Dive into the ultimate guide to mastering templating with `envsubst` and Jinja2. From Ansible to Docker to Kubernetes, we've got you covered.

Unlock the Power of envsubst and Jinja2: Your Ultimate Guide to Templating

  • Last Modified: 11 Sep, 2023

The comprehensive guide for understanding and implementing templating in DevOps. Go from templating novice to ninja with real-world examples.

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Hey, you! Yeah, you—sitting there wondering how to up your DevOps game. 🕹️ Listen up, because we’re diving deep into the world of templating today. I mean, who hasn’t gotten lost in a sea of repetitive YAML, JSON, or INI files and thought, “Seriously, isn’t there an easier way?” 🤔 Well, there is, and you’re about to become the Picasso of templating with envsubst and Jinja2. 🎨

Why Templating is Crucial in Modern DevOps

Hold onto your hats! Templating isn’t just a shiny gimmick; it’s the linchpin of scalable, maintainable, and—dare I say—enjoyable DevOps.

Remember setting up those endless Apache or Nginx server blocks? Copy-pasting code until your eyes crossed? I’ve been there, and it’s no fun. You miss one semicolon, and it’s game over, buddy. But templating? Ah, it’s the gift that keeps on giving.

CriteriaWithout TemplatingWith Templating
Speed🐢 Slow🚀 Fast
Error-prone🚨 Yes✅ No
Scalability📈 Limited🌍 Unlimited

You see, the beauty of templating lies in its dynamism. You set up your template once—be it a Docker Compose file, an Ansible playbook, or an Nginx config—and voilà! You can use and reuse it across multiple environments, tweaking just a few variables to fit the specific use case. 🤯

My ‘Aha!’ Moment with Templating
Once upon a time, I was managing a monolithic application, and each deployment was a nerve-wracking affair. One bad config could wreak havoc. Then I discovered templating, and my world changed. I set up my variables, slotted them into templates, and just like that, deployments became a walk in the park. 🌳

So, how about it? Ready to join the templating revolution and save yourself from copy-paste purgatory? 😇

Basic: Starting Simple to Get You Geared Up

Alright, folks, if you’re a templating newbie, this is where you want to start. We’re keeping it simple but saucy, laying the foundation for some serious templating wizardry. 🧙‍♂️

What is Templating?

So, you’ve heard the term “templating” being thrown around like confetti at a New Year’s Eve party, but what the heck is it? 🎉 Let’s break it down.

Definition and Why It Matters

Templating is like cooking with a recipe. You’ve got your list of ingredients (variables), and you’ve got your cooking steps (the template). Swap out the ingredients, follow the same steps, and you can cook up a storm without ever repeating yourself.

In layman’s terms, templating allows you to define a skeleton—like an HTML layout or a configuration file—where you can swap out specific values without altering the structure. That’s right, it’s like Mad Libs for code!

AspectWhy It’s Important
Code ReusabilityDo more with less
Dynamic ConfigurationsAdapt without rewrites
Error ReductionFewer typos, more peace

Let’s say you’ve got an Nginx configuration. Imagine having to copy-paste that bad boy for each new project. Nah-ah, nobody’s got time for that! Instead, you create a template with placeholders for the variables that’ll change—like the domain name, root directory, or log paths—and then just plug those in for each project. 🚀

My Personal Experience with Templating

That Time I Ditched Copy-Paste
Back when I was a rookie, I was copy-pasting Nginx configs like there was no tomorrow. Until one day, I pasted the wrong config into the wrong project. Disaster? You betcha! It took me hours to find the mistake and even more time to fix it. That’s when I said, “Enough!” and embraced templating. It was like upgrading from a flip phone to a smartphone. 📱

Believe me when I say that templating is a lifesaver. Once you go template, you never go back. 🤘

History of Templating: Unveiling the Curtain

If you’re anything like me, you’re probably thinking, “Where did this wizardry come from?” 🤔 Well, put on your historian’s hat, because we’re taking a quick journey through time. 🚀

The Need for Templating

Back in the day, coding was as manual as churning butter. You needed a separate file for every single task, and managing these files was an absolute nightmare.

