If you’re befuddled by every Poe other than Edgar Allen, after this short blog post, you’ll be confused… nevermore.
I’ve been installing a lot of POE devices recently, and the different methods for providing power over Ethernet cables can be very confusing. There are a few standards in place, and then there’s a method that isn’t a standard, but is widely used.
802.3af or Active PoE:
This is the oldest standard for providing power over Ethernet cables. It allows a maximum of 15.4 watts of power to be transmitted, and the devices (switch and peripheral) negotiate the amount of power and the wires on which the power is transmitted. If a device says it is PoE-compliant, that compliance is usually referring to 802.3af.
802.3at or PoE+:
The main difference between PoE and PoE+ is the amount of power that can be transmitted. There is still negotiation to determine the amount of power and what wires it’s transmitted on, but PoE+ supports up to 25.5 watts of power. Often, access points with multiple radios or higher-powered antennas require more power than 802.3af can supply.
Passive PoE:
This provides power over the Ethernet lines, but it doesn’t negotiate the amount of power or the wires on which the power is sent. Many devices use Passive PoE (notably, the Ubiquiti line of network hardware often uses 24v Passive PoE) to provide power to remote devices. With Passive PoE, the proprietary nature of the power specifics means that it’s often wise to use only power injectors or switches specifically designed for the devices that require Passive PoE. The power is “always on”, so it’s possible to burn out devices if they’re not prepared for electrified Ethernet wires, or if the CAT5 cabling is wired incorrectly.
Figure 1. This AP requires a Passive PoE 24v supply. It can be confusing, because even though it says it’s PoE, it won’t power on using a standard 802.3af switch.
The best practice for using power over Ethernet is either to use equipment that adheres to the 802.3af/at standards or to use the power injectors or switches specifically designed for the hardware. Usually, the standard-based PoE devices are more expensive, but the ability to use any brand PoE switch and device often makes the extra expense worthwhile. That said, there’s nothing wrong with Passive PoE, as long as the correct power is given to the correct devices.
Greps and pipes and greps and pipes and greps and pipes…
This article covers some grep and regex basics.
There are generally two types of coffee drinkers. The first type buys a can of pre-ground beans and uses the included scoop to make their automatic drip coffee in the morning. The second type picks single-origin beans from various parts of the world, accepts only beans that have been roasted within the past week and grinds those beans with a conical burr grinder moments before brewing in any number of complicated methods. Text searching is a bit like that.
For most things on the command line, people think of *.* or *.txt and are happy to use file globbing to select the files they want. When it comes to grepping a log file, however, you need to get a little fancier. The confusing part is when the syntax of globbing and regex overlap. Thankfully, it’s not hard to figure out when to use which construct.
Globbing
The command shell uses globbing for filename completion. If you type something like ls *.txt, you’ll get a list of all the files that end in .txt in the current directory. If you do ls R*.txt, you’ll get all the files that start with capital R and have the .txt extension. The asterisk is a wild card that lets you quickly filter which files you mean.
You also can use a question mark in globbing if you want to specify a single character. So, typing ls read??.txt will list readme.txt, but not read.txt. That’s different from ls read*.txt, which will match both readme.txt and read.txt, because the asterisk means “zero or more characters” in the file glob.
Here’s the easy way to remember if you’re using globbing (which is very simple) vs. regular expressions: globbing is done to filenames by the shell, and regex is used for searching text. The only frustrating exception to this is that sometimes the shell is too smart and conveniently does globbing when you don’t want it to—for example:
grep file* README.TXT
In most cases, this will search the file README.TXT looking for the regular expression file*, which is what you normally want. But if there happens to be a file in the current folder that matches the file* glob (let’s say filename.txt), the shell will assume you meant to pass that to grep, and so grep actually will see:
grep filename.txt README.TXT
Gee, thank you so much Mr. Shell, but that’s not what I wanted to do. For that reason, I recommend always using quotation marks when using grep. 99% of the time you won’t get an accidental glob match, but that 1% can be infuriating. So when using grep, this is much safer:
grep "file*" README.TXT
Because even if there is a filename.txt, the shell won’t substitute it automatically.
So, globs are for filenames, and regex is for searching text. That’s the first thing to understand. The next thing is to realize that similar syntax means different things.
Glob and Regex Conflicts
I don’t want this article to become a super in-depth piece on regex; rather, I want you to understand simple regex, especially as it conflicts with blobbing. Table 1 shows a few of the most commonly confused symbols and what they mean in each case.
Table 1. Commonly Used Symbols
Special Character
Meaning in Globs
Meaning in Regex
*
zero or more characters
zero or more of the character it follows
?
single occurrence of any character
zero or one of the character it follows but not more than 1
.
literal “.” character
any single character
To add insult to injury, you might be thinking about globs when you use grep, but just because you get the expected results doesn’t mean you got the results for the correct reason. Let me try to explain. Here is a text file called filename.doc:
The fast dog is fast.
