Question

I am trying to execute a program from the Java code. Here is my code:

public static void main(String argv[]) {
    try {
      String line;
      Process p = Runtime.getRuntime().exec(
          "/bin/bash -c ls > OutputFileNames.txt");
      BufferedReader input = new BufferedReader(
          new InputStreamReader(p.getInputStream()));
      while ((line = input.readLine()) != null) {
        System.out.println(line);
      }
      input.close();
    } catch (Exception err) {
      err.printStackTrace();
    }
}

My OS is Mac OS X 10.6.

If I remove the "> OutputFileNames.txt" from the getRuntime().exec() method, all the file names are printed on the console just fine. But I need it to be printed to a file.

Also, if I change the command to:

Process p = Runtime.getRuntime().exec(
    "cmd \c dir > OutputFileNames.txt"); 

and run it on Windows, it runs and prints the results in the file perfectly fine too.

I have read the other posts for executing another application from Java but none seemed to relate to my problem.

I would really appreciate any help I can get.

Thanks,

Answer 1

To get the redirection to work as written, you need to do this:

Process p = Runtime.getRuntime().exec(
      new String[]{"/bin/bash", "-c", "ls > OutputFileNames.txt"});

The problem you were running into is the simple-minded way that that Runtime.exec(String) splits a command line into arguments.

If you were to run that command (as is) at a shell prompt, you would have had to have entered it as:

$ /bin/bash -c "ls > OutputFileNames.txt"

because the “-c” option for “bash” requires the command line for the spawned shell as a single shell argument. But if you were to put the naked quotes into the Java String, the Runtime.exec(String) method still get the splitting wrong. The only solution is to provide the command arguments as an array.