Let’s take websites as an example. Imagine coding each HTML page individually, from scratch, for a site with hundreds of pages. Yeah, it’s a coder’s worst nightmare! But as projects grew in scale and complexity, developers needed a way to maintain their sanity. Enter templating: the knight in shining armor. 🏰

For web developers, templating meant freedom. No more individual HTML files for every page. A single layout could be created and reused, making life a gazillion times easier.
Sysadmins and DevOps professionals found their own Holy Grail in templating. Imagine managing server configurations for a hundred machines manually. With templating, they could make changes at scale effortlessly.
Automation tools like Ansible, Puppet, and Chef also jumped on the templating bandwagon, making it easier to manage configurations and deploy applications.

Who Initiated These Concepts?

You might be surprised to learn that the concept of templating isn’t exactly new. It’s rooted in programming practices that date back decades, such as:

  • DRY (Don’t Repeat Yourself): Encouraged coders to be efficient and reuse code.
  • Separation of Concerns: Pushed the idea that different parts of software should handle different responsibilities.

If you’ve ever dabbled in any form of software development, you’ll recognize these as fundamental principles. They laid the groundwork for what would eventually become templating.

Some of the earliest formal templating systems include:

  • Smarty for PHP
  • ERB (Embedded Ruby) for Ruby

But the hero we’re focusing on today, envsubst, is a Unix command-line utility. It doesn’t have a single creator; rather, it’s the result of collective wisdom in the Unix and Linux communities.

Did You Know?
The envsubst command is part of the GNU gettext package, which is an open-source project! It’s a whole community effort to make software localization easier.

So there you have it—our quick but enlightening trip through the history of templating. Ready to get back to the future and start using it?

What is envsubst? Unlocking the Mystery 🗝️

I get it. “Env-what-now?” 🤨 No worries, we’re about to clear up the fog.

Basic Description

The term envsubst stands for “Environment Substitute”. It’s a Unix command-line utility that allows you to replace variables in a given text with their corresponding environment variables.

Imagine you have a text file with some placeholders like $NAME or ${AGE}. All you need to do is set those as environment variables, and envsubst will swap them out for you. It’s like a text-based version of “Find and Replace,” but way cooler.

# Simple example: Replace $NAME in a file
echo "Hello, $NAME" > greeting.txt
export NAME=John
envsubst < greeting.txt
# Output: Hello, John

It’s simple, but don’t let that fool you. The simplicity is its power, especially when you’re working with Docker, Ansible, or NGINX configurations.

How and When It Came into Existence

Wondering who to thank for this nifty tool? envsubst is part of the GNU gettext utilities, aimed at internationalizing and localizing software.

The `gettext` package is a stalwart in the open-source community, dating back to 1995. Yep, `envsubst` has been around for a while!
Unlike some software that has a single creator or a small team, `envsubst` and the whole `gettext` package are communal efforts. They're the epitome of what the open-source community can achieve.

The tool was initially meant to help in localizing software into different languages. Over time, developers saw its potential in templating, especially for server configurations and scripts.

Personal Experience: My ‘Ah-Ha!’ Moment
When I first stumbled upon envsubst, I was like, “Eh, this is too simple. What could it possibly do for me?” Boy, was I wrong! I was dealing with an NGINX configuration, and using envsubst felt like taking off weighted training clothes. I couldn’t believe how straightforward it made the whole process. 🤩

Alright, now that we’ve got envsubst down to a T, are you ready to start working with it? 🛠️

Basic Syntax and Usage: Your First Steps 🚶‍♂️🚶‍♀️

The basic syntax of envsubst is a cakewalk. If you’ve worked with shell commands before, you’re gonna find this a breeze.

Here’s how you can use it:

# Basic Syntax
envsubst [OPTION] < INPUT-FILE
  • OPTION: This is optional. You can specify which variables to replace.
  • INPUT-FILE: The text file containing the placeholders you want to replace.

