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While automation exists to make it easier to make things repeatable, all of your systems are likely not exactly alike.
All of your systems are likely not the same. On some systems you may want to set some behavior or configuration that is slightly different from others.
Also, some of the observed behavior or state of remote systems might need to influence how you configure those systems. (Such as you might need to find out the IP address of a system and even use it as a configuration value on another system).
You might have some templates for configuration files that are mostly the same, but slightly different based on those variables.
Variables in Ansible are how we deal with differences between systems.
Once understanding variables you’ll also want to dig into Conditionals and Loops. Useful things like the “group_by” module and the “when” conditional can also be used with variables, and to help manage differences between systems.
It’s highly recommended that you consult the ansible-examples github repository to see a lot of examples of variables put to use.
Before we start using variables it’s important to know what are valid variable names.
Variable names should be letters, numbers, and underscores. Variables should always start with a letter.
“foo_port” is a great variable. “foo5” is fine too.
“foo-port”, “foo port”, “foo.port” and “12” are not valid variable names.
Easy enough, let’s move on.
We’ve actually already covered a lot about variables in another section, so far this shouldn’t be terribly new, but a bit of a refresher.
Often you’ll want to set variables based on what groups a machine is in. For instance, maybe machines in Boston want to use ‘boston.ntp.example.com’ as an NTP server.
See the Inventory document for multiple ways on how to define variables in inventory.
In a playbook, it’s possible to define variables directly inline like so:
- hosts: webservers
vars:
http_port: 80
This can be nice as it’s right there when you are reading the playbook.
It turns out we’ve already talked about variables in another place too.
As described in Playbook Roles and Include Statements, variables can also be included in the playbook via include files, which may or may not be part of an “Ansible Role”. Usage of roles is preferred as it provides a nice organizational system.
It’s nice enough to know about how to define variables, but how do you use them?
Ansible allows you to reference variables in your playbooks using the Jinja2 templating system. While you can do a lot of complex things in Jinja, only the basics are things you really need to learn at first.
For instance, in a simple template, you can do something like:
My amp goes to {{ max_amp_value }}
And that will provide the most basic form of variable substitution.
This is also valid directly in playbooks, and you’ll occasionally want to do things like:
template: src=foo.cfg.j2 dest={{ remote_install_path }}/foo.cfg
In the above example, we used a variable to help decide where to place a file.
Inside a template you automatically have access to all of the variables that are in scope for a host. Actually it’s more than that – you can also read variables about other hosts. We’ll show how to do that in a bit.
Note
ansible allows Jinja2 loops and conditionals in templates, but in playbooks, we do not use them. Ansible playbooks are pure machine-parseable YAML. This is a rather important feature as it means it is possible to code-generate pieces of files, or to have other ecosystem tools read Ansible files. Not everyone will need this but it can unlock possibilities.
Note
These are infrequently utilized features. Use them if they fit a use case you have, but this is optional knowledge.
Filters in Jinja2 are a way of transforming template expressions from one kind of data into another. Jinja2 ships with many of these. See builtin filters in the official Jinja2 template documentation.
In addition to those, Ansible supplies many more.
The following filters will take a data structure in a template and render it in a slightly different format. These are occasionally useful for debugging:
{{ some_variable | to_nice_json }}
{{ some_variable | to_nice_yaml }}
The following tasks are illustrative of how filters can be used with conditionals:
tasks:
- shell: /usr/bin/foo
register: result
ignore_errors: True
- debug: msg="it failed"
when: result|failed
# in most cases you'll want a handler, but if you want to do something right now, this is nice
- debug: msg="it changed"
when: result|changed
- debug: msg="it succeeded"
when: result|success
- debug: msg="it was skipped"
when: result|skipped
The default behavior from ansible and ansible.cfg is to fail if variables are undefined, but you can turn this off.
This allows an explicit check with this feature off:
{{ variable | mandatory }}
The variable value will be used as is, but the template evaluation will raise an error if it is undefined.
Jinja2 provides a useful ‘default’ filter, that is often a better approach to failing if a variable is not defined:
{{ some_variable | default(5) }}
In the above example, if the variable ‘some_variable’ is not defined, the value used will be 5, rather than an error being raised.
All these functions return a unique set from sets or lists.
New in version 1.4.
