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Manual:Creating a bot/pt-br

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This page is a translated version of the page Manual:Creating a bot and the translation is 20% complete.

MediaWiki Robots, ou apenas bots, são processos automáticos que interagem com a Wikipédia (e outros projetos da fundação Wikimedia) como se fossem editores humanos. Esta página tenta explicar como realizar o desenvolvimento de um bot para uso nesses projetos e, também aplicável em outras wikis baseados no MediaWiki. A explicação é voltada principalmente para aqueles que têm alguma experiência anterior em programação, mas não têm certeza de como aplicar esse conhecimento na criação de um bot MediaWiki.

Por que preciso criar um bot?

Os bots podem automatizar tarefas e executá-las muito mais rápido que os humanos. Se você tem uma tarefa simples que precisa realizar muitas vezes (um exemplo pode ser adicionar um template a todas as páginas de uma categoria com 1000 páginas), então esta é uma tarefa mais adequada para um bot do que um humano.

Considerações antes de criar um bot

Reuse existing bots

Muitas vezes é muito mais simples solicitar um trabalho de bot de um bot existente. Se você tem apenas solicitações periódicas ou se sente desconfortável com a programação, essa costuma ser a melhor solução. Alguns wikis possuem uma página dedicada onde tais solicitações podem ser feitas. Além disso, existem várias ferramentas disponíveis para qualquer pessoa. A maioria deles assume a forma de navegadores aprimorados com funcionalidades específicas do MediaWiki. The most popular of these is AutoWikiBrowser (AWB), a browser specifically designed to assist with editing on Wikipedia and other Wikimedia projects. A lista completa de ferramentas pode ser encontrada em w:Wikipedia:Tools/Editing Tools na Wikipédia em inglês. Ferramentas como o AWB muitas vezes podem ser operadas com pouco ou nenhum conhecimento de programação.

Use Toolhub to explore available tools for Wikimedia wikis.

Reutilizar a base de código

Se você decidir que precisa de seu próprio bot devido à frequência ou novidade de seus requisitos, não será necessário escrever um do zero. Many bots publish their source code, which can sometimes be reused with little additional development time. There are also a number of standard bot frameworks available for download. These frameworks comprise the vast majority of a bot's code. Since these bot frameworks are in common usage and the complex coding has been done by others and has been heavily tested, it is far easier to get bots based on these frameworks approved for use. The most popular and common of these frameworks is Pywikibot (PWB), a bot framework written in Python. It is thoroughly documented and tested and many standardized Pywikibot scripts (bot instructions) are already available. Other examples of bot frameworks can be found below. For some of these bot frameworks, such as PWB, a general familiarity with scripts is all that is necessary to run the bot successfully (it is important to update these frameworks regularly).

Important questions

Escrever um novo bot requer habilidade de programação significativa. Um bot completamente novo deve passar por testes substanciais antes de ser aprovado para operação regular. Planning is crucial to obtain an error-free, efficient, and effective program. The following initial considerations are important:

  • Will the bot be manually assisted or fully automated?
  • Will you create the bot alone, or with the help of other programmers?
  • What language will be used to implement the bot?
  • Will the bot's requests, edits, or other actions be logged? If so, will the logs be stored on local media, or on wiki pages?
  • Will the bot run inside a web browser (for example, written in JavaScript), or will it be a standalone program?
  • If the bot is a standalone program, will it run on your local computer, or on a remote server such as the Toolforge?
  • If the bot runs on a remote server, will other editors be able to operate the bot or start it running?

Como funciona um bot MediaWiki?

Overview of operation

Assim como um editor humano, um bot MediaWiki lê páginas wiki e faz alterações onde acha que precisam ser feitas. A diferença é que, embora os bots sejam mais rápidos e menos propensos à fadiga do que os humanos, eles não são tão brilhantes quanto nós. Bots are good at repetitive tasks that have easily defined patterns, where few decisions have to be made.

No caso mais típico, um bot faz login em sua própria conta e solicita páginas do wiki da mesma forma que um navegador faz – embora não exiba a página na tela, mas trabalhe nela na memória – e então programaticamente examina o código da página para ver se alguma alteração precisa ser feita.

It then makes and submits whatever edits it was designed to do, again in much the same way a browser would.