Simple, right? But there’s more! You can also pipe content directly into envsubst, like this:

echo "Welcome, $USER!" | envsubst

This will replace $USER with the value of the USER environment variable and display the string “Welcome, [Your Username]!”

Personal Experience: The Lightbulb Moment
So there I was, fumbling through Ansible playbooks, dealing with multiple template files. Then it hit me: “Wait a sec, I can pipe this into envsubst!” That’s right, I just piped my templated configuration right through envsubst and watched it transform before my eyes. Mind = blown! 🤯

Be cautious when using envsubst with shell scripts. Since it replaces $variables, it could potentially replace a variable you didn’t intend to if you’re not careful!

Now you know how to walk; are you ready to run with envsubst? 🏃‍♂️🏃‍♀️

envsubst vs. Jinja2: The Showdown 🥊

The templating world isn’t a one-horse town, my friends. While envsubst is a sleek and simple stallion, Jinja2 is like a decorated warhorse with armor and plumes. Both have their merits, and the “better” choice often comes down to your specific needs. Let’s get into the nitty-gritty.

Basic Differences and Similarities


Both envsubst and Jinja2 are used for text substitution. They take a template file, replace placeholders with actual values, and generate an output. It’s the what, not the how, that’s similar here.


Simplicity is the name of the game here. `envsubst` works directly with environment variables. No frills, no added complexity—just direct substitution. However, it's not the tool for you if you need logic like conditionals or loops.
Jinja2 is like the Swiss Army knife in your templating toolkit. It's not just about environment variables; you can pull in data from different sources. Need conditionals, loops, or even macros? Jinja2 has got you covered.

Why I Personally Lean Towards One Over the Other

My personal preference shifts like the wind, depending on the project at hand. But if I have to pick a daily driver? I’d go with Jinja2.

Now, don’t get me wrong; envsubst is like that old reliable friend who shows up for coffee every weekend. It’s easy to use and understand. But sometimes you just need more than a casual conversation over a cup of joe—you need a deep, intellectual discussion, and that’s where Jinja2 comes in.

My Eureka Moment with Jinja2
I was building this Ansible playbook to configure a fleet of Docker containers. Sure, I could’ve used envsubst for some basic substitutions, but then I hit a wall. I needed to add conditional blocks within the configuration files. Enter Jinja2 with its if-else capabilities, and voilà! My configs were dynamic, flexible, and robust. It felt like upgrading from a sedan to a sports car!

However, with great power comes great complexity. Jinja2’s extensive features mean there’s more to manage, and the learning curve is steeper. So if your project is straightforward, envsubst could actually be a faster, lighter option.

In essence, envsubst is the quick and easy solution for simpler tasks. On the other hand, Jinja2 is the tool of choice for complex, nuanced operations. Choose your weapon wisely!

Setting up a Basic Template with envsubst 🛠️

Now that we’ve cleared the air on what envsubst is and how it differs from Jinja2, let’s get to the fun part—coding! Who’s excited? I know I am! 🙌

Code Example: A Simple Text Template

Creating a template with envsubst is as simple as baking a pre-made pizza—you just need to add a few of your favorite toppings (variables).

Let’s say we’re trying to create a configuration file for an NGINX web server. The server name and the root path might change depending on the environment (prod, dev, staging, you name it). Here’s how you’d create a simple template.

Create a file named nginx.conf.template and add the following:

server {
    listen 80;
    server_name ${SERVER_NAME};

    location / {
        root ${WEB_ROOT};
        index index.html;

Yep, that’s it! Those ${SERVER_NAME} and ${WEB_ROOT} placeholders are where envsubst will plug in the values. It’s a bit like writing a Mad Libs book, where you leave blanks for nouns, adjectives, etc., and then fill them in later to create a hilarious (or in our case, functional) story.

How to Run It

Running an envsubst command is so straightforward, even my grandma could do it. Seriously, she’s getting pretty good with the command line!