To get a unique set from a list:
{{ list1 | unique }}
To get a union of two lists:
{{ list1 | union(list2) }}
To get the intersection of 2 lists (unique list of all items in both):
{{ list1 | intersect(list2) }}
To get the difference of 2 lists (items in 1 that don’t exist in 2):
{{ list1 | difference(list2) }}
To get the symmetric difference of 2 lists (items exclusive to each list):
{{ list1 | symmetric_difference(list2) }}
New in version 1.6.
To compare a version number, such as checking if the ansible_distribution_version version is greater than or equal to ‘12.04’, you can use the version_compare filter.
The version_compare filter can also be used to evaluate the ansible_distribution_version:
{{ ansible_distribution_version | version_compare('12.04', '>=') }}
If ansible_distribution_version is greater than or equal to 12, this filter will return True, otherwise it will return False.
The version_compare filter accepts the following operators:
<, lt, <=, le, >, gt, >=, ge, ==, =, eq, !=, <>, ne
This filter also accepts a 3rd parameter, strict which defines if strict version parsing should be used. The default is False, and if set as True will use more strict version parsing:
{{ sample_version_var | version_compare('1.0', operator='lt', strict=True) }}
New in version 1.6.
This filter can be used similar to the default jinja2 random filter (returning a random item from a sequence of items), but can also generate a random number based on a range.
To get a random item from a list:
{{ ['a','b','c']|random }} => 'c'
To get a random number from 0 to supplied end:
{{ 59 |random}} * * * * root /script/from/cron
Get a random number from 0 to 100 but in steps of 10:
{{ 100 |random(step=10) }} => 70
Get a random number from 1 to 100 but in steps of 10:
{{ 100 |random(1, 10) }} => 31
{{ 100 |random(start=1, step=10) }} => 51
To concatenate a list into a string:
{{ list | join(" ") }}
To get the last name of a file path, like ‘foo.txt’ out of ‘/etc/asdf/foo.txt’:
{{ path | basename }}
To get the directory from a path:
{{ path | dirname }}
To expand a path containing a tilde (~) character (new in version 1.5):
{{ path | expanduser }}
To work with Base64 encoded strings:
{{ encoded | b64decode }}
{{ decoded | b64encode }}
To take an md5sum of a filename:
{{ filename | md5 }}
To cast values as certain types, such as when you input a string as “True” from a vars_prompt and the system doesn’t know it is a boolean value:
- debug: msg=test
when: some_string_value | bool
To match strings against a regex, use the “match” or “search” filter:
vars:
url: "http://example.com/users/foo/resources/bar"
tasks:
- shell: "msg='matched pattern 1'"
when: url | match("http://example.com/users/.*/resources/.*")
- debug: "msg='matched pattern 2'"
when: url | search("/users/.*/resources/.*")
‘match’ will require a complete match in the string, while ‘search’ will require a match inside of the string.
To replace text in a string with regex, use the “regex_replace” filter:
# convert "ansible" to "able"
{{ 'ansible' | regex_replace('^a.*i(.*)$', 'a\\1') }}
# convert "foobar" to "bar"
{{ 'foobar' | regex_replace('^f.*o(.*)$', '\\1') }}
A few useful filters are typically added with each new Ansible release. The development documentation shows how to extend Ansible filters by writing your own as plugins, though in general, we encourage new ones to be added to core so everyone can make use of them.
YAML syntax requires that if you start a value with {{ foo }} you quote the whole line, since it wants to be sure you aren’t trying to start a YAML dictionary. This is covered on the YAML Syntax page.
This won’t work:
- hosts: app_servers
vars:
app_path: {{ base_path }}/22
Do it like this and you’ll be fine:
- hosts: app_servers
vars:
app_path: "{{ base_path }}/22"
There are other places where variables can come from, but these are a type of variable that are discovered, not set by the user.
Facts are information derived from speaking with your remote systems.
An example of this might be the ip address of the remote host, or what the operating system is.
