从终端和 java 运行时进程执行 linux 命令时的区别

Question

I'm looking a way to write running log of python which is executed by java app via script.

Let's say my script is:

import time

for x in range(120):
  print("Running ", x)
  time.sleep(1)

Here is my current solution:

1. Trigger script using java

String cmd = "python script.py";
var process = Runtime.getRuntime().exec(cmd, null, new File(sandboxPath));

2. Write log to new file:

String traceLogCmd = String.format("strace -p %s -s 9999 -e trace=write -o output.txt", process.pid());
Runtime.getRuntime().exec(traceLogCmd, null, new File(sandboxPath));

Now the problem is output.txt only has content whenever the python script is done executing so that I cannot tailf the output file.

Meanwhile if I execute python script.py and strace command dirrectly from terminal, the output is exactly what I expected.

Can someone correct me if I did something wrong or have a another way to get python log?

Thanks in advance.

Answer 1

Use ProcessBuilder instead of Runtime.exec() . More details: When Runtime.exec() won't

The following code will append to StringBuilder object output of the script sb.append(line); . It would not be difficult to write that content to a file.

Process p = new ProcessBuilder("sh", "-c", "python", "path-to-your-script").start();
String result = getCommandResult(p.getInputStream());

private static String getCommandResult(InputStream stream) throws IOException {

    StringBuilder sb = new StringBuilder();
    try (InputStreamReader isr = new InputStreamReader(stream);
         BufferedReader in = new BufferedReader(isr)) {

        String line;
        while ((line = in.readLine()) != null) {
            sb.append(line);
        }
    }
    return sb.toString().trim();
}