All you have to do is define those environment variables and pass the template file through envsubst. Like this:

export SERVER_NAME=mywebsite.com
export WEB_ROOT=/var/www/html
envsubst < nginx.conf.template > nginx.conf

What’s happening here? You’re setting your environment variables with the export command. Then, envsubst reads nginx.conf.template, replaces the placeholders with the actual environment variables, and outputs the new nginx.conf.

Hold On, What’s That Syntax?
If you’re scratching your head about the < and >, don’t worry—you’re not alone. These are just input and output redirection operators. The < tells envsubst to read from nginx.conf.template, and the > tells it to write the output to nginx.conf.

And voilà! You’ve got yourself a shiny, new NGINX configuration file ready for action. 🚀

Setting up a Basic Template with Jinja2 🧰

Time to switch gears and get cozy with Jinja2. If you’ve ever felt limited by envsubst, then Jinja2 will be like going from a tricycle to a mountain bike with all the bells and whistles.

Code Example: A Simple Text Template

Creating a template with Jinja2 is kinda like building a LEGO set. You’ve got your base pieces, but you can also throw in some special blocks to build something truly unique.

Let’s stick with our NGINX example for consistency. Create a file called nginx_jinja2.conf.template and add the following:

server {
    listen 80;
    server_name {{ server_name }};

    location / {
        root {{ web_root }};
        index index.html;

Notice the double curly braces? Those are Jinja2’s way of saying, “Hey, replace this spot with an actual value, pretty please!”

How to Run It

So you’ve got your Jinja2 template, but how do you turn that template into an actual configuration file? That’s where Python steps in.

First, make sure you’ve got Jinja2 installed. If not, you can install it using pip:

pip install Jinja2

Then, create a Python script that’ll do the magic:

from jinja2 import Environment, FileSystemLoader

env = Environment(loader=FileSystemLoader('./'))
template = env.get_template('nginx_jinja2.conf.template')

output = template.render(server_name='mywebsite.com', web_root='/var/www/html')
with open('nginx_jinja2.conf', 'w') as f:
Python Script Breakdown
Here, we’re using the Jinja2 Python library to load our template file. We then render the template using the render method, which takes our variables (server_name and web_root) and replaces the placeholders in the template. Finally, we write the output to a new nginx_jinja2.conf file.

And boom! 🎉 You’ve got yourself an NGINX configuration file generated with Jinja2. It’s like cooking a gourmet meal in your own kitchen.

Intermediate 🧙‍♂️

So, you’ve got the basics down, and you’re feeling pretty good about yourself, eh? That’s fantastic! But hold on, we’re about to go deeper. In this section, we’re gonna integrate envsubst with Ansible. Why? Because Ansible is like the conductor of our DevOps orchestra, and sometimes, it just makes sense to use simple, lightweight tools like envsubst within its roles. Are you pumped? ‘Cause I am! 🎉

envsubst with Ansible 🤖

Why would you want to use something as simple as envsubst when you’ve got the might of Ansible at your disposal? Well, simplicity can be a superpower, my friends!

Using envsubst in Ansible Roles

Ansible roles are like recipe books for DevOps. They tell Ansible exactly what steps to follow to achieve a specific setup. But sometimes, those recipes need a little personal touch. That’s where envsubst comes in.

Here’s how you can include envsubst in an Ansible role.

Create a task in your Ansible role that looks something like this:

- name: Generate NGINX configuration file using envsubst
  shell: |
    export SERVER_NAME={{ server_name }}
    export WEB_ROOT={{ web_root }}
    envsubst < /path/to/nginx.conf.template > /path/to/nginx.conf
In this Ansible task, we're using the `shell` module to run a shell command. We set environment variables (`SERVER_NAME` and `WEB_ROOT`) directly from our Ansible variables. Then, we use `envsubst` to substitute these values into our template and output the final NGINX configuration.
You might be thinking, "Hey, Ansible already has template modules, why use `envsubst`?" The answer is simple: sometimes you don't need a sledgehammer to crack a nut. `envsubst` is quick, lightweight, and gets the job done without much fuss.
If you're building something that requires quick configuration changes or you're dealing with a simple, static setup, `envsubst` within an Ansible role can be incredibly useful. It’s faster and easier to debug.