To see what information is available, try the following:
ansible hostname -m setup
This will return a ginormous amount of variable data, which may look like this, as taken from Ansible 1.4 on a Ubuntu 12.04 system:
"ansible_all_ipv4_addresses": [
"REDACTED IP ADDRESS"
],
"ansible_all_ipv6_addresses": [
"REDACTED IPV6 ADDRESS"
],
"ansible_architecture": "x86_64",
"ansible_bios_date": "09/20/2012",
"ansible_bios_version": "6.00",
"ansible_cmdline": {
"BOOT_IMAGE": "/boot/vmlinuz-3.5.0-23-generic",
"quiet": true,
"ro": true,
"root": "UUID=4195bff4-e157-4e41-8701-e93f0aec9e22",
"splash": true
},
"ansible_date_time": {
"date": "2013-10-02",
"day": "02",
"epoch": "1380756810",
"hour": "19",
"iso8601": "2013-10-02T23:33:30Z",
"iso8601_micro": "2013-10-02T23:33:30.036070Z",
"minute": "33",
"month": "10",
"second": "30",
"time": "19:33:30",
"tz": "EDT",
"year": "2013"
},
"ansible_default_ipv4": {
"address": "REDACTED",
"alias": "eth0",
"gateway": "REDACTED",
"interface": "eth0",
"macaddress": "REDACTED",
"mtu": 1500,
"netmask": "255.255.255.0",
"network": "REDACTED",
"type": "ether"
},
"ansible_default_ipv6": {},
"ansible_devices": {
"fd0": {
"holders": [],
"host": "",
"model": null,
"partitions": {},
"removable": "1",
"rotational": "1",
"scheduler_mode": "deadline",
"sectors": "0",
"sectorsize": "512",
"size": "0.00 Bytes",
"support_discard": "0",
"vendor": null
},
"sda": {
"holders": [],
"host": "SCSI storage controller: LSI Logic / Symbios Logic 53c1030 PCI-X Fusion-MPT Dual Ultra320 SCSI (rev 01)",
"model": "VMware Virtual S",
"partitions": {
"sda1": {
"sectors": "39843840",
"sectorsize": 512,
"size": "19.00 GB",
"start": "2048"
},
"sda2": {
"sectors": "2",
"sectorsize": 512,
"size": "1.00 KB",
"start": "39847934"
},
"sda5": {
"sectors": "2093056",
"sectorsize": 512,
"size": "1022.00 MB",
"start": "39847936"
}
},
"removable": "0",
"rotational": "1",
"scheduler_mode": "deadline",
"sectors": "41943040",
"sectorsize": "512",
"size": "20.00 GB",
"support_discard": "0",
"vendor": "VMware,"
},
"sr0": {
"holders": [],
"host": "IDE interface: Intel Corporation 82371AB/EB/MB PIIX4 IDE (rev 01)",
"model": "VMware IDE CDR10",
"partitions": {},
"removable": "1",
"rotational": "1",
"scheduler_mode": "deadline",
"sectors": "2097151",
"sectorsize": "512",
"size": "1024.00 MB",
"support_discard": "0",
"vendor": "NECVMWar"
}
},
"ansible_distribution": "Ubuntu",
"ansible_distribution_release": "precise",
"ansible_distribution_version": "12.04",
"ansible_domain": "",
"ansible_env": {
"COLORTERM": "gnome-terminal",
"DISPLAY": ":0",
"HOME": "/home/mdehaan",
"LANG": "C",
"LESSCLOSE": "/usr/bin/lesspipe %s %s",
"LESSOPEN": "| /usr/bin/lesspipe %s",
"LOGNAME": "root",
"LS_COLORS": "rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.lz=01;31:*.xz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.axv=01;35:*.anx=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.axa=00;36:*.oga=00;36:*.spx=00;36:*.xspf=00;36:",
"MAIL": "/var/mail/root",
"OLDPWD": "/root/ansible/docsite",
"PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"PWD": "/root/ansible",
"SHELL": "/bin/bash",
"SHLVL": "1",
"SUDO_COMMAND": "/bin/bash",
"SUDO_GID": "1000",
"SUDO_UID": "1000",
"SUDO_USER": "mdehaan",
"TERM": "xterm",
"USER": "root",
"USERNAME": "root",
"XAUTHORITY": "/home/mdehaan/.Xauthority",
"_": "/usr/local/bin/ansible"
},
"ansible_eth0": {
"active": true,
"device": "eth0",
"ipv4": {
"address": "REDACTED",
"netmask": "255.255.255.0",
"network": "REDACTED"
},
"ipv6": [
{
"address": "REDACTED",
"prefix": "64",
"scope": "link"
}
],
"macaddress": "REDACTED",
"module": "e1000",
"mtu": 1500,
"type": "ether"
},