The faster dogs are faster.
A sick dog should see a dogdoc.
This file is filename.doc
If you type:
grep "fast*" filename.doc
The first two lines will match. Whether you’re thinking globs or regex, that makes sense. But if you type:
grep "dogs*" filename.doc
The first three lines will match, but if you’re thinking in globs, that doesn’t make sense. Since grep uses regular expressions (regex) when searching files, the asterisk means “zero or more occurrences of the previous character”, so in the second example, it matches dog and dogs, because having zero “s” characters matches the regex.
And let’s say you typed this:
grep "*.doc" filename.doc
This will match the last two lines. The asterisk doesn’t actually do anything in this command, because it’s not following any character. The dot in regex means “any character”, so it will match the “.doc”, but it also will match “gdoc” in “dogdoc”, so both lines match.
The moral of the story is that grep never uses globbing. The only exception is when the shell does globbing before passing the command on to grep, which is why it’s always a good idea to use quotation marks around the regular expression you are trying to grep for.
Use fgrep to Avoid Regex
If you don’t want the power of regex, it can be very frustrating. This is especially true if you’re actually looking for some of the special characters in a bunch of text. You can use the fgrep command (or grep -F, which is the same thing) in order to skip any regex substitutions. Using fgrep, you’ll search for exactly what you type, even if they are special characters. Here is a text file called file.txt:
I really hate regex.
All those stupid $, {}, and \ stuff ticks me off.
Why can't text be text?
If you try to use regular grep like this:
grep "$," file.txt
you’ll get no results. That’s because the “$” is a special character (more on that in a bit). If you’d like to grep for special characters without escaping them, or knowing the regex code to get what you want, this will work fine:
grep -F "$," file.txt
And, grep will return the second line of the text file because it matches the literal characters. It’s possible to build a regex query to search for special characters, but it can become complicated quickly. Plus, fgrep is much, much faster on a large text file.
Some Simple, Useful Regex
Okay, now that you know when to use globbing and when to use regular expressions, let’s look at a bit of regex that can make grepping much more useful. I find myself using the caret and dollar sign symbols in regex fairly often. Caret means “at the beginning of the line”, and dollar sign means “at the end of the line”. I used to mix them up, so my silly method to remember is that a farmer has to plant carrots at the beginning of the season in order to sell them for dollars at the end of the season. It’s silly, but it works for me!
Here’s a sample text file named file.txt:
chickens eat corn
corn rarely eats chickens
people eat chickens and corn
chickens rarely eat people
If you were to type:
grep "chickens" file.txt
you will get all four lines returned, because “chickens” is in each line. But if you add some regex to the mix:
grep "^chickens" file.txt
you’ll get both the first and fourth line returned, because the word “chickens” is at the beginning of those lines. If you type:
grep "corn$" file.txt
you will see the first and third lines, because they both end with “corn”. However, if you type:
grep "^chickens.*corn$" file.txt
you will get only the first line, because it is the only one that begins with chickens and ends with corn. This example might look confusing, but there are three regular expressions that build the search. Let’s look at each of them.
First, ^chickens means the line must start with chickens.
Second, .* means zero or more of any character, because remember, the dot means any character, and the asterisk means zero or more of the previous character.
Third, corn$ means the line must end with corn.
When you’re building regular expressions, you just mush them all together like that in a long string. It can become confusing, but if you break down each piece, it makes sense. In order for the entire regular expression to match, all of the pieces must match. That’s why only the first line matches the example regex statement.
A handful of other common regex characters are useful when grepping text files. Remember just to mush them together to form the entire regular expression:
\ — the backslash negates the “special-ness” of special characters, which means you actually can search for them with regex. For example, \$ will search for the $ character, instead of looking for the end of a line.
\s — this construct means “whitespace”, which can be a space or spaces, tabs or newline characters. To find the word pickle surrounded by whitespace, you could search for \spickle\s, and that will find “pickle” but not “pickles”.
.* — this is really just a specific use of the asterisk, but it’s a very common combination, so I mention it here. It basically means “zero or more of any characters”, which is what was used in the corn/chicken example above.
| — this means “or” in regex. So hi|hello will match either “hi” or “hello”. It’s often used in parentheses to separate it from other parts of the regular expression. For example, (F|f)rankfurter will search for the word frankfurter, whether or not it’s capitalized.
[] — brackets are another way to specify “or” options, but they support ranges. So the regex [Ff]rankfurter is the same as the above example. Brackets support ranges though, so ^[A-Z] will match any line that starts with a capital letter. It also supports numbers, so [0-9]$ will match any line that ends in a digit.
Your Mission
You can do far more complicated things with regular expressions. These basic building blocks are usually enough to get the sort of text you need out of a log file. If you want to learn more, by all means, either do some googling on regex, or get a book explaining all the nitty-gritty goodness. If you want me to write more about it, drop a comment and let me know.