Practical Example

Now, let’s assume you’ve got a multi-tier application that uses different configurations for development, staging, and production environments.

Create a playbook nginx_setup.yml:

- hosts: web_servers
    - role: nginx_setup
        server_name: mywebsite.com
        web_root: /var/www/html

And within your nginx_setup role, have a task like the one above for envsubst.

To run the playbook:

ansible-playbook -i inventory.ini nginx_setup.yml
Real-World Usage
I’ve used this approach in projects where we needed fast, uncomplicated configuration changes that didn’t warrant the use of heavier templating engines. And let me tell you, it works like a charm!

And there you have it—a practical example of how you can use envsubst in your Ansible roles to make your life just a tad bit easier.

Jinja2 with Ansible 🎨

If envsubst is the chisel to your configuration sculpting, then Jinja2 is the chainsaw. Way more powerful, but you’ve gotta know how to handle it.

Using Jinja2 Templates in Ansible Roles

Jinja2 is the go-to templating engine for Ansible. Its flexibility is like a Swiss army knife for DevOps, making it a top pick for complex configurations. Setting up a Jinja2 template in Ansible is straightforward.

  1. Create a Jinja2 template file, let’s say nginx.j2, and add your variables using the double curly-brace syntax:
server {
    listen 80;
    server_name {{ server_name }};
    root {{ web_root }};
  1. Now, in your Ansible role, use the template module to apply this Jinja2 template:
- name: Generate NGINX configuration using Jinja2
    src: nginx.j2
    dest: /path/to/nginx.conf
We're using Ansible's `template` module, which internally uses Jinja2 to replace variables in our `nginx.j2` file. The updated configuration then gets saved to `/path/to/nginx.conf`.
Jinja2 allows for more complex logic within your templates like conditionals and loops. So if you need to do some heavy-lifting, Jinja2 is your guy.

Practical Example

Alright, time to roll up those sleeves and get into a practical example.

Create a playbook nginx_jinja_setup.yml:

- hosts: web_servers
    - role: nginx_jinja_setup
        server_name: mywebsite.com
        web_root: /var/www/html

Within your nginx_jinja_setup role, add a task like the one mentioned above for Jinja2. To run the playbook:

ansible-playbook -i inventory.ini nginx_jinja_setup.yml

This is a more dynamic approach and provides more control over the configuration. I’ve often used this approach when the configuration logic got a bit too complicated for envsubst to handle efficiently.

Using envsubst with Docker 🐳

Ah, Docker, the DevOps darling! But how often have you found yourself drowning in docker-compose.yaml files for different environments? Here’s where envsubst comes to the rescue.

Templating Docker Compose Files

Docker doesn’t have built-in support for templates, but envsubst is a neat little trick you can use. Say you have a docker-compose.template.yaml file like this:

version: '3'
    image: nginx:${NGINX_VERSION}

What’s cool about this is you can easily substitute environment variables and generate environment-specific compose files.

Run this shell command to generate a `docker-compose.yaml` file with all the variables replaced. ```bash export NGINX_VERSION=1.19 export SERVER_NAME=mywebsite.com envsubst < docker-compose.template.yaml > docker-compose.yaml ```
`envsubst` is incredibly useful in Docker for generating quick, environment-specific files without needing an additional tool. One shell command, and you're ready to roll!

Practical Example

Time to put the pedal to the metal! 🚗

  1. First, create a docker-compose.template.yaml file like the one above.
  2. Export your environment variables. Something like:
export NGINX_VERSION=1.19
export SERVER_NAME=mywebsite.com
  1. Run envsubst to generate your docker-compose.yaml:
envsubst < docker-compose.template.yaml > docker-compose.yaml
  1. Bring up your Docker services:
docker-compose up -d

I’ve personally found this technique to be a lifesaver for quick-and-dirty setup tasks. It keeps my repo clean and makes environment-specific deployment a breeze. However, for more complex needs, I’d switch to a more powerful templating engine.

