httpuv server that prints out headers

server.R

library(httpuv)

s <- startServer("0.0.0.0", 5000,
  list(
    call = function(req) {
      txt <- as.list(req$HEADERS)
      txt <- paste(capture.output(str(txt)), collapse = "\n")

      txt <- paste0(req$REQUEST_METHOD, " ", req$PATH_INFO, " \n", txt)
      cat(txt, "\n\n")

      list(
        status = 200L,
        headers = list(
          'Content-Type' = 'text/plain'
        ),
        body = txt
      )
    }
  )
)

# Output looks like this:
#
# GET / 
# List of 9
#  $ accept                   : chr "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8"
#  $ accept-encoding          : chr "gzip, deflate, br"
#  $ accept-language          : chr "en-US,en;q=0.9"
#  $ cache-control            : chr "max-age=0"
#  $ connection               : chr "keep-alive"
#  $ cookie                   : chr "_gauges_unique=1"
#  $ host                     : chr "localhost:5000"
#  $ upgrade-insecure-requests: chr "1"
#  $ user-agent               : chr "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.109 Safari/537.36"
#