Constructing with Nix on Replit

As of Also can 2021, Replit supports all programming languages thru the energy of Nix. But Nix can create a complete bunch more than exact allow us to employ new languages. In this article, we are going to hide several various employ-conditions for Nix on Replit, at the side of:

  • Developing a producing-grade web stack, with a database and increase for more than one web servers.
  • Running third-occasion packages in Replit.
  • Taking half in DOOM in a repl.

What is Nix?

Nix is a instrument designed for managing packages and blueprint configurations. It has some similarities to kit managers it is probably you’ll perchance perchance also indulge in outmoded in the past, corresponding to Homebrew on macOS, APT on Debian-primarily based fully Linux distributions, Python’s pip, or Node.js’s NPM. When you indulge in not outmoded a kit supervisor before, it be in total an app retailer.

When you happen to install a kit with a historical kit supervisor – speak you have to install a browser admire Firefox – it will win some data, unpack them in various locations in your blueprint, and flee some configuration scripts. This could occasionally perchance perchance also additionally install the complete extra packages and libraries Firefox needs to flee. Many of those can indulge in dependencies themselves, and so it will install those too. In the atomize, placing in a single kit can require many more installations and consequence in in vogue changes to your blueprint.

Most continuously, packages will likely be incompatible with every various, attributable to counting on various versions of the identical dependency. As an instance, App #1 depends on libxyz 1.4 and App #2 depends on libxyz 1.5. Putting in App #2 upgrades libxyz and breaks App #1. Downgrading libxyz fixes App #1 nonetheless breaks App #2.

Here is believed as dependency hell, and it be belief to be one of the most issues Nix used to be designed to solve. Whereas a typical kit supervisor could perchance perchance install Vim to /usr/bin/vim, nix will install it to a itemizing that appears admire this:


The itemizing name has three parts: the kit name, the version, and a hash of the total kit data, corresponding to configuration alternate solutions. Two packages on two various systems with the identical hash will likely be identical, nonetheless two packages with the identical name and version nonetheless various hashes can indulge in slight differences. Thanks to this, we can install more than one versions of the identical kit, neatly fixing our App #1 and App #2 dependency hell.

Whereas a kit supervisor admire APT could perchance perchance scatter the contents of an installed program across /bin, /and so on, /usr, and various directories that require special privileges to put in writing to, Nix keeps the complete lot in /nix/retailer. This lets us securely install packages as a non-privileged person.

Programs in Nix are built the utilization of derivations, which it is probably you’ll perchance perchance factor in as private scripts. All derivations are written in the Nix language, a purposeful programming language, corresponding to Haskell or F#. When you indulge in not outmoded a purposeful language before, the most valuable component to understand is that there could be not truly any chronic impart, i.e. it is probably you’ll perchance perchance not outline variables exterior of capabilities. Functional languages are tranquil of capabilities that engage some enter and form some output. Every time a feature is done with a given enter, it will return the identical output. This requires a various scheme to historical imperative programming languages nonetheless enhances predictability and reproducibility of output, two very honest qualities for a private blueprint.

To study more about Nix, are trying the next sources:

How create we employ Nix on Replit?

Every repl you private is backed by a Docker container. The leisure you create from your repl, from running code to executing instructions in the shell, will happen in the context of the runner person in this container. For causes of security, this person doesn’t indulge in root privileges, and attributable to this fact can not install packages the utilization of a historical kit supervisor admire APT. However the runner in a Nix repl can install packages with Nix.

This opens up an monumental array of potentialities. In outdated tutorials, we’ve got targeted on the utilization of repls to flee custom code in various languages. The Replit Database makes it that it is probably you’ll perchance perchance factor in to achieve capabilities with chronic storage, nonetheless Nix enables us to employ a typical DBMS admire MySQL or Postgres. We are able to also additionally install webservers and even graphical packages admire Inkscape and LibreOffice.

Really, Nix turns our repl correct into a fully fledged server. In the subsequent few sections, we are going to hide a number of the issues we can create which skill that.

Deploying a producing web stack on Replit with Nix

The first component we are going to private with Nix is a producing web stack with the next parts:

  • Python Flask as the applying server.
  • Waitress as the Web Server Gateway Interface.
  • NGINX as the on-line server.
  • Postgres as the SQL database.

Why Waitress?