I really, really encourage you to practice using regex. The best way to learn is to do, so make a few text files and see if the regex statements you create give you the results you expect. Thankfully, grep highlights the “match” it finds in the line it returns. That means if you’re getting more results than you expect, you’ll see why the regex matched more than you expected, because grep will show you.
The most important thing to remember is that grep doesn’t do globbing—that wild-card stuff is for filenames on the shell only. Even if globbing with grep seems to work, it’s probably just coincidence (look back at the dog/dogs example here if you don’t know what I’m talking about). Have fun grepping!
Roles are the most complicated and yet simplest aspect of Ansible to learn.
I’ve mentioned before that Ansible’s ad-hoc mode often is overlooked as just a way to learn how to use Ansible. I couldn’t disagree with that mentality any more fervently than I already do. Ad-hoc mode is actually what I tend to use most often on a day-to-day basis. That said, using playbooks and roles are very powerful ways to utilize Ansible’s abilities. In fact, when most people think of Ansible, they tend to think of the roles feature, because it’s the way most Ansible code is shared. So first, it’s important to understand the relationship between ad-hoc mode, playbooks and roles.
Ad-hoc Mode
This is a bit of a review, but it’s easy to forget once you start creating playbooks. Ad-hoc mode is simply a one-liner that uses an Ansible module to accomplish a given task on a set of computers. Something like:
ansible cadlab -b -m yum -a "name=vim state=latest"
will install vim on every computer in the cadlab group. The -b signals to elevate privilege (“become” root), the -m means to use the yum module, and the -a says what actions to take. In this case, it’s installing the latest version of vim.
Usually when I use ad-hoc mode to install packages, I’ll follow up with something like this:
ansible cadlab -b -m service -a "name=httpd state=started
↪enabled=yes"
That one-liner will make sure that the httpd service is running and set to start on boot automatically (the latter is what “enabled” means). Like I said at the beginning, I most often use Ansible’s ad-hoc mode on a day-to-day basis. When a new rollout or upgrade needs to happen though, that’s when it makes sense to create a playbook, which is a text file that contains a bunch of Ansible commands.
Playbook Mode
I described playbooks in my last article. They are YAML- (Yet Another Markup Language) formatted text files that contain a list of things for Ansible to accomplish. For example, to install Apache on a lab full of computers, you’d create a file something like this:
Mind you, this isn’t the most elegant playbook. It contains a single play that tries to install httpd with yum and apache2 with apt. If the lab is a mix of CentOS and Ubuntu machines, one or the other of the installation methods will fail. That’s why the ignore_errors command is in each task. Otherwise, Ansible would quit when it encountered an error. Again, this method works, but it’s not pretty. It would be much better to create conditional statements that would allow for a graceful exit on incompatible platforms. In fact, playbooks that are more complex and do more things tend to evolve into a “role” in Ansible.
Roles
Roles aren’t really a mode of operation. Actually, roles are an integral part of playbooks. Just like a playbook can have tasks, variables and handlers, they can also have roles. Quite simply, roles are just a way to organize the various components referenced in playbooks. It starts with a folder layout:
Ansible looks for a roles folder in the current directory, but also in a system-wide location like /etc/ansible/roles, so you can store your roles to keep them organized and out of your home folder. The advantage of using roles is that your playbooks can look as simple as this:
---
- hosts: cadlab
roles:
- webserver
And then the “webserver” role will be applied to the group “cadlab” without needing to type any more information inside your playbook. When a role is specified, Ansible looks for a folder matching the name “webserver” inside your roles folder (in the current directory or the system-wide directory). It then will execute the tasks inside webserver/tasks/main.yml. Any handlers mentioned in that playbook will be searched for automatically in webserver/handlers/main.yml. Also, any time files are referenced by a template module or file/copy module, the path doesn’t need to be specified. Ansible automatically will look inside webserver/files/ or /webserver/templates/ for the files.
Basically, using roles will save you lots of path declarations and include statements. That might seem like a simple thing, but the organization creates a standard that not only makes it easy to figure out what a role does, but also makes it easy to share your code with others. If you always know any files must be stored in roles/rolename/files/, it means you can share a “role” with others and they’ll know exactly what to do with it—namely, just plop it in their own roles folder and start using it.
Sharing Roles: Ansible Galaxy
One of the best aspects of current DevOps tools like Chef, Puppet and Ansible is that there is a community of people willing to share their hard work. On a small scale, roles are a great way to share with your coworkers, especially if you have roles that are customized specifically for your environment. Since many of environments are similar, roles can be shared with an even wider audience—and that’s where Ansible Galaxy comes into play.
