gistfile1.txt

$ cat group_vars/all
foo: bar

$ cat test.yml
---
# file: test.yml
- hosts: localhost
  tasks:
  - name: Create foo
    template: src=foo.j2 dest=/tmp/foo


$ cat foo.j2
Hopefully this says bar = ${foo}


$ ansible-playbook -i /etc/ansible/hosts test.yml -CD

PLAY [localhost] **************************************************************

GATHERING FACTS ***************************************************************

ok: [localhost]

TASK: [Create foo] ************************************************************
--- before: /tmp/foo
+++ after: /Users/chris/work/configurator/ansible/foo.j2
@@ -1,0 +1,1 @@
+Hopefully this says bar = ${foo}

changed: [localhost]

PLAY RECAP ********************************************************************
localhost                  : ok=2    changed=1    unreachable=0    failed=0


=========================================================================================
===================== NOW adding a vars_files directive to playbook =====================
=========================================================================================

$ cat test.yml
---
# file: test.yml
- hosts: localhost
  tasks:
  - name: Create foo
    template: src=foo.j2 dest=/tmp/foo
  vars_files:  # Not certain if this is required, but it doesn't seem to hurt¬
    - "group_vars/all"


$ ansible-playbook -i /etc/ansible/hosts test.yml -CD

PLAY [localhost] **************************************************************

GATHERING FACTS ***************************************************************

ok: [localhost]

TASK: [Create foo] ************************************************************
--- before: /tmp/foo
+++ after: /Users/chris/work/configurator/ansible/foo.j2
@@ -1,0 +1,1 @@
+Hopefully this says bar = bar

changed: [localhost]

PLAY RECAP ********************************************************************
localhost                  : ok=2    changed=1    unreachable=0    failed=0

For those seeing similar issues, There were 2 discoveries here.

1: the {{}} variable syntax > ${}
2: The group_vars/all file is searched for relative to the specified inventory location (/etc/ansible/group_vars/all by default apparently)