Execute external program from Java

Question

Hi, 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(new String[]{
          "/bin/bash", "-c", "executable -o filename.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.

Now, the executable I am trying to run is supposed to spit the output to filename.txt. If I take this command and run it on the terminal, it works fine and the filename.txt gets populated also. But, from my java program the file is not created.

if instead I use executable > filename.txt then the filename.txt is created but is empty. Not sure what's wrong here. The executable I am trying to run is Xtide (if that helps).

I would really appreciate any help I can get.

Thanks,

Answer 1

You cannot redirect output to file and read the output in java. It's one or the other. What you want is this:

      Process p = Runtime.getRuntime().exec(new String[]{
          "/bin/bash", "-c", "executable -o filename.txt"});
      p.waitFor();
      BufferedReader input = new BufferedReader(
          new InputStreamReader(new FileInputStream("filename.txt")));
      while ((line = input.readLine()) != null) {
        System.out.println(line);
      }

The main changes are:

• p.waitFor(), since process execution is asynchronous, so you have to wait for it to complete.
• The data is read from the file rather than from the output of the process (since this will be empty.)

Answer 2

The answer from mdma works (and I voted it up), but you might also want to consider the version where you do read the output stream directly from executable:

Process p = Runtime.getRuntime().exec(new String[]{
          "/bin/bash", "-c", "executable"});
p.waitFor();
BufferedReader input = new BufferedReader(new InputStreamReader(p.getInputStream())_;
while ((line = input.readLine()) != null) {
    System.out.println(line);
}

Answer 3

Correct me if I am wrong, but the symptoms are as follows:

• exec("/usr/bash", "-c", "executable > filename.txt") creates an empty file.
• exec("/usr/bash", "-c", "executable -o filename.txt") does not create a file.
• One or both of the above gives an exit code of 255 when you look at it.
• When you run the command from the command line as executable -o filename.txt or executable > filename.txt it works as expected.

In the light of the above, I think that the most likely cause is that /bin/bash is not finding the executable when you launch it from Java. The fact that the first example does create an empty file means that /bin/bash is doing something. But if you try to run

$ unknown-command > somefile.txt

from a bash shell prompt you will get an error message saying that the command cannot be found and an empty "something.txt" file. (You would not see the error message in your Java app because it is being written to stderr, and you are not capturing it.) The reason that the empty "something.txt" file is created is that it is opened by the shell before it attempts to fork and exec the "executable".

If this is the problem, then the simple solution is to use the absolute pathname for the executable.

Also, if you are not doing any command line redirection or other shell magic, there is no need to run the executable in a new bash instance. Rather, just do this:

Process p = Runtime.getRuntime().exec("executable", "-o", filename.txt");

then wait for the process to complete and check the exit code before trying to read the file contents.