I’ll be honest, part of the draw for me with Ansible is the sci-fi theme in the naming convention. I know I’m a bit silly in that regard, but just naming something Ansible or Ansible Galaxy gets my attention. This might be one of those “built by nerds, for nerds” sort of things. I’m completely okay with that. If you head over to the Galaxy site, you’ll find the online repository for shared roles—and there are a ton.
For simply downloading and using other people’s roles, you don’t need any sort of account on Ansible Galaxy. You can search on the website by going to Galaxy and clicking “Browse Roles” on the left side of the page (Figure 1). There are more than 13,000 roles currently uploaded to Ansible Galaxy, so I highly recommend taking advantage of the search feature! In Figure 2, you’ll see I’ve searched for “apache” and sorted by “downloads” in order to find the most popular roles.
Figure 1. Click that link to browse and search for roles.
Figure 2. Jeff Geerling’s roles are always worth checking out.
Many of the standard roles you’ll find that are very popular are written by Jeff Geerling, whose user name is geerlingguy. He’s an Ansible developer who has written at least one Ansible book that I’ve read and possibly others. He shares his roles, and I encourage you to check them out—not only for using them, but also for seeing how he codes around issues like conditionally choosing the correct module for a given distribution and things like that. You can click on the role name and see all the code involved. You might notice that if you want to examine the code, you need to click on the GitHub link. That’s one of the genius moves of Ansible Galaxy—all roles are stored on a user’s GitHub page as opposed to an Ansible Galaxy server. Since most developers keep their code on GitHub, they don’t need to remember to upload to Ansible Galaxy as well.
Incidentally, if you ever desire to share your own Ansible roles, you’ll need to use a GitHub user name to upload them, because again, roles are all stored on GitHub. But that’s getting ahead of things; first you need to learn how to use roles in your environment.
Using ansible-galaxy to Install Roles
It’s certainly possible to download an entire repository and then unzip the contents into your roles folder. Since they’re just text files and structured folders, there’s not really anything wrong with doing it that way. It’s just far less convenient than using the tools built in to Ansible.
There is a search mechanism on the Ansible command line for searching the Ansible Galaxy site, but in order to find a role I want to use, I generally go to the website and find it, then use the command-line tools to download and install it. Here’s an example of Jeff Geerling’s “apache” role. In order to use Ansible to download a role, you need to do this:
sudo ansible-galaxy install geerlingguy.apache
Notice two things. First, you need to execute this command with root privilege. That’s because the ansible-galaxy command will install roles in your system-wide roles folder, which isn’t writable (by default) by your regular user account. Second, take note of the format of roles named on Ansible Galaxy. The format is username.rolename, so in this case, geerlingguy.apache, which is also how you reference the role inside your playbooks.
If you want to see roles listed with the correct format, you can use ansible-galaxy‘s search command, but like I said, I find it less than useful because it doesn’t sort by popularity. In fact, I can’t figure out what it sorts by at all. The only time I use the command-line search feature is if I also use grep to narrow down roles by a single person. Anyway, Figure 3 shows what the results of ansible-galaxy search look like. Notice the username.rolename format.
Figure 3. I love the command line, but these search results are frustrating.
Once you install a role, it is immediately available for you to use in your own playbooks, because it’s installed in the system-wide roles folder. In my case, that’s /etc/ansible/roles (Figure 4). So now, if I create a playbook like this:
---
- hosts: cadlab
roles:
- geerlingguy.apache
Apache will be installed on all my cadlab computers, regardless of what distribution they’re using. If you want to see how the role (which is just a bunch of tasks, handlers and so forth) works, just pick through the folder structure inside /etc/ansible/roles/geerlingguy.apache/. It’s all right there for you to use or modify.
Figure 4. Easy Peasy, Lemon Squeezy
Creating Your Own Roles
There’s really no magic here, since you easily can create a roles folder and then create your own roles manually inside it, but ansible-galaxy does give you a shortcut by creating a skeleton role for you. Make sure you have a roles folder, then just type:
ansible-galaxy init roles/rolename
and you’ll end up with a nicely created folder structure for your new role. It doesn’t do anything magical, but as someone who has misspelled “Templates” before, I can tell you it will save you a lot of frustration if you have clumsy fingers like me.
Sharing Your Roles
If you get to the point where you want to share you roles on Ansible Galaxy, it’s fairly easy to do. Make sure you have your role on GitHub (using git is beyond the scope of this article, but using git and GitHub is a great way to keep track of your code anyway). Once you have your roles on GitHub, you can use ansible-galaxy to “import” them into the publicly searchable Ansible Galaxy site. You first need to authenticate:
ansible-galaxy login
Before you try to log in with the command-line tool, be sure you’ve visited the Ansible Galaxy website and logged in with your GitHub account. You can see in Figure 5 that at first I was unable to log in. Then I logged in on the website, and after that, I was able to log in with the command-line tool successfully.
Figure 5. It drove me nuts trying to figure out why I couldn’t authenticate.