Because bots access pages the same way people do, bots can experience the same kind of difficulties that human users do. They can get caught in edit conflicts, have page timeouts, or run across other unexpected complications while requesting pages or making edits. Because the volume of work done by a bot is larger than that done by a live person, the bot is more likely to encounter these issues. Thus, it is important to consider these situations when writing a bot.

APIs for bots

Para fazer alterações nas páginas do wiki, um bot necessariamente precisa recuperar as páginas do wiki e enviar as edições de volta. Existem diversas Interfaces de Programação de Aplicativos (API) disponíveis para essa finalidade.

  • MediaWiki Action API (api.php). Este serviço web foi escrito especificamente para permitir que processos automatizados, como bots, façam consultas e publiquem alterações. Os dados são retornados no formato JSON (veja formatos de saída para mais detalhes).
    Status: Recurso integrado do MediaWiki, disponível em todos os servidores Wikimedia. Outros wikis que não sejam da Wikimedia podem desabilitar ou restringir o acesso de gravação.
    Há também um API sandbox para quem deseja testar os recursos do api.php.
  • Special:Export can be used to obtain bulk export of page content in XML form. See Manual:Parameters to Special:Export for arguments;
    Status: Recurso integrado do MediaWiki, disponível em todos os servidores Wikimedia.
  • Raw (Wikitext) page processing: sending a action=raw or a action=raw&templates=expand GET request to index.php will give the unprocessed wikitext source code of a page. For example: https://en.wikipedia.org/w/index.php?title=Help:Creating_a_bot&action=raw. An API query with action=query&prop=revisions&rvprop=content or action=query&prop=revisions&rvprop=content&rvexpandtemplates=1 is roughly equivalent, and allows for retrieving additional information.
    Status: Built-in feature of MediaWiki, available on all Wikimedia servers.

Some web servers are configured to grant requests for compressed (gzip) content.

Isso pode ser feito incluindo uma linha "Accept-Encoding: gzip" no cabeçalho da solicitação HTTP; se o cabeçalho de resposta HTTP contiver "Content-Encoding: gzip", o documento estará no formato gzip, caso contrário, estará no formato normal descompactado.

Note that this is specific to the web server and not to the MediaWiki software. Other sites employing MediaWiki may not have this feature. If you are using an existing bot framework, it should handle low-level operations like this.

Logging in

Os bots aprovados precisam estar logados para fazer edições. Embora um bot possa fazer solicitações de leitura sem fazer login, os bots que concluíram os testes devem fazer login para todas as atividades. Bots logados a partir de uma conta com a flag bot (veja #Bot Flag abaixo) podem obter mais resultados por consulta na API Mediawiki (api.php). Most bot frameworks should handle login and cookies automatically, but if you are not using an existing framework, you will need to follow these steps.

For security, login data must be passed using the HTTP POST method. Because parameters of HTTP GET requests are easily visible in URL, logins via GET are disabled.

To log a bot in using the MediaWiki API, 2 POST requests are needed:

Request 1 – this is a GET request to obtain a login token

This will return a "logintoken" parameter in JSON form, as documented at API:Login. Other output formats are available. It will also return HTTP cookies as described below.

Request 2 – this is a POST to complete the login

where TOKEN is the token from the previous result. The HTTP cookies from the previous request must also be passed with the second request.

A successful login attempt will result in the Wikimedia server setting several HTTP cookies. The bot must save these cookies and send them back every time it makes a request (this is particularly crucial for editing). On the English Wikipedia, the following cookies should be used: enwikiUserID, enwikiToken, and enwikiUserName. The enwikisession cookie is required to actually send an edit or commit some change, otherwise the MediaWiki:Session fail preview error message will be returned.

Main-account login via action=login is deprecated and may stop working without warning. To continue using bot code which logs in with action=login, see Special:BotPasswords.

Editing; edit tokens

MediaWiki uses a system of edit tokens for making edits to MediaWiki pages, as well as other operations that modify existing content such as rollback. The token looks like a long hexadecimal number followed by '+\', for example:

d41d8cd98f00b204e9800998ecf8427e+\

A função dos tokens de edição é evitar o "sequestro de edição", onde os usuários são induzidos a fazer uma edição clicando em um único link.

O processo de edição envolve duas solicitações HTTP. Primeiro, deve ser feita uma solicitação de token de edição. Em seguida, deve ser feita uma segunda solicitação HTTP que envia o novo conteúdo da página junto com o token de edição recém-obtido. Não é possível fazer uma edição em uma única solicitação HTTP.

