Making a WebApp from a shell script

I've recently written a shellscript for a personal need and thought it would be useful to other people. But how could I make this usable to non-tech people?

I've converted it to a simple web application:

• a form
• the script

Let's go through the steps that make this possible.

Select a light and convenient Web server

The Web server needs to be lightweight and able to execute code. mini_httpd is the perfect choice for this!

We'll run it as into a Docker container. Let's create a file named Dockerfile with the following contents:

FROM alpine:latest

RUN apk add --update mini_httpd bash

COPY . /app

EXPOSE 80

CMD [ "mini_httpd", "-h", "0.0.0.0", "-dd", "/app", "-r", "-c", "**.sh", "-D" ]

The CMD line starts the Web server:

• with document root configured in /app folder
• in a chroot for higher security
• that will run **.sh files (all files ending with .sh extension)

You can now build and run this Docker image:

docker build . -t shellscript-cgi
docker run --rm -t -p 8080:80 shellscript-cgi

If you load http://localhost:8080 in your Web browser, you'll see the index of the working dir:

Index of ./

.                                   29Nov2022 20:39           4096
..                                  29Nov2022 20:43           4096
Dockerfile                          29Nov2022 20:37            162    

mini_httpd/1.30 26Oct2018

Add a simple script

Let's add a non-interactive script. This script will return the output of a command (in this example, we'll get the system uptime).

Create a file named uptime.sh with the following contents:

#!/usr/bin/env sh

echo "Content-type: text/plain"
echo

uptime

Make it executable:

chmod +x uptime.sh

Then stop the running Docker container (Ctrl-C), rebuild the image and run the new container:

docker build . -t shellscript-cgi
docker run --rm -t -p 8080:80 shellscript-cgi

Now if you load http://localhost:8080/uptime.sh you'll get the system uptime:

 20:49:47 up 7 days, 13:39,  0 users,  load average: 5.09, 2.89, 1.96

Handle input arguments

Now let's deal with input arguments. We'll only cover GET arguments in this article.

Parsing of input arguments will be done through ColasNahaboo's Cgibashopts. Download the cgibashopts into your working directory.

Now create the hello.sh script with the following contents:

#!/bin/bash

source cgibashopts

echo "Content-type: text/plain; charset=utf-8"
echo

echo "Hello, $FORM_name"

Make it executable:

chmod +x hello.sh

Then stop the running Docker container (Ctrl-C), rebuild the image and run the new container:

docker build . -t shellscript-cgi
docker run --rm -t -p 8080:80 shellscript-cgi

Now if you load http://localhost:8080/hello.sh?name=Sébastien you'll get a nice welcome message 🙂 :

Hello, Sébastien!

Going further

In this article we only went through simple cases. Real-life applications will probably have more advanced needs.

The script I was talking about at the beginning of this article is a PDF watermarking tool. This tool deals with:

• POST request
• file upload
• advanced external commands (image & PDF manipulation)

You can look at its code there: https://code.pipoprods.org/web/watermark-pdf.