Once you’re logged in, you can add your role by typing:
The process takes a while, so you can add the -no-wait option if you want, and the role will be imported in the background. I really don’t recommend doing this until you have created roles worth sharing. Keep in mind, there are more than 13,000 roles on Ansible Galaxy, so there are many “re-inventions of the wheel” happening.
From Here?
Well, it’s taken me four articles, but I think if you’ve been following along, you should be to the point where you can take it from here. Playbooks and roles are usually where people focus their attention in Ansible, but I also encourage you to take advantage of ad-hoc mode for day-to-day maintenance tasks. Ansible in some ways is just another DevOps configuration management tool, but for me, it feels the most like the traditional problem-solving solution that I used Bash scripts to accomplish for decades. Perhaps I just like Ansible because it thinks the same way I do. Regardless of your motivation, I encourage you to learn Ansible enough so you can determine whether it fits into your workflow as well as it fits into mine.
If you’d like more direct training on Ansible (and other stuff) from yours truly, visit me at my DayJob as a trainer for CBT Nuggets. You can get a full week free if you head over to https://cbt.gg/shawnp0wers and sign up for a trial!
Playbooks make Ansible even more powerful than before.
To be quite honest, if Ansible had nothing but its ad-hoc mode, it still would be a powerful and useful tool for automating large numbers of computers. In fact, if it weren’t for a few features, I might consider sticking with ad-hoc mode and adding a bunch of those ad-hoc commands to a Bash script and be done with learning. Those few additional features, however, make the continued effort well worth it.
Tame the Beast with YAML
Ansible goes out of its way to use an easy-to-read configuration file for making “playbooks”, which are files full of separate Ansible “tasks”. A task is basically an ad-hoc command written out in a configuration file that makes it more organized and easy to expand. The configuration files use YAML, which stands for “Yet Another Markup Language”. It’s an easy-to-read markup language, but it does rely on whitespace, which isn’t terribly common with most config files. A simple playbook looks something like this:
---
- hosts: webservers
become: yes
tasks:
- name: this installs a package
apt: name=apache2 update_cache=yes state=latest
- name: this restarts the apache service
service: name=apache2 enabled=yes state=restarted
The contents should be fairly easy to identify. It’s basically two ad-hoc commands broken up into a YAML configuration file. There are a few important things to notice. First, every filename ends with .yaml, and every YAML file must begin with three hyphen characters. Also, as mentioned above, whitespace matters. Finally, when a hyphen should precede a section and when it should just be spaced appropriately often is confusing. Basically every new section needs to start with a – symbol, but it’s often hard to tell what should be its own section. Nevertheless, it starts to feel natural as you create more and more playbooks.
The above playbook would be executed by typing:
ansible-playbook filename.yaml
And that is the equivalent of these two commands:
ansible webservers -b -m apt -a "name=apache2
↪update_cache=yes state=latest"
ansible webservers -b -m service -a "name=apache2
↪enabled=yes state=restarted"
Handling Your Handlers
But a bit of organization is really only the beginning of why playbooks are so powerful. First off, there’s the idea of “Handlers”, which are tasks that are executed only when “notified” that a task has made a change. How does that work exactly? Let’s rewrite the above YAML file to make the second task a handler:
On the surface, this looks very similar to just executing multiple tasks. When the first task (installing Apache) executes, if a change is made, it notifies the “enable apache” handler, which makes sure Apache is enabled on boot and currently running. The significance is that if Apache is already installed, and no changes are made, the handler never is called. That makes the code much more efficient, but it also means no unnecessary interruption of the already running Apache process.
There are other subtle time-saving issues with handlers too—for example, multiple tasks can call a handler, but it executes only a single time regardless of how many times it’s called. But the really significant thing to remember is that handlers are executed (notified) only when an Ansible task makes a change on the remote system.
Just the Facts, Ma’am
Variable substitution works quite simply inside a playbook. Here’s a simple example:
It should be fairly easy to understand what’s happening above. Note that I did put the entire module action section in quotes. It’s not always required, but sometimes Ansible is funny about unquoted variable substitutions, so I always try to put things in quotes when variables are involved.
The really interesting thing about variables, however, are the “Gathered Facts” about every host. You might notice when executing a playbook that the first thing Ansible does is “Gathering Facts…”, which completes without error, but doesn’t actually seem to do anything. What’s really happening is that system information is getting populated into variables that can be used inside a playbook. To see the entire list of “Gathered Facts”, you can execute an ad-hoc command:
ansible webservers -m setup
You’ll get a huge list of facts generated from the individual hosts. Some of them are particularly useful. For example, ansible_os_family will return something like “RedHat” or “Debian” depending on which distribution you’re using. Ubuntu and Debian systems both return “Debian”, while Red Hat and CentOS will return “RedHat”. Although that’s certainly interesting information, it’s really useful when different distros use different tools—for example, apt vs. yum.
