Piping a subprocesses output directly to stdout (java/clojure)

Question

I'm looking for a way to start up a subprocess in clojure (java is fine to) and have the output of it get sent directly to stdout in real-time. The closest I've been able to find is the Conch library for clojure, which allows you to send the output to *out*, but it doesn't actually display the output until the process is done running.

Answer 1

Not sure if there is a convenient Clojure wrapper for this:

(->> (.. Runtime getRuntime (exec "ls") getInputStream)                                                                                                                   
    java.io.InputStreamReader.                                                                                                                                            
    java.io.BufferedReader.                                                                                                                                               
    line-seq                                                                                                                                                              
    (map println))  

It's worth noting in practice that you need to read both stdin and stderr regularly or the process can hang when one of the buffers fill.

Answer 2

I marked Arthur's answer as correct since it led me to the solution, and it basically is what someone would want in the basic case. I ended up building out a larger method which does more though, and figured I'd put it here in case anyone else found it useful

(defn- print-return-stream     
    [stream]
    (let [stream-seq (->> stream    
                          (java.io.InputStreamReader.)    
                          (java.io.BufferedReader.)       
                          line-seq)]                      
        (doall (reduce
            (fn [acc line]     
                (println line) 
                (if (empty? acc) line (str acc "\n" line))) 
            ""                 
            stream-seq))))     

(defn exec-stream              
    "Executes a command in the given dir, streaming stdout and stderr to stdout,
    and once the exec is finished returns a vector of the return code, a string of
    all the stdout output, and a string of all the stderr output"
    [dir command & args]       
    (let [runtime  (Runtime/getRuntime)
          proc     (.exec runtime (into-array (cons command args)) nil (File. dir)) 
          stdout   (.getInputStream proc) 
          stderr   (.getErrorStream proc) 
          outfut   (future (print-return-stream stdout))
          errfut   (future (print-return-stream stderr))
          proc-ret (.waitFor proc)]       
        [proc-ret @outfut @errfut]      
        ))