Using Jinja2 with Docker 🎭

While envsubst is your Swiss Army knife, Jinja2 is the high-end chef’s knife set with more gadgets than you knew you needed. Let’s see how it adds flavor to your Docker life.

How to Use Third-Party Libraries to Template Docker Files

For Jinja2 and Docker to play nicely together, you’ll need a third-party helper like jinja2-cli. It’s a Python package you can install easily through pip.

Practical Example

Alright, let’s do some real-world stuff here.

  1. Step 1: Create a Jinja2 template file for Docker Compose, something like docker-compose.j2. Populate it with your typical YAML configuration, but add Jinja2 template variables where you need customization. For example:
version: '3'
    image: nginx:{{ nginx_version }}
      - SERVER_NAME={{ server_name }}
  1. Step 2: Create a separate YAML file to store your variable values. Let’s call it variables.yml.
nginx_version: 1.19
server_name: mywebsite.com
  1. Step 3: Render the Jinja2 template using the jinja2-cli.
jinja2 docker-compose.j2 variables.yml --format=yaml > docker-compose.yaml
  1. Step 4: Fire up your Docker services.
docker-compose up -d

I’ve found that Jinja2 really shines when you need more complex logic, like loops and conditionals, in your Docker templates. With great power comes… you know the drill.

envsubst vs Jinja2: The Smackdown 🥊

You’ve met both contestants: envsubst, the speedy utility guy, and Jinja2, the heavyweight with more moves than a chess game. Let’s see how they compare head-to-head.

Basic Differences and Similarities

At its core, both envsubst and Jinja2 aim to simplify your life by allowing variable substitution in files. But how they do it and what else they offer is where things get interesting.

  1. Complexity: envsubst is easier to use but offers less functionality. Jinja2 has a steeper learning curve but offers way more features like loops, conditionals, and filters.
  2. Dependencies: envsubst comes pre-installed on most UNIX systems. Jinja2 usually requires you to install additional packages or libraries.
  3. Integration: Jinja2 is deeply integrated into tools like Ansible, while envsubst is more of a stand-alone utility.

Both can:

  1. Perform variable substitution
  2. Work with different types of text files (not just YAML or JSON)
  3. Be used in automation scripts

Why I Personally Lean Towards One Over the Other

This is where personal preference kicks in. For quick-and-dirty tasks, I absolutely love envsubst. It’s fast, lightweight, and does the job. But when things get complex, Jinja2 is my go-to. The additional features more than justify the initial time spent learning the ropes.

My Two Cents 🤑

If you’re just starting out, envsubst is a safe bet. You won’t get lost in its simplicity. But if you’re dealing with complex templates that require advanced functionality, then Jinja2 will serve you better. It’s like comparing a bicycle to a motorcycle. Both will get you where you’re going, but one offers a lot more speed and excitement along the way.

Pros and Cons: envsubst vs Jinja2 🏆

Ah, the classic “Pros and Cons” list! Who doesn’t love a good ol’ comparison? Especially when we’re talking about tools that could make or break your workflow, am I right?

Ease of UseSimple and straightforwardMore features, so steeper learning curve
Installation NeededComes pre-installed on most UNIX systemsRequires installation
SpeedFast due to limited feature setSlower due to more complexity
UniversalityWorks on virtually any file typeGenerally used with specific file types
Resource ConsumptionLowHigher
FeaturesLimited (just text replacement)Rich (loops, conditionals, filters, and more)
IntegrationNot natively integrated with DevOps toolsSeamlessly works with tools like Ansible
ValidationNo validation for typos or errorsSyntax and undefined variables trigger warnings
Community SupportLimitedStrong, with many third-party modules available

So, which one’s better? Well, there’s no one-size-fits-all answer here. If you’re looking for quick, simple text replacement, envsubst might be your BFF. But if you want a tool that can wear multiple hats and juggle flaming chainsaws (metaphorically, of course), then Jinja2 is where it’s at.