Getting Verbose
One of the frustrations of moving from Ansible ad-hoc commands to playbooks is that in playbook mode, Ansible tends to keep fairly quiet with regard to output. With ad-hoc mode, you often can see what is going on, but with a playbook, you know only if it finished okay, and if a change was made. There are two easy ways to change that. The first is just to add the -v flag when executing ansible-playbook. That adds verbosity and provides lots of feedback when things are executed. Unfortunately, it often gives so much information, that usefulness gets lost in the mix. Still, in a pinch, just adding the -v flag helps.
If you’re creating a playbook and want to be notified of things along the way, the debug module is really your friend. In ad-hoc mode, the debug module doesn’t make much sense to use, but in a playbook, it can act as a “reporting” tool about what is going on. For example:
The above will show you something like Figure 1, which is incredibly useful when you’re trying to figure out the sort of systems you’re using. The nice thing about the debug module is that it can display anything you want, so if a value changes, you can have it displayed on the screen so you can troubleshoot a playbook that isn’t working like you expect it to work. It is important to note that the debug module doesn’t do anything other than display information on the screen for you. It’s not a logging system; rather, it’s just a way to have information (customized information, unlike the verbose flag) displayed during execution. Still, it can be invaluable as your playbooks become more complex.
Figure 1. Debug mode is the best way to get some information on what’s happening inside your playbooks.
If This Then That
Conditionals are a part of pretty much every programming language. Ansible YAML files also can take advantage of conditional execution, but the format is a little wacky. Normally the condition comes first, and then if it evaluates as true, the following code executes. With Ansible, it’s a little backward. The task is completely spelled out, then a when statement is added at the end. It makes the code very readable, but as someone who’s been using if/then mentality his entire career, it feels funny. Here’s a slightly more complicated playbook. See if you can parse out what would happen in an environment with both Debian/Ubuntu and Red Hat/CentOS systems:
Hopefully the YAML format makes that fairly easy to read. Basically, it’s a playbook that will install Apache on hosts using either yum or apt based on which type of distro they have installed. Then handlers make sure the newly installed packages are enabled and running.
It’s easy to see how useful a combination of gathered facts and conditional statements can be. Thankfully, Ansible doesn’t stop there. As with other configuration management systems, it includes most features of programming and scripting languages. For example, there are loops.
Play It Again, Sam
If there is one thing Ansible does well, it’s loops. Quite frankly, it supports so many different sorts of loops, I can’t cover them all here. The best way to figure out the perfect sort of loop for your situation is to read the Ansible documentation directly.
For simple lists, playbooks use a familiar, easy-to-read method for doing multiple tasks. For example:
---
- hosts: webservers
become: yes
tasks:
- name: install a bunch of stuff
apt: "name={{ item }} state=latest update_cache=yes"
with_items:
- apache2
- vim
- chromium-browser
This simple playbook will install multiple packages using the apt module. Note the special variable named item, which is replaced with the items one at a time in the with_items section. Again, this is pretty easy to understand and utilize in your own playbooks. Other loops work in similar ways, but they’re formatted differently. Just check out the documentation for the wide variety of ways Ansible can repeat similar tasks.
Templates
One last module I find myself using often is the template module. If you’ve ever used mail merge in a word processor, templating works similarly. Basically, you create a text file and then use variable substitution to create a custom version on the fly. I most often do this for creating HTML files or config files. Ansible uses the Jinja2 templating language, which is conveniently similar to standard variable substitution in playbooks themselves. The example I almost always use is a custom HTML file that can be installed on a remote batch of web servers. Let’s look at a fairly complex playbook and an accompanying HTML template file.
Here’s the template file, which must end in .j2 (it’s the file referenced in the last task above):
<html><center>
<h1>This computer is running {{ ansible_os_family }},
and its hostname is:</h1>
<h3>{{ ansible_hostname }}</h3>
{# this is a comment, which won't be copied to the index.html file #}
</center></html>
This also should be fairly easy to understand. The playbook takes a few different things it learned and installs Apache on the remote systems, regardless of whether they are Red Hat- or Debian-based. Then, it starts the web servers and makes sure the web server starts on system boot. Finally, the playbook takes the template file, index.html.j2, and substitutes the variables while copying the file to the remote system. Note the {# #} format for making comments. Those comments are completely erased on the remote system and are visible only in the .j2 file on the Ansible machine.
The Sky Is the Limit!
I’ll finish up this series in my next article, where I plan to cover how to build on your playbook knowledge to create entire roles and take advantage of the community contributions available. Ansible is a very powerful tool that is surprisingly simple to understand and use. If you’ve been experimenting with ad-hoc commands, I encourage you to create playbooks that will allow you to do multiple tasks on a multitude of computers with minimal effort. At the very least, play around with the “Facts” gathered by the ansible-playbook app, because those are things unavailable to the ad-hoc mode of Ansible. Until next time, learn, experiment, play and have fun!
