Java: is there a way to run a system command and print the output during execution?

Question

I have a python script and it takes a long time to finish. I would like to run it from Java, but also output the script\'s output while it is executing, so that I can tell i

Answer 1

I've searched and only found examples where we output the output after the system command has finished, rather than during its execution.

That's weird, because your example should be dumping the output as the command is executing.

Instead of using BufferedReader, you could try reading directly from the InputStream instead as the required conditions for readLine might not be being met until after the process exits.

I'd also recommend that you use a ProcessBuilder over Process directly, as, apart from anything else, it allows you to redirect the output from the error stream into the input stream, allowing you to read just one stream instead of two...

This might also be an issue with Python and how it flushes it output buffers...

For example, rather then waiting for the BufferedReader to decide when to return, try printing each character from the stream as it occurs/is reported

            ProcessBuilder pb = new ProcessBuilder("test.py");
            pb.redirectError();
            Process p = pb.start();

            InputStream is = null;
            try {
                is = p.getInputStream();
                int in = -1;
                while ((in = is.read()) != -1) {
                    System.out.print((char)in);
                }
            } finally {
                try {
                    is.close();
                } catch (Exception e) {
                }
            }

Update

Doing a little reading, Python seems to be buffering its out out before sending it to the stdout. I don't think you can fix this on the a Java side, but need to alter either the way Python is run or the script works.

See How to flush output of Python print? for more details