so_question.R

library(shiny)
runApp(list(
  ui = bootstrapPage(
    uiOutput('intext'),
    uiOutput('variables')
  ),
  server = function(input, output){
    outVar <- reactive({
      mytext = input$text
      if (!is.null(mytext)){
        vars <- all.vars(parse(text = mytext))
        vars <- as.list(vars)
        return(vars)
      }
    })
    output$intext = renderUI({
      textInput("text", "Enter Formula", "a=b+c")
    })
    output$variables = renderUI({
      selectInput('variables2', 'Variables', outVar())
    })
  }
))