If you’d like more direct training on Ansible (and other stuff) from yours truly, visit me at my DayJob as a trainer for CBT Nuggets. You can get a full week free if you head over to https://cbt.gg/shawnp0wers and sign up for a trial!
Finally, an automation framework that thinks like a sysadmin. Ansible, you’re hired.
In my last article, I described how to configure your server and clients so you could connect to each client from the server. Ansible is a push-based automation tool, so the connection is initiated from your “server”, which is usually just a workstation or a server you ssh in to from your workstation. In this article, I explain how modules work and how you can use Ansible in ad-hoc mode from the command line.
Ansible is supposed to make your job easier, so the first thing you need to learn is how to do familiar tasks. For most sysadmins, that means some simple command-line work. Ansible has a few quirks when it comes to command-line utilities, but it’s worth learning the nuances, because it makes for a powerful system.
Command Module
This is the safest module to execute remote commands on the client machine. As with most Ansible modules, it requires Python to be installed on the client, but that’s it. When Ansible executes commands using the Command Module, it does not process those commands through the user’s shell. This means some variables like $HOME are not available. It also means stream functions (redirects, pipes) don’t work. If you don’t need to redirect output or to reference the user’s home directory as a shell variable, the Command Module is what you want to use. To invoke the Command Module in ad-hoc mode, do something like this:
ansible host_or_groupname -m command -a "whoami"
Your output should show SUCCESS for each host referenced and then return the user name that the user used to log in. You’ll notice that the user is not root, unless that’s the user you used to connect to the client computer.
If you want to see the elevated user, you’ll add another argument to the ansible command. You can add -b in order to “become” the elevated user (or the sudo user). So, if you were to run the same command as above with a “-b” flag:
ansible host_or_groupname -b -m command -a "whoami"
you should see a similar result, but the whoami results should say root instead of the user you used to connect. That flag is important to use, especially if you try to run remote commands that require root access!
Shell Module
There’s nothing wrong with using the Shell Module to execute remote commands. It’s just important to know that since it uses the remote user’s environment, if there’s something goofy with the user’s account, it might cause problems that the Command Module avoids. If you use the Shell Module, however, you’re able to use redirects and pipes. You can use the whoami example to see the difference. This command:
ansible host_or_groupname -m command -a "whoami > myname.txt"
should result in an error about > not being a valid argument. Since the Command Module doesn’t run inside any shell, it interprets the greater-than character as something you’re trying to pass to the whoami command. If you use the Shell Module, however, you have no problems:
ansible host_or_groupname -m shell -a "whom > myname.txt"
This should execute and give you a SUCCESS message for each host, but there should be nothing returned as output. On the remote machine, however, there should be a file called myname.txt in the user’s home directory that contains the name of the user. My personal policy is to use the Command Module whenever possible and to use the Shell Module if needed.
The Raw Module
Functionally, the Raw Module works like the Shell Module. The key difference is that Ansible doesn’t do any error checking, and STDERR, STDOUT and Return Code is returned. Other than that, Ansible has no idea what happens, because it just executes the command over SSH directly. So while the Shell Module will use /bin/sh by default, the Raw Module just uses whatever the user’s personal default shell might be.
Why would a person decide to use the Raw Module? It doesn’t require Python on the remote computer—at all. Although it’s true that most servers have Python installed by default, or easily could have it installed, many embedded devices don’t and can’t have Python installed. For most configuration management tools, not having an agent program installed means the remote device can’t be managed. With Ansible, if all you have is SSH, you still can execute remote commands using the Raw Module. I’ve used the Raw Module to manage Bitcoin miners that have a very minimal embedded environment. It’s a powerful tool, and when you need it, it’s invaluable!
Copy Module
Although it’s certainly possible to do file and folder manipulation with the Command and Shell Modules, Ansible includes a module specifically for copying files to the server. Even though it requires learning a new syntax for copying files, I like to use it because Ansible will check to see whether a file exists, and whether it’s the same file. That means it copies the file only if it needs to, saving time and bandwidth. It even will make backups of existing files! I can’t tell you how many times I’ve used scp and sshpass in a Bash FOR loop and dumped files on servers, even if they didn’t need them. Ansible makes it easy and doesn’t require FOR loops and IP iterations.
The syntax is a little more complicated than with Command, Shell or Raw. Thankfully, as with most things in the Ansible world, it’s easy to understand—for example:
This will look in the current directory (on the Ansible server/workstation) for a file called updated.conf and then copy it to each host. On the remote system, the file will be put in /etc/ntp.conf, and if a file already exists, and it’s different, the original will be backed up with a date extension. If the files are the same, Ansible won’t make any changes.
