Why does the same command work in a bash script but not in Java Runtime directly?

Question

The command "cat ~/desktop/b.mpg ~/desktop/b2.mpg > ~desktop/intermediate_all.mpg" does not seem to work via Java Runtime alone (as seen in the example below);

public class Test {
    public static void main(final String[] args)  {
        String[] cmd = {"cat ~/desktop/b.mpg ~/desktop/b2.mpg > ~desktop/intermediate_all.mpg"};
        try { Runtime.getRuntime().exec(cmd);  }
        catch (IOException e) { e.printStackTrace();}
    }
}

However, when put into a .sh file like in this second example it works just fine....

public class Test {
    public static void main(final String[] args)  {
        try { Runtime.getRuntime().exec("/users/nn/desktop/configure.sh"); }
        catch (IOException e) { e.printStackTrace();}
    }
}

Can anybody please tell me what the fundamental process is being lost when moving from a bash script to straight Java Runtime? FYI, I am using OSX, have already tried using absolute filepaths, and know about Process Builder (which has the same effect) is preferred to using Java Runtim--as has been stated a thousand times on this forum already, so lets avoid beating the dead horse on that one.

Thanks

Answer 1

The command being executed is cat with arguments. The command and its arguments must be separate elements of the array.

Also, you can't redirect using Runtime.exec() - you must use ProcessBuilder :

Try this:

ProcessBuilder pb = new ProcessBuilder("cat", "~/desktop/b.mpg", "~/desktop/b2.mpg");
pb.redirectOutput(new File("~/desktop/intermediate_all.mpg"));
Process p = pb.start();

It is likely that the shell location ~ will not be understood, so you may have to use the full absolute path for the files

Answer 2

Try this:

Runtime.getRuntime().exec(new String[] {
    "/bin/bash", "-c", 
    "cat ~/desktop/b.mpg ~/desktop/b2.mpg > ~/desktop/intermediate_all.mpg" })

Answer 3

Because in the second case, you are effectively running "bash", "-c", "cat etc >file" where Bash takes care of parsing the redirection for you. Redirection is a feature of the shell, not of cat ; if you run the raw process without a shell, the features of the shell are not available to you.

Answer 4

在您的java代码〜/ desktop / b.mpg~/ desktop / b2.mpg> ~desktop / intermediate_all.mpg中，您必须在>〜/ desktop / intermediate_all.mpg之后给出完整路径