"ansible_form_factor": "Other",
"ansible_fqdn": "ubuntu2",
"ansible_hostname": "ubuntu2",
"ansible_interfaces": [
"lo",
"eth0"
],
"ansible_kernel": "3.5.0-23-generic",
"ansible_lo": {
"active": true,
"device": "lo",
"ipv4": {
"address": "127.0.0.1",
"netmask": "255.0.0.0",
"network": "127.0.0.0"
},
"ipv6": [
{
"address": "::1",
"prefix": "128",
"scope": "host"
}
],
"mtu": 16436,
"type": "loopback"
},
"ansible_lsb": {
"codename": "precise",
"description": "Ubuntu 12.04.2 LTS",
"id": "Ubuntu",
"major_release": "12",
"release": "12.04"
},
"ansible_machine": "x86_64",
"ansible_memfree_mb": 74,
"ansible_memtotal_mb": 991,
"ansible_mounts": [
{
"device": "/dev/sda1",
"fstype": "ext4",
"mount": "/",
"options": "rw,errors=remount-ro",
"size_available": 15032406016,
"size_total": 20079898624
}
],
"ansible_os_family": "Debian",
"ansible_pkg_mgr": "apt",
"ansible_processor": [
"Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz"
],
"ansible_processor_cores": 1,
"ansible_processor_count": 1,
"ansible_processor_threads_per_core": 1,
"ansible_processor_vcpus": 1,
"ansible_product_name": "VMware Virtual Platform",
"ansible_product_serial": "REDACTED",
"ansible_product_uuid": "REDACTED",
"ansible_product_version": "None",
"ansible_python_version": "2.7.3",
"ansible_selinux": false,
"ansible_ssh_host_key_dsa_public": "REDACTED KEY VALUE"
"ansible_ssh_host_key_ecdsa_public": "REDACTED KEY VALUE"
"ansible_ssh_host_key_rsa_public": "REDACTED KEY VALUE"
"ansible_swapfree_mb": 665,
"ansible_swaptotal_mb": 1021,
"ansible_system": "Linux",
"ansible_system_vendor": "VMware, Inc.",
"ansible_user_id": "root",
"ansible_userspace_architecture": "x86_64",
"ansible_userspace_bits": "64",
"ansible_virtualization_role": "guest",
"ansible_virtualization_type": "VMware"
In the above the model of the first harddrive may be referenced in a template or playbook as:
{{ ansible_devices.sda.model }}
Similarly, the hostname as the system reports it is:
{{ ansible_hostname }}
Facts are frequently used in conditionals (see Conditionals) and also in templates.
Facts can be also used to create dynamic groups of hosts that match particular criteria, see the About Modules documentation on ‘group_by’ for details, as well as in generalized conditional statements as discussed in the Conditionals chapter.
If you know you don’t need any fact data about your hosts, and know everything about your systems centrally, you can turn off fact gathering. This has advantages in scaling Ansible in push mode with very large numbers of systems, mainly, or if you are using Ansible on experimental platforms. In any play, just do this:
- hosts: whatever
gather_facts: no
New in version 1.3.
As discussed in the playbooks chapter, Ansible facts are a way of getting data about remote systems for use in playbook variables.
Usually these are discovered automatically by the ‘setup’ module in Ansible. Users can also write custom facts modules, as described in the API guide. However, what if you want to have a simple way to provide system or user provided data for use in Ansible variables, without writing a fact module?
For instance, what if you want users to be able to control some aspect about how their systems are managed? “Facts.d” is one such mechanism.
Note
Perhaps “local facts” is a bit of a misnomer, it means “locally supplied user values” as opposed to “centrally supplied user values”, or what facts are – “locally dynamically determined values”.
If a remotely managed system has an “/etc/ansible/facts.d” directory, any files in this directory ending in ”.fact”, can be JSON, INI, or executable files returning JSON, and these can supply local facts in Ansible.
