Detect non-empty STDIN in Clojure

Question

How do you detect non-empty standard input (*in*) without reading from it in a non-blocking way in Clojure?

At first, I thought calling using the java.io.Reader#ready() method would do, but (.ready *in*) returns false even when standard input is provided.

Answer 1

Is this what you are looking for? InputStream .available

(defn -main [& args]
  (if (> (.available System/in) 0)
    (println "STDIN: " (slurp *in*))
    (println "No Input")))

---

$ echo "hello" | lein run
STDIN:  hello

$ lein run
No Input

---

Update: It does seem that .available is a race condition checking STDIN. n alternative is to have a fixed timeout for STDIN to become available otherwise assume no data is coming from STDIN

Here is an example of using core.async to attempt to read the first byte from STDIN and append it to the rest of the STDIN or timeout.

(ns stdin.core
  (:require
   [clojure.core.async :as async :refer [go >! timeout chan alt!!]])
  (:gen-class))

(defn -main [& args]
  (let [c (chan)]
    (go (>! c (.read *in*)))
    (if-let [ch (alt!! (timeout 500) nil
                       c ([ch] (if-not (< ch 0) ch)))]
      (do
        (.unread *in* ch)
        (println (slurp *in*)))

      (println "No STDIN"))))