library(shiny)
library(tidyverse)
library(readxl)
server <- function(input, output) {

  names <- reactive({ read_excel(input$file1$name, sheet = 1,
                      range = "A1:P1", col_names = TRUE)
                    })
}

library(shiny)
 library(shinythemes)
 ui <- fluidPage(

  theme=shinytheme("readable"),

  # App title ----
  titlePanel("App_1"),

  # Sidebar layout with input and output definitions ----
  sidebarLayout(

    # Sidebar panel for inputs ----
    sidebarPanel(


      # Input: Select a file ----
      fileInput("file1", "Choose File",
                multiple = FALSE),

      # Horizontal line ----
      tags$hr(),


      checkboxGroupInput("show_vars", "Columns in diamonds to show:", names(names) )
    )
  )
)

observe({
 if (is.null(input$file1))
  return(NULL)
 cnames <- colnames(names())
 updateCheckboxGroupInput(session, inputId = "show_vars", choices = cnames)
})