For instance assume a /etc/ansible/facts.d/preferences.fact:
[general]
asdf=1
bar=2
This will produce a hash variable fact named “general” with ‘asdf’ and ‘bar’ as members. To validate this, run the following:
ansible <hostname> -m setup -a "filter=ansible_local"
And you will see the following fact added:
"ansible_local": {
"preferences": {
"general": {
"asdf" : "1",
"bar" : "2"
}
}
}
And this data can be accessed in a template/playbook as:
{{ ansible_local.preferences.general.asdf }}
The local namespace prevents any user supplied fact from overriding system facts or variables defined elsewhere in the playbook.
If you have a playbook that is copying over a custom fact and then running it, making an explicit call to re-run the setup module can allow that fact to be used during that particular play. Otherwise, it will be available in the next play that gathers fact information. Here is an example of what that might look like:
- hosts: webservers
tasks:
- name: create directory for ansible custom facts
file: state=directory recurse=yes path=/etc/ansible/facts.d
- name: install custom impi fact
copy: src=ipmi.fact dest=/etc/ansible/facts.d
- name: re-read facts after adding custom fact
setup: filter=ansible_local
In this pattern however, you could also write a fact module as well, and may wish to consider this as an option.
Another major use of variables is running a command and using the result of that command to save the result into a variable. Results will vary from module to module. Use of -v when executing playbooks will show possible values for the results.
The value of a task being executed in ansible can be saved in a variable and used later. See some examples of this in the Conditionals chapter.
While it’s mentioned elsewhere in that document too, here’s a quick syntax example:
- hosts: web_servers
tasks:
- shell: /usr/bin/foo
register: foo_result
ignore_errors: True
- shell: /usr/bin/bar
when: foo_result.rc == 5
Registered variables are valid on the host the remainder of the playbook run, which is the same as the lifetime of “facts” in Ansible. Effectively registered variables are just like facts.
We already talked about facts a little higher up in the documentation.
Some provided facts, like networking information, are made available as nested data structures. To access them a simple {{ foo }} is not sufficient, but it is still easy to do. Here’s how we get an IP address:
{{ ansible_eth0["ipv4"]["address"] }}
OR alternatively:
{{ ansible_eth0.ipv4.address }}
Similarly, this is how we access the first element of an array:
{{ foo[0] }}
Even if you didn’t define them yourself, Ansible provides a few variables for you automatically. The most important of these are ‘hostvars’, ‘group_names’, and ‘groups’. Users should not use these names themselves as they are reserved. ‘environment’ is also reserved.
Hostvars lets you ask about the variables of another host, including facts that have been gathered about that host. If, at this point, you haven’t talked to that host yet in any play in the playbook or set of playbooks, you can get at the variables, but you will not be able to see the facts.
If your database server wants to use the value of a ‘fact’ from another node, or an inventory variable assigned to another node, it’s easy to do so within a template or even an action line:
{{ hostvars['test.example.com']['ansible_distribution'] }}
Additionally, group_names is a list (array) of all the groups the current host is in. This can be used in templates using Jinja2 syntax to make template source files that vary based on the group membership (or role) of the host:
{% if 'webserver' in group_names %}
# some part of a configuration file that only applies to webservers
{% endif %}
groups is a list of all the groups (and hosts) in the inventory. This can be used to enumerate all hosts within a group. For example:
{% for host in groups['app_servers'] %}
# something that applies to all app servers.
{% endfor %}
A frequently used idiom is walking a group to find all IP addresses in that group:
{% for host in groups['app_servers'] %}
{{ hostvars[host]['ansible_eth0']['ipv4']['address'] }}
{% endfor %}
An example of this could include pointing a frontend proxy server to all of the app servers, setting up the correct firewall rules between servers, etc.
Additionally, inventory_hostname is the name of the hostname as configured in Ansible’s inventory host file. This can be useful for when you don’t want to rely on the discovered hostname ansible_hostname or for other mysterious reasons. If you have a long FQDN, inventory_hostname_short also contains the part up to the first period, without the rest of the domain.
play_hosts is available as a list of hostnames that are in scope for the current play. This may be useful for filling out templates with multiple hostnames or for injecting the list into the rules for a load balancer.
Don’t worry about any of this unless you think you need it. You’ll know when you do.
Also available, inventory_dir is the pathname of the directory holding Ansible’s inventory host file, inventory_file is the pathname and the filename pointing to the Ansible’s inventory host file.
It’s a great idea to keep your playbooks under source control, but you may wish to make the playbook source public while keeping certain important variables private. Similarly, sometimes you may just want to keep certain information in different files, away from the main playbook.