Alright, so you’ve made it to the advanced section! High five 🖐️! Up until now, we’ve been sipping envsubst and Jinja2 like a fine wine. But it’s time to take off the training wheels and go full sommelier mode. Ready? Let’s start scripting with envsubst.

Scripting with envsubst

You ever have that moment when you’re using a tool, and you think, “Man, this would be perfect if it just had XYZ feature!”? Well, you’re in luck. We’re about to take envsubst to the next level by extending its capabilities with some good ol’ shell scripting.

Extending envsubst capabilities with shell scripts

envsubst is pretty great out of the box, but it’s kinda like a Swiss Army knife with only one blade. With a little scripting, though, you can turn it into a full-blown multi-tool.

Here’s the thing: envsubst is simple, which is both a blessing and a curse. It’s fast and low-resource, but it’s not designed to do complex stuff like loops and conditional logic. That’s where scripting comes in.

# Extend envsubst with some custom logic

# Perform some custom logic here. E.g., setting a variable based on a condition
if [[ $USER == 'admin' ]]; then
  export WELCOME_MESSAGE="Hello, Master Admin!"
  export WELCOME_MESSAGE="Hello, $USER."

# Substitute variables in the template
envsubst < "$template_file" > "$temp_file"

# Perform some more custom logic here if needed

# Output the transformed template
cat "$temp_file"

# Clean up
rm "$temp_file"

In this example, we create a shell script that takes a template file as an argument. It then sets a WELCOME_MESSAGE variable based on whether the user is an ‘admin’ or not. Finally, it runs envsubst and outputs the transformed template. Simple, but effective!

Practical example

Okay, theory is nice, but let’s get our hands dirty with a practical example. Let’s say you have a template for an NGINX config file where you want to welcome the user specifically.

Your template file (nginx_template.conf):

server {
  listen 80;
  root /var/www/html;
  location / {
    echo "${WELCOME_MESSAGE}";

Just run the script with this template, and you’ll get an NGINX config file that greets the user based on their role. Isn’t that neat?

The power of envsubst isn’t just in what it can do—it’s in how you can build on it. It’s like a blank canvas waiting for your scripting masterpiece.

Scripting with Jinja2

Hold onto your hats, folks! We’re diving deep into the scripting world of Jinja2. If envsubst is a Swiss Army knife, then Jinja2 is like a well-stocked toolbox with your favorite Python tunes playing in the background. Ready to add some logic to your Jinja2 templates? Let’s jam.

Adding logic to your Jinja2 templates with Python

You know what makes Jinja2 and Python a match made in heaven? Python’s ability to handle complex logic effortlessly. You can use Python to define variables, loops, and conditions right within the Jinja2 template or manage them externally before rendering the template.

Here’s a quick example to inject some Python logic into a Jinja2 template:

from jinja2 import Environment, FileSystemLoader

# Define the template environment
env = Environment(loader=FileSystemLoader('templates'))

# Load the template
template = env.get_template('my_template.j2')

# Define Python logic
variables = {
    'users': ['Alice', 'Bob', 'Charlie'],
    'admin': 'Alice'

# Render the template with the Python variables
output = template.render(variables)

In this Python script, we’re using Jinja2’s Environment and FileSystemLoader to load a template file. Then, we define a variables dictionary to inject user-specific logic into the template. Finally, we render the template with these variables.

Practical example

Alright, let’s see this in action with a more elaborate example. Imagine you’re building a website and want to generate HTML pages for a list of users with different access levels.

Your Jinja2 template (my_template.j2):

    <h1>User List</h1>
      {% for user in users %}
          {{ user }}
          {% if user == admin %}
          {% endif %}
      {% endfor %}

When you run the Python script with this template, you’ll generate an HTML file listing all the users, and it’ll even mark the admin! How cool is that? 🤩

Jinja2 plus Python is like a dynamic duo for your templating needs. The sky is really the limit on what you can do.