Um token de edição permanece o mesmo durante uma sessão de login, portanto, o token de edição precisa ser recuperado apenas uma vez e pode ser usado para todas as edições subsequentes.

To obtain an edit token, follow these steps:

  • MediaWiki API (api.php). Make a request with the following parameters (see API:Edit - Create&Edit pages).
    • action=query
    • meta=tokens

    The token will be returned in the edittoken attribute of the response.

If the edit token the bot receives does not have the hexadecimal string (i.e., the edit token is just '+\') then the bot most likely is not logged in.

Isso pode ser devido a vários fatores: falha na autenticação com o servidor, queda de conexão, algum tipo de tempo limite ou erro ao armazenar ou retornar os cookies corretos.

If it is not because of a programming error, just log in again to refresh the login cookies. The bots must use assertion to make sure that they are logged in.

Edit conflicts

Edit conflicts occur when multiple, overlapping edit attempts are made on the same page. Almost every bot will eventually get caught in an edit conflict of one sort or another, and should include some mechanism to test for and accommodate these issues.

Bots that use the Mediawiki API (api.php) should retrieve the edit token, along with the starttimestamp and the last revision "base" timestamp, before loading the page text in preparation for the edit; prop=info|revisions can be used to retrieve both the token and page contents in one query (example). When submitting the edit, set the starttimestamp and basetimestamp attributes, and check the server responses for indications of errors. For more details, see API:Edit - Create&Edit pages.

Generally speaking, if an edit fails to complete the bot should check the page again before trying to make a new edit, to make sure the edit is still appropriate. Further, if a bot rechecks a page to resubmit a change, it should be careful to avoid any behavior that could lead to an infinite loop and any behavior that could even resemble edit warring.

Visão geral do desenvolvimento do bot

Actually, coding or writing a bot is only one part of developing a bot.

The development cycle below is a recommendation from English Wikipedia.

Overview of English Wikipedia bot development cycle

On Wikimedia wikis, ensure that your bot follows any potential bot policies of the wiki.

Idea

  • The first task in creating a MediaWiki bot is extracting the requirements or coming up with an idea.
  • Make sure an existing bot isn't already doing what you think your bot should do.

Specification

  • Specification is the task of precisely describing the software to be written, possibly in a rigorous way. You should come up with a detailed proposal of what you want it to do. Try to discuss this proposal with some editors and refine it based on feedback. Even a great idea can be made better by incorporating ideas from other editors.
  • In the most basic form, your specified bot must meet the following criteria:
  • The bot is harmless (it must not make edits that could be considered disruptive to the smooth running of the wiki)
  • The bot is useful (it provides a useful service more effectively than a human editor could)
  • The bot does not waste server resources.

Software architecture

  • Think about how you might create it and which programming language(s) and tools you would use. Architecture is concerned with making sure the software system will meet the requirements of the product as well as ensuring that future requirements can be addressed. Certain programming languages are better suited to some tasks than others, for more details see the section on programming languages below.

Implementation

Implementation (or coding) involves turning design and planning into code. It may be the most obvious part of the software engineering job, but it is not necessarily the largest portion. In the implementation stage you should:

  • Create an account for your bot. Go to the sign up page when logged in to create the account, linking it to yours. (If you do not create the bot account while logged in, it might be blocked on some wikis according to their policies)
  • Create a user page for your bot. Your bot's edits must not be made under your own account. Your bot will need its own account with its own username and password.
  • Add the same information to the user page of the bot. It would be a good idea to add a link to the approval page (whether approved or not) for each function.
  • Code your bot in your chosen programming language.

Testing

A good way of testing your bot as you are developing is to have it show the changes (if any) it would have made to a page, rather than actually editing the live wiki. Some bot frameworks (such as pywikibot) have pre-coded methods for showing diffs.

Documentation

An important (and often overlooked) task is documenting the internal design of your bot for the purpose of future maintenance and enhancement. This is especially important if you are going to allow clones of your bot. Ideally, you should post the source code of your bot on its userpage or in a revision control system (see #Open-source bots) if you want others to be able to run clones of it. This code should be well documented (usually using comments) for ease of use.