Once it is probably you’ll perchance perchance also indulge in got built one thing with Python and Flask, corresponding to belief to be one of our outdated tutorials, that it is probably you’ll indulge in outmoded Flask’s pattern server to work in conjunction with your application. While this server is stunning for pattern and debugging, it be optimised for a single person and will get somewhat stupid if more than one person tries to employ it.

Flask development server

We are able to toughen the flee of our Flask apps by the utilization of a producing-grade WSGI server slightly than Flask’s default, corresponding to Gunicorn or Waitress.


What if we’re attempting to host more than exact a Flask app? Let’s indulge in in thoughts an e-commerce residing at The precious retailer application is powered by Flask, nonetheless our advertising division would fetch to start up a blog at Lets private blogging functionality into our e-commerce residing, nonetheless it would be remarkable quicker and simpler to employ a separate application, corresponding to Wordpress or Ghost.

Here is where a fully featured web server, corresponding to NGINX or Apache, is available in. These web servers will likely be configured to aid several various capabilities and instruct material directories at various locations on a few domains. They’re also remarkable faster at serving static instruct material than even a producing-grade WSGI server, so even single-app deployments engage pleasure in the utilization of them.

Why Postgres?

We’ve outmoded Replit’s Database for chronic storage in different outdated tutorials. While it be easy to employ in supported languages admire Python, it doesn’t indulge in the energy and suppleness of a mainstream SQL database, and we can not proceed the utilization of it if we ever switch our code out of Replit. Postgres is a smartly-liked SQL database outmoded by many, from limited startups to tech giants admire Apple, Reddit and Spotify. We are able to employ it too if we install it on Nix.

Repl overview

We’ve made a Nix repl containing the manufacturing web stack available right here:

Start it now, or fork it to your profile, and we are going to plod over the scheme in which it works. Make certain the repl’s config data are showing.

Show configuration

The first file we are going to encounter at is replit.nix. Here is the nasty Nix file that tells our repl what packages to install. In the default Nix repl, it appears admire this:

{ pkgs }: {
    deps = [

The first line, { pkgs }: , defines an anonymous feature that takes a single argument, pkgs. After we flee our repl, this option will likely be known as and its contents done. In this case, its contents is a listing of packages to install, one item prolonged. Therefore, all this option does is install cowsay, a program that prints an ASCII cow.


By distinction, the replit.nix file in our manufacturing web stack repl is more challenging. It appears admire this:

{ pkgs }: 

    nginxModified = pkgs.nginx.overrideAttrs (oldAttrs: rec {
        configureFlags = oldAttrs.configureFlags ++ [

in {
    deps = [


You ought to aloof recognise some similarities between this code and the default replit.nix. We’re aloof defining an anonymous feature that takes pkgs, nonetheless now we’re placing in more than one kit.

The total packages in deps are straight from Nix’s kit repository, with the exception of for nginxModified. We want to make some changes to nginx to win it to flee in our repl. Nix’s language and blueprint configuration skills make this remarkable more useful to create than if we had been the utilization of a various kit supervisor that did not increase recompiling packages.

Nix’s let ... in { ... } control building is outmoded after we’re attempting to outline native variables outmoded in a given feature. We outline the variables after let and then employ them after in. Let’s engage a more in-depth encounter at the definition of nginxModified:

    nginxModified = pkgs.nginx.overrideAttrs (oldAttrs: rec {
        configureFlags = oldAttrs.configureFlags ++ [

Here we’re taking pkgs.nginx and calling overrideAttrs to trade the configuration flags that are residing when compiling NGINX. We want to add a few flags that trade the paths NGINX makes employ of to paths that are accessible in our repl. Tag that we’ve got created the total directories in the anticipated locations.

The derivation that runs after we install pkgs.nginx will likely be chanced on right here. Our version will create the identical issues, nonetheless with a few extra items in configureFlags.

That is it for replit.nix. Now let’s engage a encounter at .replit. This file defines what characterize will win done after we click on the “Bustle” button, and what custom atmosphere variables will likely be available to our repl.

flee = "sh start"

PGDATA = "/house/runner/${REPL_SLUG}/data"

We are going to lift out the shell script start after we press “Bustle”, and we’ve got got outlined PGDATA, an atmosphere variable Postgres makes employ of to encounter its data itemizing. Let’s encounter at start subsequent:

# start up Postgres
pg_ctl cease

cp postgresql.conf.tpl data/postgresql.conf


sed -i "s/replace_unix_dir/${socker_dir}/" data/postgresql.conf

pg_ctl -l /house/runner/${REPL_SLUG}/postgresql.log start up

createdb -h
psql -h -c "private database appdb;"

# start up nginx
pkill nginx

nginx -e /house/runner/$REPL_SLUG/logs/error.log -c /house/runner/$REPL_SLUG/nginx.conf

# start up Flask app

In bellow, we start up Postgres, then NGINX, and then our Python Flask application.

