However, the description is extendable with advanced inputs and properties for sophisticated GUIs. Pline supports both simple input types like text, files, checkboxes or selection lists, and advanced ones that merge or modify values from linked inputs (see Figure \ref{890860} for an advanced case).
Input arguments are often related. A CLI program may require one or more sets of arguments where the list of compulsory or optional arguments, their values or the effect of those values is defined by the user input for a set of other arguments. In the Pline JSON format, the network of linked inputs is described by setting rules for the input properties that support it. For example, instead of a fixed default value for an input, a rule can derive the value from another input. These rules are written as conditionals – english-like if-else sentences or Javascript statements where the action of the rule is defined by the property that the rule is attached to. In addition to setting the default value, Pline supports conditionals for dynamically formatting or fixing an input value, for specifying an output file name and for enabling or disabling input elements and element groups. Together with other advanced features like input filters, error messages and merged values, the Pline API allows for describing even highly complex command-line interfaces. To customize the resulting interface, the API also specifies properties for adding icons, labels, documentation, HTML markup and CSS rules.

Interface rendering

Pline includes an interface generator that implements the JSON specification and translates the program description into a graphical user interface. The generator is written in JavaScript, which runs natively inside any modern web browser, and is used by including it in a webpage as a library. The library takes a Pline plugin description as input (given as raw JSON text or a URL), parses the information into an internal data model, and outputs a graphical interface that can be placed into any container element in a web page. In practice, the process involves just two Javascript commands (addPlugin() for the plugin import, plugin.draw() for interface drawing) and supports multiple input programs and output interfaces in a single web page.
A Pline-generated GUI is not static – the HTML interface is bound to an internal data model and event listeners that enforce the dependency rules between the program parameters and adjust the interface according to user interactions. User input is tracked in real-time: as soon as a tick-box is clicked or a number is typed, the interface updates accordingly, e.g. by hiding, revealing, or changing the values of all the linked input elements. The conditionals in the Pline JSON therefore provide a quick way to construct sophisticated interfaces that hide invalid inputs and guide the user through program configuration options. In addition to generating standalone GUIs, Pline can chain multiple interfaces together, forming a pipeline – a set of commands executed in succession. The information about input and output files in the program description is used to control the data flow between the pipeline steps. The current state of a single interface or a full pipeline can be stored to a file and distributed as a reusable Pline pipeline with pre-filled input values.
A graphical interface generated with Pline records user input and forms a complete terminal command for launching the CLI program or pipeline together with user-supplied input files. Since the web browser security sandbox prevents direct command-line access, the program launch data is passed on to a backend server for execution. Pline includes a lightweight python script that acts as a server module to launch the commands, either on a local computer or over the web. The Pline server accepts the command data sent by the interface via an HTTP POST request, sanitizes the input, and manages the execution process. It also supports HTTP requests to send execution status updates back to the interface, pause, cancel or resume running pipelines, or send email notifications after a command or pipeline has finished.
Pline interfaces are designed to be installation-free and work across many different operating systems. The JSON program description files are platform-agnostic, the interface generator runs on any device with a modern web browser (including mobile devices), and the server module supports both Python 2.7 and 3 environments (which is preinstalled on most MacOS and Linux systems). However, command-line executables are compiled to run on a specific operating system, so a Pline plugin should include an executable for each target system.