You can do this by using an external variables file, or files, just like this:
---
- hosts: all
remote_user: root
vars:
favcolor: blue
vars_files:
- /vars/external_vars.yml
tasks:
- name: this is just a placeholder
command: /bin/echo foo
This removes the risk of sharing sensitive data with others when sharing your playbook source with them.
The contents of each variables file is a simple YAML dictionary, like this:
---
# in the above example, this would be vars/external_vars.yml
somevar: somevalue
password: magic
Note
It’s also possible to keep per-host and per-group variables in very similar files, this is covered in Patterns.
In addition to vars_prompt and vars_files, it is possible to send variables over the Ansible command line. This is particularly useful when writing a generic release playbook where you may want to pass in the version of the application to deploy:
ansible-playbook release.yml --extra-vars "version=1.23.45 other_variable=foo"
This is useful, for, among other things, setting the hosts group or the user for the playbook.
Example:
---
- hosts: '{{ hosts }}'
remote_user: '{{ user }}'
tasks:
- ...
ansible-playbook release.yml --extra-vars "hosts=vipers user=starbuck"
As of Ansible 1.2, you can also pass in extra vars as quoted JSON, like so:
--extra-vars '{"pacman":"mrs","ghosts":["inky","pinky","clyde","sue"]}'
The key=value form is obviously simpler, but it’s there if you need it!
As of Ansible 1.3, extra vars can be loaded from a JSON file with the “@” syntax:
--extra-vars "@some_file.json"
Also as of Ansible 1.3, extra vars can be formatted as YAML, either on the command line or in a file as above.
Note
This behavior is infrequently used in Ansible. You may wish to skip this section. The ‘group_by’ module as described in the module documentation is a better way to achieve this behavior in most cases.
Sometimes you will want to do certain things differently in a playbook based on certain criteria. Having one playbook that works on multiple platforms and OS versions is a good example.
As an example, the name of the Apache package may be different between CentOS and Debian, but it is easily handled with a minimum of syntax in an Ansible Playbook:
---
- hosts: all
remote_user: root
vars_files:
- "vars/common.yml"
- [ "vars/{{ ansible_os_family }}.yml", "vars/os_defaults.yml" ]
tasks:
- name: make sure apache is running
service: name={{ apache }} state=running
Note
The variable ‘ansible_os_family’ is being interpolated into the list of filenames being defined for vars_files.
As a reminder, the various YAML files contain just keys and values:
---
# for vars/CentOS.yml
apache: httpd
somethingelse: 42
How does this work? If the operating system was ‘CentOS’, the first file Ansible would try to import would be ‘vars/CentOS.yml’, followed by ‘/vars/os_defaults.yml’ if that file did not exist. If no files in the list were found, an error would be raised. On Debian, it would instead first look towards ‘vars/Debian.yml’ instead of ‘vars/CentOS.yml’, before falling back on ‘vars/os_defaults.yml’. Pretty simple.
To use this conditional import feature, you’ll need facter or ohai installed prior to running the playbook, but you can of course push this out with Ansible if you like:
# for facter
ansible -m yum -a "pkg=facter ensure=installed"
ansible -m yum -a "pkg=ruby-json ensure=installed"
# for ohai
ansible -m yum -a "pkg=ohai ensure=installed"
Ansible’s approach to configuration – separating variables from tasks, keeps your playbooks from turning into arbitrary code with ugly nested ifs, conditionals, and so on - and results in more streamlined & auditable configuration rules – especially because there are a minimum of decision points to track.
A lot of folks may ask about how variables override another. Ultimately it’s Ansible’s philosophy that it’s better you know where to put a variable, and then you have to think about it a lot less.
Avoid defining the variable “x” in 47 places and then ask the question “which x gets used”. Why? Because that’s not Ansible’s Zen philosophy of doing things.
There is only one Empire State Building. One Mona Lisa, etc. Figure out where to define a variable, and don’t make it complicated.
However, let’s go ahead and get precedence out of the way! It exists. It’s a real thing, and you might have a use for it.
If multiple variables of the same name are defined in different places, they win in a certain order, which is:
* -e variables always win
* then comes "most everything else"
* then comes variables defined in inventory
* then comes facts discovered about a system
* then "role defaults", which are the most "defaulty" and lose in priority to everything.
