Ansible allows variable substitution in plays through Jinja2 templating. If a variable starts a string, the string needs to be quoted, either by single quotes or double quotes, so as not to be treated as a mapping by YAML, e.g.
- hosts: app_servers
vars:
app_path: "{{ base_path }}/22"
or
- hosts: app_servers
vars:
app_path: '{{ base_path }}/22'
The Story
I had been aware of the whole variable substitution thing, but under the impression that quoting can only be done with double quotes, probably because of the official example. I was reading some code by a colleague of mine and realized single quote also works under such circumstances. I then digged a little further and here is what I've got.
The Rationale
- Single quotes or double quotes are for the YAML interpreter. The difference is that certain escaping sequences could be applied in double quotes for special meanings, like "\n" for a new line, but in single quotes they will be kept as is.
- Jinja2 variable substitution, denoted by {{}}, is not escaping. It is for the Jinja2 interpreter, one level higher in Ansible's processing stack.
- Quoting a Jinja2 variable started string is merely so that it is not treated as a mapping by the YAML interpreter. So if a string contains variables but doesn't start with any, it doesn't even need quoting.
