Talking with nodes

Now we’re good to go. Let’s play with the command we saw in the previous chapter: ansible. This command is the first one of three that ansible provides which interact with nodes.

Doing something useful

In the previous command, -m ping means “use module ping”. This module is one of many available with ansible. ping module is really simple, it doesn’t need any arguments. Modules that take arguments pass them via -a switch. Let’s see a few other modules.

Shell module

This module lets you execute a shell command on the remote host:

ansible -i hosts -m shell -a 'uname -a' host0.example.org

Output should look like:

host0.example.org | CHANGED | rc=0 >>
Linux scw-e06da7 4.4.0-128-generic #154-Ubuntu SMP Fri May 25 14:15:18 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Copy module

No surprise, with this module you can copy a file from the controlling machine to the node. Lets say we want to copy our /etc/motd to /tmp of our target node:

ansible -i hosts -m copy -a 'src=/etc/hosts dest=/tmp/' host0.example.org

Output should look similar to:

host0.example.org | success >> {
    "changed": true,
    "dest": "/tmp/hosts",
    "group": "root",
    "md5sum": "d41d8cd98f00b204e9800998ecf8427e",
    "mode": "0644",
    "owner": "root",
    "size": 0,
    "src": "/root/.ansible/tmp/ansible-1362910475.9-246937081757218/motd",
    "state": "file"
}

Ansible (more accurately copy module executed on the node) replied back a bunch of useful information in JSON format. We’ll see how that can be used later.

We’ll see other useful modules below. Ansible has a huge module list that covers almost anything you can do on a system. If you can’t find the right module, writing one is pretty easy (it doesn’t even have to be Python, it just needs to speak JSON).

Many hosts, same command

Let’s try that on other hosts too.

Lets say we want to get some facts about the node, and, for instance, know which Ubuntu version we have deployed on nodes, it’s pretty easy:

ansible -i hosts -m shell -a 'grep DISTRIB_RELEASE /etc/lsb-release' all

all is a shortcut meaning ‘all hosts found in inventory file’. It would return:

host0.example.org | CHANGED | rc=0 >>
DISTRIB_RELEASE=16.04

host1.example.org | CHANGED | rc=0 >>
DISTRIB_RELEASE=16.04

host2.example.org | CHANGED | rc=0 >>
DISTRIB_RELEASE=16.04

Many more facts

It would quickly become cumbersome if we wanted more information (ip addresses, RAM size, etc…). The solution comes from another really handy module (weirdly) called setup: it specializes in node’s facts gathering.

Try it out:

ansible -i hosts -m setup host0.example.org

replies with lots of information:

"ansible_facts": {
    "ansible_all_ipv4_addresses": [
        "192.168.0.60"
    ],
    "ansible_all_ipv6_addresses": [],
    "ansible_architecture": "x86_64",
    "ansible_bios_date": "01/01/2007",
    "ansible_bios_version": "Bochs"
    },
    ---snip---
    "ansible_virtualization_role": "guest",
    "ansible_virtualization_type": "kvm"
},
"changed": false,
"verbose_override": true

It’s been truncated for brevity, but you can find many interesting bits in the returned data. You may also filter returned keys, in case you’re looking for something specific.

For instance, let’s say you want to know how much memory you have on all your hosts, easy with:

ansible -i hosts -m setup -a 'filter=ansible_memtotal_mb' all

host0.example.org | SUCCESS => {
    "ansible_facts": {
        "ansible_memtotal_mb": 1996
    },
    "changed": false
}
host2.example.org | SUCCESS => {
    "ansible_facts": {
        "ansible_memtotal_mb": 1996
    },
    "changed": false
}
host1.example.org | SUCCESS => {
    "ansible_facts": {
        "ansible_memtotal_mb": 1996
    },
    "changed": false
}

Notice that hosts replied in different order compared to the previous output. This is because ansible parallelizes communications with hosts!

BTW, when using the setup module, you can use * in the filter= expression. It will act like a shell glob.

Selecting hosts

We saw that all means ‘all hosts’, but ansible provides a lot of other ways to select hosts:

host0.example.org:host1.example.org would run on host0.example.org and host1.example.org
host*.example.org would run on all hosts starting with ‘host’ and ending with ‘.example.org’ (just like a shell glob too)

There are other ways that involve groups, we’ll see that in example03.