Our Postgres code first stops any existing conditions of Postgres, then calls initdb, that will private a brand new database at the itemizing specified by $PGDATA if none exists. We then reproduction our Postgres configuration file into our data itemizing and employ sed to beget in its unix_socket_directories payment, one more itemizing we’ve got got to trade to win issues working in a repl.

The file postgressql.conf.tpl is prolonged and mostly unimportant. The correct section of that file that will likely be relevant for in vogue employ is the next lines under the heading “Connection Settings”:

listen_addresses = ''      
port = 5432             

These lines residing the database to hear on the native loopback on TCP port 5432. This could occasionally perchance perchance allow various packages in our repl to join to it over the network, with out having it uncovered to the get or various repls. Here is relevant for the subsequent few lines of our script:

pg_ctl -l /house/runner/${REPL_SLUG}/postgresql.log start up

createdb -h
psql -h -c "private database appdb;"

The first line begins our database, and the last two private a Postgres occasion usable by runner and interior that, a database named appdb. Each of those lines will fail on subsequent runs of start (if the database has already been created), so we create not want to hassle about overwriting our database at any time after we flee our repl.

The code for starting NGINX is more useful:

pkill nginx

nginx -e /house/runner/$REPL_SLUG/logs/error.log -c /house/runner/$REPL_SLUG/nginx.conf

First, we assassinate any existing nginx processes, and then we start up NGINX, telling it to put in writing errors to logs/error.log and employ the configuration file nginx.conf. Love postgresql.conf, this configuration file is mostly unimportant for in vogue employ. The following changes indulge in been fabricated from Nix’s default NGINX configuration file:

  1. The choice pid specifies a repl-accessible PID file region:

     pid        /house/runner/REPL-NAME-HERE/logs/;
  2. The choice access_log in the http block specifies a repl-accessible win admission to log file region:

     access_log  /house/runner/REPL-NAME-HERE/logs/win admission to.log;
  3. The server block has been changed to host our Python server, as detailed under.

         server {
             hear       8080;
             server_name  localhost;
             region / {
                 proxy_pass 8181;
             error_page   500 502 503 504  /50x.html;
             region = /50x.html {
                 root   html;

In NGINX, server blocks are what you use to residing up websites on particular person domains. Each arena (e.g. or subdomain (e.g. can indulge in its hold server block. To private a server block that will outline what our repl hosts, we employ the next NGINX directives:

    hear  8080
    server_name  localhost

Here we’ve got residing NGINX up to flee our server on TCP port 8080, as per Replit’s web hosting guidelines. Internal our server block, we can indulge in a few region blocks. These characterize NGINX what instruct material to host at various URLs. We are able to employ the proxy_pass directive to aid the contents of one more webserver running in our repl, or the root directive to aid static data. That you just can study more about configuring NGINX in the decent NGINX Newbie’s Files.

The last component we create in start is start up up our Python Flask server:


The code for ought to aloof encounter acquainted whenever you happen to also can indulge in got outmoded Flask before:

from flask import Flask
from waitress import aid
import psycopg2

app = Flask(__name__)

def index():

    connection = psycopg2.join(

    cursor = connection.cursor()
    cursor.lift out('SELECT version()')
    db_version = cursor.fetchone()

    return f"Whats up from Python! PostgreSQL database version: {db_version}"

#app.flee(host='', port=8181) # dev server
aid(app, host='', port=8181, url_scheme='https') # manufacturing server

In this code, we’ve got created a Flask application that connects to our Postgres database and is served on the native loopback address at TCP port 8181 by the Waitress WSGI server.

When you flee the repl now, it is probably you’ll perchance look a page showing version data about our PostgreSQL database.

