Replace Line In YAML While Keeping Indentation Using Ansible

2021-08-08 (updated 2025-10-25)
Notes | 1 min read
#ansible #yaml

In theory Ansible should be declarative and have full control over the systems we touch with it.

In practice, this is unfortunately not always the case.

This combination of tasks loads a given yaml file from the remote host, combines a “overwrite dict” onto it, and writes the file back to disk.

- name: Load yaml file contents as fact
  ansible.builtin.slurp:
    src: /etc/some-file.yaml
  register: yaml_file

- name: Parse yaml file contents
  ansible.builtin.set_fact:
    yaml_file_content: "{{ yaml_file.content | b64decode | from_yaml }}"

- name: Create the keys/values that should be overwritten
  ansible.builtin.set_fact:
    yaml_file_content_overwrite:
      some:
        key: "Overwrite value"
      another: "Also overwritten"

- name: Write yaml file with changed values
  ansible.builtin.copy:
    content: "{{ yaml_file_content | combine(yaml_file_content_overwrite, recursive=true) | to_yaml }}"
    dest: /etc/some-file.yaml

I like keeping my learning public. The below is my very old and naive solution I did based on regex.

With this nifty task we can replace the value of a key (given as yaml_key) to a new value (given as new_value) while preserving it’s indentation.

- name: Replace values in YAML file while keeping their indentation
  lineinfile:
    backup: true
    backrefs: true
    state: present
    path: foo.yaml
    regexp: '^(\s*){{ yaml_key }}:.*'
    line: '\1{{ yaml_key }}: {{ new_value }}'