I tend to use the Copy Module when updating configuration files. It would be perfect for updating configuration files on Bitcoin miners, but unfortunately, the Copy Module does require that the remote machine has Python installed. Nevertheless, it’s a great way to update common files on many remote machines with one simple command. It’s also important to note that the Copy Module supports copying remote files to other locations on the remote filesystem using the remote_src=true directive.
File Module
The File Module has a lot in common with the Copy Module, but if you try to use the File Module to copy a file, it doesn’t work as expected. The File Module does all its actions on the remote machine, so src and dest are all references to the remote filesystem. The File Module often is used for creating directories, creating links or deleting remote files and folders. The following will simply create a folder named /etc/newfolder on the remote servers and set the mode:
ansible host_or_groupname -b -m file \
-a "path=/etc/newfolder state=directory mode=0755"
You can, of course, set the owner and group, along with a bunch of other options, which you can learn about on the Ansible doc site. I find I most often will either create a folder or symbolically link a file using the File Module. To create a symlink:
Notice that the state directive is how you inform Ansible what you actually want to do. There are several state options:
link — create symlink.
directory — create directory.
hard — create hardlink.
touch — create empty file.
absent — delete file or directory recursively.
This might seem a bit complicated, especially when you easily could do the same with a Command or Shell Module command, but the clarity of using the appropriate module makes it more difficult to make mistakes. Plus, learning these commands in ad-hoc mode will make playbooks, which consist of many commands, easier to understand (I plan to cover this in my next article).
File Management
Anyone who manages multiple distributions knows it can be tricky to handle the various package managers. Ansible handles this in a couple ways. There are specific modules for apt and yum, but there’s also a generic module called “package” that will install on the remote computer regardless of whether it’s Red Hat- or Debian/Ubuntu-based.
Unfortunately, while Ansible usually can detect the type of package manager it needs to use, it doesn’t have a way to fix packages with different names. One prime example is Apache. On Red Hat-based systems, the package is “httpd”, but on Debian/Ubuntu systems, it’s “apache2”. That means some more complex things need to happen in order to install the correct package automatically. The individual modules, however, are very easy to use. I find myself just using apt or yum as appropriate, just like when I manually manage servers. Here’s an apt example:
ansible host_or_groupname -b -m apt \
-a "update_cache=yes name=apache2 state=latest"
With this one simple line, all the host machines will run apt-get update (that’s the update_cache directive at work), then install apache2’s latest version including any dependencies required. Much like the File Module, the state directive has a few options:
latest — get the latest version, upgrading existing if needed.
absent — remove package if installed.
present — make sure package is installed, but don’t upgrade existing.
The Yum Module works similarly to the Apt Module, but I generally don’t bother with the update_cache directive, because yum updates automatically. Although very similar, installing Apache on a Red Hat-based system looks like this:
ansible host_or_groupname -b -m yum \
-a "name=httpd state=present"
The difference with this example is that if Apache is already installed, it won’t update, even if an update is available. Sometimes updating to the latest version isn’t want you want, so this stops that from accidentally happening.
Just the Facts, Ma’am
One frustrating thing about using Ansible in ad-hoc mode is that you don’t have access to the “facts” about the remote systems. In my next article, where I plan to explore creating playbooks full of various tasks, you’ll see how you can reference the facts Ansible learns about the systems. It makes Ansible far more powerful, but again, it can be utilized only in playbook mode. Nevertheless, it’s possible to use ad-hoc mode to peek at the sorts information Ansible gathers. If you run the setup module, it will show you all the details from a remote system:
ansible host_or_groupname -b -m setup
That command will spew a ton of variables on your screen. You can scroll through them all to see the vast amount of information Ansible pulls from the host machines. In fact, it shows so much information, it can be overwhelming. You can filter the results:
ansible host_or_groupname -b -m setup -a "filter=*family*"
That should just return a single variable, ansible_os_family, which likely will be Debian or Red Hat. When you start building more complex Ansible setups with playbooks, it’s possible to insert some logic and conditionals in order to use yum where appropriate and apt where the system is Debian-based. Really, the facts variables are incredibly useful and make building playbooks that much more exciting.
But, that’s for another article, because you’ve come to the end of the second installment. Your assignment for now is to get comfortable using Ansible in ad-hoc mode, doing one thing at a time. Most people think ad-hoc mode is just a stepping stone to more complex Ansible setups, but I disagree. The ability to configure hundreds of servers consistently and reliably with a single command is nothing to scoff at. I love making elaborate playbooks, but just as often, I’ll use an ad-hoc command in a situation that used to require me to ssh in to a bunch of servers to do simple tasks. Have fun with Ansible; it just gets more interesting from here!
If you’d like more direct training on Ansible (and other stuff) from yours truly, visit me at my DayJob as a trainer for CBT Nuggets. You can get a full week free if you head over to https://cbt.gg/shawnp0wers and sign up for a trial!
