Execing Processes in Mercury

Our example demonstrates how to execute external processes in a way that replaces the current process, similar to the Unix exec function. Here’s how we can achieve this in Java:

import java.io.IOException;

public class ExecingProcesses {
    public static void main(String[] args) {
        // For our example we'll exec 'ls'. Java doesn't require an
        // absolute path to the binary, so we can use it directly.
        String binary = "ls";

        // ProcessBuilder requires arguments as separate strings.
        // We'll give 'ls' a few common arguments. Note that the first
        // argument should be the program name.
        String[] command = {binary, "-a", "-l", "-h"};

        try {
            // Create a new ProcessBuilder with our command
            ProcessBuilder pb = new ProcessBuilder(command);

            // Inherit the environment of the current process
            pb.inheritIO();

            // Start the process. This replaces the current Java process
            // with the new process.
            Process process = pb.start();

            // Wait for the process to complete
            int exitCode = process.waitFor();

            // If we reach here, it means the process has completed
            // This code won't be reached if the exec is successful
            System.out.println("Process exited with code " + exitCode);

        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

When we run our program, it will be replaced by ls:

$ java ExecingProcesses
total 16
drwxr-xr-x  4 user  136B Oct 3 16:29 .
drwxr-xr-x 91 user  3.0K Oct 3 12:50 ..
-rw-r--r--  1 user  1.3K Oct 3 16:28 ExecingProcesses.java

Note that Java doesn’t offer a direct equivalent to the Unix exec system call. Instead, we use ProcessBuilder and its inheritIO() method to achieve a similar effect. This approach starts a new process and redirects its input/output streams to those of the current Java process.

Also, Java doesn’t have a direct equivalent to Go’s syscall.Exec. The Java process will continue to run after starting the new process, unlike in the Go example where the original process is completely replaced. However, by using process.waitFor(), we ensure that our Java program doesn’t exit until the executed command completes.

Remember that Java’s approach to process execution is more focused on creating child processes rather than replacing the current process. This design choice aligns with Java’s platform-independent nature and its security model.

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Execing Processes in Mercury