Queries/Complaints

You should be ready to respond to queries about or objections to your bot on your user talk page, especially if it is operating in a potentially sensitive area.

Maintenance

Maintaining and enhancing your bot to cope with newly discovered bugs or new requirements can take far more time than the initial development of the software. Not only may it be necessary to add code that does not fit the original design, but just determining how software works at some point after it is completed may require significant effort (this is another reason to document your code as you go along).

General guidelines for running a bot

In addition to the official bot policy, which covers the main points to consider when developing your bot, there are a number of more general advisory points to consider when developing your bot.

Bot best practices

  • Set a custom User-Agent header for your bot (per the Wikimedia User-Agent policy, if your bot will be operating on Wikimedia wikis). If you don't, your bot may encounter errors and may end up blocked at the server level.
  • Use the maxlag parameter with a maximum lag of 5 seconds. This will enable the bot to run quickly when server load is low, and throttle the bot when server load is high.
    • If writing a bot in a framework that does not support maxlag, limit the total requests (read and write requests together) to no more than 10/minute.
  • Use the MediaWiki API whenever possible, and set the query limits to the largest values that the server permits, to minimize the total number of requests that must be made.
  • Edit (write) requests are more expensive in server time than read requests. Be edit-light and design your code to keep edits to a minimum.
    • Try to consolidate edits. One single large edit is better than 10 smaller ones.
  • Enable HTTP persistent connections and compression in your HTTP client library, if possible.
  • Do not make multi-threaded requests. Wait for one server request to complete before beginning another
  • Back off upon receiving errors from the server. Errors such as timeouts are often an indication of heavy server load. Use a sequence of increasingly longer delays between repeated requests.
  • Make use of assertion to ensure your bot is logged in.
  • Test your code thoroughly before making large automated runs. Individually examine all edits on trial runs to verify they are perfect.

Common bot features you should consider implementing

Manual assistance

If your bot is doing anything that requires judgment or evaluation of context (e.g., correcting spelling) then you should consider making your bot manually-assisted, which means that a human verifies all edits before they are saved. This significantly reduces the bot's speed, but it also significantly reduces errors.

Disabling the bot

It is good bot policy to have a feature to disable the bot's operation if it is requested. Remember that if your bot goes bad, it is your responsibility to clean up after it! You could have the bot refuse to run if a message has been left on its talk page, on the assumption that the message may be a complaint against its activities; this can be checked using the API meta=userinfo query (example on English Wikipedia). Or you could have a page that will turn the bot off if text on the page is changed (e.g. require the page be empty, contain only the word "True", or something similar); this can be checked by loading the page contents before each edit.

Signature

Just like a human, if your bot makes edits to a talk page in MediaWiki, it should sign its post with four tildes (~~~~). Signatures usually belong only on talk namespaces.

Bot Flag

A bot's edits will be visible at Special:RecentChanges, unless the edits are set to indicate a bot. Once the bot has been approved and given its bot flag permission, one can add the "bot-True" to the API call - see API:Edit#Parameters in order to hide the bot's edits in Special:RecentChanges.

In Python, using either mwclient or wikitools, then adding Bot=True to the edit/save command will set the edit as a bot edit - e.g.

PageObject.edit(text=pagetext, bot=True, summary=pagesummary)

Monitoring the bot status

If the bot is fully automated and performs regular edits, you should periodically check it runs as specified, and its behavior has not been altered by software changes.

Open-source bots

Many bot operators choose to make their code open source, and occasionally it may be required before approval for particularly complex bots. Making your code open source has several advantages:

  • It allows others to review your code for potential bugs. As with prose, it is often difficult for the author of code to adequately review it.
  • Others can use your code to build their own bots. A user new to bot writing may be able to use your code as an example or a template for their own bots.
  • It encourages good security practices, rather than security through obscurity.
  • If you leave the project, it allows other users to run your bot tasks without having to write new code.

Open-source code, while rarely required, is typically encouraged in keeping with the open and transparent nature of wikis, though there are some cases when code should not be made public. For example, the open proxy-finding code of ProcseeBot could be used for malicious purposes on other sites.

Making code open source can add some extra work to coding. One has to make sure that sensitive information such as passwords is separated into a file that isn't made public.

