R/Shiny app as a simple GET request handler

app.R

# Save it as /srv/shiny-server/your_app_name/app.R

library(shiny)

ui <- function(request) {
  query <- parseQueryString(request$QUERY_STRING)
  
  txt <- query$txt
  a <- as.numeric(query$a)
  b <- as.numeric(query$b)
  
  if (is.null(txt) || length(a)==0 || length(b)==0)
    httpResponse(
      status = 400,
      content_type = "text/plain; charset=UTF-8",
      content = 'One of the GET parameters (txt or a or b -- each case sensitive!) not provided!'
    ) else
    httpResponse(
      status = 200,
      content_type = "text/plain; charset=UTF-8",
      content = paste(toupper(txt),
                      a + b,
                      sep='\n')
    )
}

server <- function(input, output, session) {}

shinyApp(ui = ui, server = server)

# test:
# https://localhost:8787/your_app_name/?txt=hello%20world&a=12&b=13
# or
# https://your.shinyserver.domain/your_app_name/?txt=hello%20world&a=12&b=13