From this nasty, it is probably you’ll perchance perchance private and configure a producing-ready web application. Strive the next solutions:

  • Implement the code from belief to be one of our outdated Flask-primarily based fully tutorials, corresponding to this PDF narrative generator, this technical competition residing, or this PDF retailer. Thought whenever you happen to can adapt the instruct material to employ Postgres slightly than the Replit Database.
  • Implement your hold Flask web application, the utilization of Postgres as a database.
  • Add a 2d application listening on a various loopback port and available from a various URL. This could occasionally perchance perchance be your hold Python or Node.js carrying out, or a deployment of start-source application corresponding to Ghost or Wordpress.

A pair of various stuff it is probably you’ll perchance perchance create with Nix and Replit

We started with a fancy example to present you an belief of the energy and capacity of Nix, nonetheless there are a few various stuff it is probably you’ll perchance perchance create with it that create not require the identical quantity of residing up.

Jupyter Notebook

That you just can flee a Jupyter Notebook in a Nix repl. Add pkgs.jupyter to the dep. listing in replit.nix, residing flee in .replit to sh start, and private a start script with the next contents:

mkdir data
jupyter notebook --ip --port 8080 --notebook-dir /house/runner/$REPL_SLUG/data

Notebooks are web-primarily based fully interactive pattern environments that could perchance enable you to combine runnable code, text notes, mathematical equations, and charts and graphs. They’re most continuously outmoded by data scientists.

Jupyter notebook

VSCode Server

That you just can flee a Visual Studio Code Server, that will can enable you to employ a non-public, customised version of the smartly-liked text editor from wherever, merely by navigating to your repl’s URL.

VS Code server

Alternative web stacks

Barely than NGINX, it is probably you’ll perchance perchance employ Apache, and slightly than Postgres, it is probably you’ll perchance perchance are trying MySQL or even a NoSQL database admire MongoDB. And naturally, it is probably you’ll perchance perchance employ exact about any mainstream programming language to put in writing your web application code slightly than Python, corresponding to JavaScript, Ruby or Java.

Other programming languages

That you just can residing up and employ a programming language that isn’t very formally supported by Replit, corresponding to Racket (nixpkgs.racket), Prolog (nixpkgs.swiProlog), or even COBOL (nixpkgs.gnu-cobol).


We are able to win the smartly-liked MS-DOS emulator DOSBox working in a Nix repl by exact placing in the lawful kit and running it. From there, we can flee any DOS program and employ it in our repl’s VNC window.

Here is DOOM in a repl:


Other DOS packages it is probably you’ll perchance perchance are trying:

  • Liero, a keyboard-sharing worm combating game.
  • WordStar 4.0, the discover processor George RR Martin makes employ of to put in writing the Sport of Thrones guide sequence.

Standard guidelines for building with Nix repls

As we’ve got viewed above, Nix enables us to employ repls for more than exact writing and testing code, nonetheless some packages require a swish quantity of configuration to win working. Listed right here are a few in vogue guidelines for getting packages working in Nix repls:

  • First, predict the kit it is probably you’ll perchance perchance like on the Nix kit search online page and add it to deps in replit.nix. When you create not look any errors in the console after running your repl, it perchance labored. Some packages want to be manually started by specifying a flee characterize in .replit, whereas others will start up automatically.
  • Many of the installation errors it is probably you’ll perchance stumble upon will bellow to file paths that create not exist in your repl, or that you just create not indulge in permission to win admission to. These can in total be mounted whenever you happen to can configure your kit to encounter those data in your repl’s file listing as a replace, which is hosted on disk at /house/runner/REPL-NAME-HERE/. Reckoning on the kit, it is probably you’ll perchance be ready to create this with a custom configuration file, admire we did with Postgres, or it is probably you’ll perchance perchance want to alter the skill Nix installs it, as we did with NGINX.
  • That you just can scrutinize data and directories that put not look like viewed in your repl’s filepane from the shell, the utilization of typical Unix instructions admire ls, cd and cat. This involves data and directories in the Nix retailer, at /nix/retailer. This could occasionally perchance perchance most continuously be vital for debugging.
  • That you just can scrutinize a listing of running processes with the shell characterize ps aux, and cease them with pkill .
  • Loading the Nix atmosphere will most continuously engage a in point of fact very prolonged time, especially whenever you happen to also can indulge in custom derivations.
  • When you’re developing a web server, focus on to our steering on deploying HTTP servers.

As a final reminder, make sure that to abide by Replit’s Terms of Service when the utilization of Nix repls, and steer definite of placing in packages that employ excessive blueprint sources (corresponding to cryptocurrency miners) or are intentionally malicious.

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