server.R

# server.R

library("shiny")

shinyServer(
  function(input, output){

    output$displayCounter <- renderText({
        c("value of counter:", updateCounter())
    })

    updateCounter <- reactive({
          if (input$counter == 0) {k <- 0}
          else {k <- input$counter-input$subcounter}
            write(k, 'file'="counter.txt")
            return(k)
    })
    
  }
)

ui.R

# ui.R

library("shiny")

shinyUI(
  pageWithSidebar(
    headerPanel("Counter Demo with invisible sidebarPanel")
    ,
    sidebarPanel(
      tags$head(
        tags$style(type='text/css'
          # sidebarPanel appearance
          , ".row-fluid .span4 {width: 0px; height: 0px;}"
          , ".row-fluid .well {padding: 0px; border: 0px;}"
          , ".row-fluid .span4 {background-color: rgb(0, 0, 0);}"
          # button appearance
          , ".btn {padding: 15px; font-size: 120%;}"
        )
      )    
    )
    ,
    mainPanel(    
      helpText("This demo illustrates how to use an actionButton to add to and subtract from a counter")
      ,
      actionButton("counter","Raise counter")
      , tags$br(), tags$br()
      ,
      actionButton("subcounter","Lower counter")
      , tags$br(), tags$br()
      ,
      uiOutput("displayCounter")
    ) 
  )
)