There are several options available for users wishing to make their code open. Some users choose to put the code in a subpage of the bot's userspace, although this can be a hassle to maintain if not automated and results in the code being multi-licensed under the wiki's licensing terms in addition to any other terms you may specify. Another solution is to use a revision control system such as SVN, Git, or Mercurial. Wikipedia has articles comparing the different software options and websites for code hosting, many of which have no cost. Wikimedia also offers Git code repository hosting for its users and running Wikimedia related software tools via Wikimedia Cloud Services.

Programming languages and libraries

See also: API:Client code

Bots can be written in almost any programming language. The choice of a language often depends on the experience of the bot writer (which languages are familiar) or on the availability of pre-developed libraries to perform the desired task. The following list includes some languages that have libraries to assist with bot tasks.

Awk

Perl

If located on a webserver, you can start your program running and interface with your program while it is running via the w:Common Gateway Interface from your browser. If your internet service provider provides you with webspace, the chances are good that you have access to a perl build on the webserver from which you can run your Perl programs.

Libraries:

  • MediaWiki::API – Basic interface to the API, allowing scripts to automate editing and extraction of data from MediaWiki driven sites.
  • MediaWiki::Bot – A fairly complete MediaWiki bot framework written in Perl. Provides a higher level of abstraction than MediaWiki::API. Plugins provide administrator and steward functionality. Currently unsupported.

PHP

PHP can also be used for programming bots. MediaWiki developers are already familiar with PHP, since that is the language MediaWiki and its extensions are written in. PHP is an especially good choice if you wish to provide a webform-based interface to your bot. For example, suppose you wanted to create a bot for renaming categories. You could create an HTML form into which you will type the current and desired names of a category. When the form is submitted, your bot could read these inputs, then edit all the articles in the current category and move them to the desired category. (Obviously, any bot with a form interface would need to be secured somehow from random web surfers.)

The PHP bot functions table on English Wikipedia may provide some insight into the capabilities of the major bot frameworks.

PHP Bot frameworks
Key people[php 1] Name PHP Version last update Uses API[php 2] Exclusion compliant Admin functions Plugins Repository Notes
Cyberpower678, Addshore, and Jarry1250 Peachy 5.2.1 2015 Yes Yes Yes Yes GitHub Large framework, currently undergoing rewrite. Documentation currently non-existent, so poke w:User:Cyberpower678 for help.
Addshore mediawiki-api-base 7.4+ 2021 Yes N/A N/A extra libs GitHub Base library for interaction with the mediawiki api, provides you with ways to handle logging in, out and handling tokens as well as easily getting and posting requests.
Addshore mediawiki-api 7.4+ 2021 Yes No some extra libs GitHub Build on top of mediawiki-api-base this adds more advanced services for the api such as RevisionGetter, UserGetter, PageDeleter, RevisionPatroller, RevisionSaver etc.
Kaspo Phpwikibot Unknown 2009 Partial No No No Google Code Uses a single class.
Jarry1250 Wikibot 5 2009 Yes Yes No No enwiki Used solely by LivingBot. A fork of Phpwikibot. Uses a single class.
Foxy Loxy PHPediaWiki 5 2009 Yes No Yes No SourceForge Fork of SxWiki
Nzhamstar, Xymph, Waldyrious Wikimate 5.3-5.6,
7.x, 8.x
2023 Yes No No No GitHub Supports main article and file stuff. Authentication, checking if pages exist, reading and editing pages/sections. Getting file information, downloading and uploading files. Aims to be easy to use.
Kaleb Heitzman MediaWIkiBot 5 2012 Yes No No No GitHub Supports the entire API including uploading and importing. Also supports Semantic MediaWiki. Single Class that creates dynamic methods to work with any of the API calls.
Edward Z. Yang Wikipedia Bot in PHP Unknown 2005 No No No No enwiki "Probably stale" source code
Cobi wikibot.classes 5 2010 Yes Yes No No enwiki Used by multiple large bots (e.g. ClueBot and SoxBot). Uses several classes.
Valerio Bozzolan boz-mw 5.6 2019 Yes N/A Yes extra libs GitHub Object-oriented. 80+ classes also to handle Wikidata. Inline documentation. Support for file uploading.
  1. Does not include those who worked on frameworks forked to create listed framework.
  2. Where possible. Excludes uploading images and other such tasks which are not currently supported by the API.

Python

Python is a popular interpreted language with object-oriented features.

