ansible-playbook

A playbook (YAML) lists tasks that describe a state, not steps: template renders a Jinja2 file from the host's variables and changes the target only if it differs. Run twice, a good playbook reports changed=0 the second time (idempotence). --check changes nothing and reports what would change, --diff shows the lines, so together they find drift: a device that no longer matches its intended config. The PLAY RECAP sums up each host: ok, changed, unreachable, failed.

ansible-playbook command usage

ansible-playbook PLAYBOOK [--check] [--diff] [--limit HOST]

Options

--check

dry run: report what would change, change nothing (handlers are skipped)

--diff

show the lines each change would add (+) or remove (-)

--limit HOST

run on these hosts only

-v

more detail on each task

ansible-playbook examples

ansible-playbook /etc/ansible/site.yml --check --diff

Shows, line by line, what the playbook would change on each router, changing nothing.

ansible-playbook /etc/ansible/site.yml

Pushes the intended config and prints a recap with changed= for each router.

Pitfalls

• changed=0 means the devices match the files, not that the files are right: a typo is pushed as faithfully as the fix.
• A task built from shell commands is only idempotent if it compares before it changes (changed_when).

Run it on real machines