Third-Party Libraries with Jinja2

Third-party libraries and Jinja2 go together like bread and butter, or let’s say, like code and coffee for us tech folks! 😉 If you’re craving even more functionality than vanilla Jinja2, fret not. There’s an array of third-party libraries waiting to make your life easier.

Examples of integrating other libraries for more powerful templating

Okay, so one of my all-time favorite libraries to use with Jinja2 is Ansible. I mean, who doesn’t love the automation power of Ansible combined with the templating genius of Jinja2? But there are others too. You might want to check out pybars3 if you’re a fan of Handlebars.js or maybe J2cli, a CLI utility that renders Jinja2 templates; it’s super handy when you want to avoid writing a Python script.

You can find these libraries on Python’s package index, just a simple pip install away!

Practical example

Integrating Ansible with Jinja2

Here’s a nifty example using Ansible and Jinja2. Imagine you want to dynamically set up NGINX configurations for multiple environments like staging, production, etc. You’d have a Jinja2 template for an NGINX config file (nginx_config.j2):

server {
    listen {{ nginx_port }};
    server_name {{ server_name }};

And in your Ansible playbook, you’d have something like:

- hosts: webservers
    nginx_port: 8080
    server_name: localhost
  - name: Generate NGINX config file
      src: nginx_config.j2
      dest: /etc/nginx/sites-available/default

Running this playbook will render the nginx_config.j2 template and place the resulting NGINX config file at its destined location.

Remember, when you’re using third-party libraries, you’re also inheriting their issues and bugs. Always read their documentation and understand their limitations.

Integrating pybars3 with Jinja2

If you’re a fan of Handlebars.js, you’ll appreciate how pybars3 allows you to use similar syntax in your Python projects. It works pretty well alongside Jinja2 for more complex logic.

First, you’ll need to install it:

pip install pybars3

Let’s say you’ve got a Handlebars template, saved as handlebars_template.hbs:

Hello, {{name}}! You have {{count}} new messages.

You can render this template with pybars3 like so:

from pybars import Compiler

compiler = Compiler()
source = "Hello, {{name}}! You have {{count}} new messages."
template = compiler.compile(source)

output = template({'name': 'John', 'count': 5})
print(output)  # Output will be: Hello, John! You have 5 new messages.

Pretty nifty, huh?

Integrating J2cli with Jinja2

Now, let’s talk J2cli. This is a real lifesaver when you don’t want to write a full-blown Python script just to render a Jinja2 template.

First things first, install it:

pip install j2cli

Imagine you have a Jinja2 template file called hello.j2:

Hello, {{ name }}!

To render this template using J2cli, you’d simply run:

j2 hello.j2 data.json

Here, data.json would be a JSON file containing the variable data, like so:

  "name": "Jane"

Run the command, and you’ll see Hello, Jane! printed right there in your terminal. Sweet and simple, isn’t it?

Summary and TL;DR

Dive into the world of templating and you’ll never look back. From envsubst’s minimalist approach to Jinja2’s feature-rich environment, templating solutions are here to save you from the monotony of manually managing configurations.

  • 📋 envsubst: Fast, simple, and does one thing really well. It’s the Usain Bolt of templating.
  • 🎨 Jinja2: A little more complex but incredibly versatile, like a Swiss Army knife.

My personal take? If you’re working in a simple environment or just want to do some quick text substitution, envsubst will be your go-to. But if you need something more dynamic and customizable, then Jinja2—especially with Ansible—will be your bread and butter.

Whether you’re just starting your DevOps journey or are a seasoned pro looking to streamline your pipeline, understanding these templating options can be a game-changer.

TL;DR: If you need quick and dirty text substitution, envsubst is your guy. If you need an all-in-one templating solution with bells and whistles, Jinja2 is for you. Both have their place in the DevOps toolkit, and your choice should depend on your specific needs and the complexity of your environment.

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