - /etc/
  - /home/
  - /var/lib/

backups:
  - name: primary
    output: /mnt/data/extern
    includes: *backup_includes
  - name: secondary
    output: /mnt/data/intern
    includes: *backup_includes
  - name: remote
    output: "ssh://user@host:/mnt/data/backup/client1"
    includes: *backup_includes
  - name: grab-n-go
    output: /mnt/usb-drive
    includes:
      - /home/markus/documents/important

Apart from & and * there’s also the <<: operator to clone an item and allow overriding of its child attributes:

backup_item: &backup_item
  includes:
    - /etc/
    - /home/
    - /var/lib/
  excludes: []

backups:
  - <<: *backup_item
    name: primary
    output: /mnt/data/extern
  - <<: *backup_item
    name: secondary
    output: /mnt/data/intern
  - <<: *backup_item
    name: remote
    output: "ssh://user@host:/mnt/data/backup/client1"
    excludes:
      - /var/lib/