Java running bash command while feeding it input and getting output

Question

I've got to write down some java code that runs an external process. This process gets an input line from stdin and gives an output line on stdout.

The code I wrote is as follows

try
{            
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("./" + args[0]);
    Process proc2 = rt.exec(c);
    int exitVal = proc2.exitValue();
    System.out.println("Process exitValue: " + exitVal);
} 
catch (Throwable t){
}

Where args[0] is the external process and c is the input (String) I need to feed to the process. If it was running correctly, I would get an exitVal=0, instead I get nothing. Also, what I really want to print is the output of the external process (which would be "6" in the given example).

catch (Throwable t){}... never do that javapractices.com/topic/TopicAction.do?Id=16 — artbristol
– artbristol, Commented Mar 28, 2012 at 12:56

wds · Accepted Answer · 2012-03-28 12:57:15Z

2

You must use getInputStream() and getOutputStream() on the process to communicate it, see the javadocs.

If you do not read all data from the process, it might hang and even deadlock. For a short introduction on common problems, see this blog post. At the very least, you should be reading all data from the process output.

answered Mar 28, 2012 at 12:57

wds

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Comments

AlexR · Accepted Answer · 2012-03-28 12:52:16Z

0

You cannot "get nothing". Method exitValue() returns primitive type, therefore it even cannot be null. But this method can stuck however. This happens because your process is still running.

Is is possible that it is running because you did not "feed" it? In this case the only thing you have to do is to call proc2.getOutputStream() and write what you need to this stream. If you want to read the process' output use proc2.getInputStream() and read from the stream.

BTW, take a look on ProcessBuilder. It provides better, more "object oriented" API thant simple Runtime.exec().

answered Mar 28, 2012 at 12:52

AlexR

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1 Comment

Athanasia Notta Over a year ago

Thank you for your reply! Still no success using the ProcessBuilder :/ The problem remains the same... The way the executable works is: you run it (Enter) and then it waits for a string (you enter the string -> Enter).. It's basically one process, I only need a way to let this process know that it's waiting for a stdin.

wytten · Accepted Answer · 2012-03-28 13:02:19Z

0

You can feed the program input with '<' redirection:

{
  Process p = Runtime.getRuntime().exec("wc < pom.xml");
  p.waitFor();
  System.out.println(p.exitValue());
  BufferedReader bri = new BufferedReader
    (new InputStreamReader(p.getInputStream()));
  String line;
  while ((line = bri.readLine()) != null) {
    System.out.println(line);
  }
  bri.close();
}

answered Mar 28, 2012 at 13:02

wytten

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1 Comment

Athanasia Notta Over a year ago

Thank you for your reply! Thing is i don't want to feed my executable a file, but a string.

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