Markdown preview via shiny, pandoc and rmarkdown

server.R

library(shiny)
library(rmarkdown)

shinyServer(function(input, output) {

    output$html = reactive({

        t <- tempfile()
        cat(input$markdown, file = t)

        ## convert input to html
        t <- render(
            input         = t,
            output_format = 'html_document')

        ## read results
        res <- readLines(t)

        ## cleanup
        unlink(sub('.html$', '*', t))

        ## return
        paste(res, collapse = '\n')

    })

})

ui.R

library(shiny)
 
shinyUI(fluidPage(
 
    titlePanel('Try markdown'),
 
    fluidRow(
        column(6,
               tags$textarea(
                   'Hello markdown!',
                   id    = 'markdown',
                   rows  = 25,
                   style = 'width:100%;')),
 
        column(6,
            htmlOutput('html')))
 
))

pander version: https://gist.github.com/daroczig/e56e0e8ad3e8a3eed31e