Libraries
Please help update this table.
Python Bot frameworks
Key people[py 1] Name Python Version last update Uses API[py 2] Exclusion compliant Admin functions Plugins Repository Notes
xqt Pywikibot Python 3.7 ou superior or PyPy 2023 Yes Yes Yes Yes

The most used Python bot framework. Includes ready to use scripts.

Myst WikibaseIntegrator Python 3.7 or higher 2022 Yes Not applicable Yes No GitHub Only to interact with Wikibase instances like Wikidata
Mr.Z-man wikitools 2 2016 GitHub Incompatible with Python 3. (downloads)
Bryan mwclient 2021 GitHub An API-based framework
The Earwig mwparserfromhell 2021 GitHub A Python parser for MediaWiki text
  1. Does not include those who worked on frameworks forked to create listed framework.
  2. Where possible. Excludes uploading images and other such tasks which are not currently supported by the API.

Microsoft .NET

Microsoft .NET is a set of languages including C#, C++/CLI, Visual Basic .NET, J#, JScript .NET, IronPython, and Windows PowerShell. The Microsoft Visual Studio integrated development environment is often used, or the free Microsoft Visual Studio Express versions. Using Mono Project, .NET programs can also run on Linux, Unix, BSD, Solaris and Mac OS X as well as under Windows.

Libraries:

  • DotNetWikiBot Framework – a full-featured client API on .NET, that allows to build programs and web robots easily to manage information on MediaWiki-powered sites. Now translated to several languages. Detailed compiled documentation is available in English.
  • WikiFunctions .NET library – Bundled with AWB, is a library of stuff useful for bots, such as generating lists, loading/editing articles, connecting to the recent changes IRC channel and more.

Java

Java programs are generally developed with an IDE, such as Eclipse or NetBeans; development using a command line console (with the javac and java programs) is also an option.

Libraries:

  • Java Wiki Bot Framework – A Java wiki bot framework
  • wiki-java – A Java wiki bot framework that is only one file
  • WPCleaner – The library used by the WPCleaner tool
  • jwiki – A simple and easy-to-use Java wiki bot framework

JavaScript (Node.js)

JavaScript is a scripting language used mainly on web pages, such as for user scripts added to your vector.js or your monobook.js pages. Using Node.js it is possible to use JavaScript server-side, such as for developing bots.

Please help to update this table.
NodeJS Bot frameworks
Key people[js 1] Name Nodejs Version last update Uses API[js 2] Exclusion compliant Admin functions Package Repository Notes
SD0001 mwn 10+ 2021 Yes Yes Yes npm GitHub Large library with classes for working with page titles and wikitext. Works with TypeScript also. Promise-based API (asyncawait). Limited wikitext parsing capabilities.
kanashimi wikiapi 0.10–15.x 2021 Yes Yes Partial npm GitHub JavaScript MediaWiki API for node.js with modern ECMAScript 2016 asyncawait and wikitext parser.
MediaWiki module 2014 GitHub Provides a framework of standard requests (e.g. log in, log out, etc.) as well as a general wrapper method for the MediaWiki API and includes throttling. The module can also be added to your Wikimedia .js page and used as library for on-wiki JS calls.
  1. Does not include those who worked on frameworks forked to create listed framework.
  2. Where possible. Excludes uploading images and other such tasks which are not currently supported by the API.

Ruby

Ruby is a popular dynamic, object-oriented programming language.

Libraries:

  • MediaWiki::Butt - Ruby framework for the API in active development. Tested with versions as up-to-date as CurseGamepedia is.
  • mediawiki/ruby/api, Ruby API client library. Last updated December 2017, no longer maintained, but still works.
  • MediaWiki::Gateway – Ruby framework for the API. Last updated January 2016. No longer in active development, tested up to MediaWiki 1.22, compatible with Wikimedia wikis. Unknown if still works.
  • wikipedia-client - Ruby framework using the API. Last updated March 2018. Unknown if still works.

Common Lisp

  • CL-MediaWiki implements MediaWiki API as a Common Lisp package. Is planned to use JSON as a query data format. Supports maxlag and assertion.

Haskell

VBScript

VBScript is a scripting language based on the Visual Basic programming language. There are no published bot frameworks for VBScript, but some examples of bots that use it can be seen below:

Examples:

Bash

Bash is a Unix shell.