Let's start with an example ： Suppose I want to get the host IP by 172.25.250.9 Fully qualified domain name （FQDN), But I can't log in to this host , Then you can use the hostvars Magic variable （ I will share my understanding of the word magic later ） In this dictionary ansible_fact This sub dictionary （ It will be explained later ） To get .
First of all, let's take a look at hostvars What's in it , We can use the following command to view hostvars This dictionary ：
ansible locahost -m debug -a “var=hostvars"
localhost | SUCCESS => {
"hostvars": {
"172.25.250.10": {
"ansible_check_mode": false,
"ansible_diff_mode": false,
"ansible_facts": {},
"ansible_forks": 5,
"ansible_inventory_sources": [
"/home/greg/ansible/inventory"
],
"ansible_password": "redhat",
"ansible_playbook_python": "/usr/libexec/platform-python",
"ansible_user": "root",
"ansible_verbosity": 0,
"ansible_version": {
"full": "2.8.0",
"major": 2,
"minor": 8,
"revision": 0,
"string": "2.8.0"
},
"group_names": [
"test"
],
"groups": {
"all": [
"172.25.250.9",
"172.25.250.10",
"172.25.250.13",
"172.25.250.11",
"172.25.250.12"
],
"balancers": [
"172.25.250.13"
],
"dev": [
"172.25.250.9"
],
"prod": [
"172.25.250.11",
"172.25.250.12"
],
"test": [
"172.25.250.10"
],
"ungrouped": [],
"webservers": [
"172.25.250.11",
"172.25.250.12"
]
},
"inventory_dir": "/home/greg/ansible",
"inventory_file": "/home/greg/ansible/inventory",
"inventory_hostname": "172.25.250.10",
"inventory_hostname_short": "172",
"omit": "__omit_place_holder__24431a21232ab0a628ff5c537f472e6b49d1e14a",
"playbook_dir": "/home/greg/ansible"
}, "172.25.250.11": {
"ansible_check_mode": false,
"ansible_diff_mode": false,
"ansible_facts": {},
....................
"172.25.250.12": {
"ansible_check_mode": false,
"ansible_diff_mode": false,
"ansible_facts": {},
"ansible_forks": 5,
"ansible_inventory_sources": [
"/home/greg/ansible/inventory"
],
..........................
"172.25.250.13": {
"ansible_check_mode": false,
"ansible_diff_mode": false,
"ansible_facts": {},
"ansible_forks": 5,
"ansible_inventory_sources": [
"/home/greg/ansible/inventory"
],
............................
"172.25.250.9": {
"ansible_check_mode": false,
"ansible_diff_mode": false,
"ansible_facts": {},
"ansible_forks": 5,
"ansible_inventory_sources": [
"/home/greg/ansible/inventory"
],
......................
"inventory_dir": "/home/greg/ansible",
"inventory_file": "/home/greg/ansible/inventory",
"inventory_hostname": "172.25.250.9",
"inventory_hostname_short": "172",
"omit": "__omit_place_holder__24431a21232ab0a628ff5c537f472e6b49d1e14a",
"playbook_dir": "/home/greg/ansible"
}
}
} As you can see from the output , stay hostvars There are 5 Key value pairs （‘172.25.250.9’： value 1,‘172.25.250.13： value 2 Form like this . Because there are... In my environment 5 Controlled nodes , So there are five key value pairs ）, There is one in the value of each key value pair ”ansible_facts“ Key , The value corresponding to this key is generated f Of acts（ Information about remote host nodes ）.
Tip: hostvars There are also groups This magic variable （ You can see from the top output ）, It contains 5 Of nodes IP, That is to say all This group ,（ All nodes are automatically divided into all In the group , Corresponding , If a group is not added to the division all Groups other than groups , Then the host will be automatically divided into ungrouped In the group , in other words ,ungrouped The hosts in the group are not joined to other groups （ except all Group ））
Okay , Now we can say ：fqdn Can pass ansible_facts From a certain element in it , Since we know hostvars There are ansible_facts, Now let's take a look at ansible_facts There's something in it .
ansible localhost -m setup -a "filter=*fqdn*"
# localhost： The execution node is native
# -m： module
# setup： Generate ansible_facts
# -a： append
# filter： The command composed of the preceding parameters will output all the output of one brain ,filter Parameter is used to filter these outputs
# *fqdn*： Regular expressions , contain fqdn The item
Tip: Can be executed directly “ansible localhost -m setup” To see all of ansible_facts, Because it is too redundant, it will not be shown here .
As can be seen from the picture above ,ansible_facts Inside ansible_fqdn It stores localhost（ The upper left corner shows localhost） Of fqdn.
So we can pass hostvars---->ansible_facts------->ansible_fqdn To get fqdn, however hostvars There are many different hosts ansible_facts, So we need to specify which host it belongs to ansible_facts, Only in this way can we get the correct fqdn, So we can get the following results ：
hostvars["172.25.250.9"]["ansible_facts"]["ansible_fqdn"]
Yes “ magic ” The understanding of the ：
If you want to configure your database server using the value of a ‘fact’ from another node, or the value of an inventory variable assigned to another node, you can use
hostvarsin a template or on an action line
If we want to use information from other nodes , Then you can jinja Or on the command line hostvars This magic variable , My personal understanding is ： Its “ magic ” This is reflected in the fact that it is this machine , But you can get information from other hosts , So it's amazing .
With
hostvars, you can access variables defined for any host in the play, at any point in a playbook. You can access Ansible facts using thehostvarsvariable too, but only after you have gathered (or cached) facts.
We can also go through hostvars Variable acquisition ansible_facts.
Document links ：
Discovering variables: facts and magic variables — Ansible Documentation
Search the document for ：“Information about Ansible” You can navigate to .
Here is a question for the netizens , I have just come into contact with ansible, incomprehension jinja Why can we not define these magic variables first when calling them in the template , That means I want to know ansible How to call jinja Of documents , Or is it jinja How to get those without implementation definitions in jinja Magic variables in the file （ for example groups） Of ？ Please preach and teach , I am obliged .
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