Note
In versions prior to 1.5.4, facts discovered about a system were in the “most everything else” category above.
That seems a little theoretical. Let’s show some examples and where you would choose to put what based on the kind of control you might want over values.
First off, group variables are super powerful.
Site wide defaults should be defined as a ‘group_vars/all’ setting. Group variables are generally placed alongside your inventory file. They can also be returned by a dynamic inventory script (see Dynamic Inventory) or defined in things like Ansible Tower from the UI or API:
---
# file: /etc/ansible/group_vars/all
# this is the site wide default
ntp_server: default-time.example.com
Regional information might be defined in a ‘group_vars/region’ variable. If this group is a child of the ‘all’ group (which it is, because all groups are), it will override the group that is higher up and more general:
---
# file: /etc/ansible/group_vars/boston
ntp_server: boston-time.example.com
If for some crazy reason we wanted to tell just a specific host to use a specific NTP server, it would then override the group variable!:
---
# file: /etc/ansible/host_vars/xyz.boston.example.com
ntp_server: override.example.com
So that covers inventory and what you would normally set there. It’s a great place for things that deal with geography or behavior. Since groups are frequently the entity that maps roles onto hosts, it is sometimes a shortcut to set variables on the group instead of defining them on a role. You could go either way.
Remember: Child groups override parent groups, and hosts always override their groups.
Next up: learning about role variable precedence.
We’ll pretty much assume you are using roles at this point. You should be using roles for sure. Roles are great. You are using roles aren’t you? Hint hint.
Ok, so if you are writing a redistributable role with reasonable defaults, put those in the ‘roles/x/defaults/main.yml’ file. This means the role will bring along a default value but ANYTHING in Ansible will override it. It’s just a default. That’s why it says “defaults” :) See Playbook Roles and Include Statements for more info about this:
---
# file: roles/x/defaults/main.yml
# if not overridden in inventory or as a parameter, this is the value that will be used
http_port: 80
if you are writing a role and want to ensure the value in the role is absolutely used in that role, and is not going to be overridden by inventory, you should put it in roles/x/vars/main.yml like so, and inventory values cannot override it. -e however, still will:
---
# file: roles/x/vars/main.yml
# this will absolutely be used in this role
http_port: 80
So the above is a great way to plug in constants about the role that are always true. If you are not sharing your role with others, app specific behaviors like ports is fine to put in here. But if you are sharing roles with others, putting variables in here might be bad. Nobody will be able to override them with inventory, but they still can by passing a parameter to the role.
Parameterized roles are useful.
If you are using a role and want to override a default, pass it as a parameter to the role like so:
roles:
- { name: apache, http_port: 8080 }
This makes it clear to the playbook reader that you’ve made a conscious choice to override some default in the role, or pass in some configuration that the role can’t assume by itself. It also allows you to pass something site-specific that isn’t really part of the role you are sharing with others.
This can often be used for things that might apply to some hosts multiple times, like so:
roles:
- { role: app_user, name: Ian }
- { role: app_user, name: Terry }
- { role: app_user, name: Graham }
- { role: app_user, name: John }
That’s a bit arbitrary, but you can see how the same role was invoked multiple Times. In that example it’s quite likely there was no default for ‘name’ supplied at all. Ansible can yell at you when variables aren’t defined – it’s the default behavior in fact.
So that’s a bit about roles.
There are a few bonus things that go on with roles.
Generally speaking, variables set in one role are available to others. This means if you have a “roles/common/vars/main.yml” you can set variables in there and make use of them in other roles and elsewhere in your playbook:
roles:
- { role: common_settings }
- { role: something, foo: 12 }
- { role: something_else }
Note
There are some protections in place to avoid the need to namespace variables. In the above, variables defined in common_settings are most definitely available to ‘something’ and ‘something_else’ tasks, but if “something’s” guaranteed to have foo set at 12, even if somewhere deep in common settings it set foo to 20.
So, that’s precedence, explained in a more direct way. Don’t worry about precedence, just think about if your role is defining a variable that is a default, or a “live” variable you definitely want to use. Inventory lies in precedence right in the middle, and if you want to forcibly override something, use -e.
If you found that a little hard to understand, take a look at the ansible-examples repo on our github for a bit more about how all of these things can work together.
See also