#include <fcntl.h>
#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

int main() {
  // The program that will get run, and the file to write to. It is similar to
  // the command-line:
  // echo "Hello, world!" > ./hello.txt
  char *const prog[] = { "echo", "Hello, world!", NULL };
  const char *filename = "./hello.txt";

  // Create a child process like normal. The parent process will wait for the
  // child process, and the child process will execute its program with the
  // standard output redirected to the file.
  pid_t pid = fork();

  if(pid == 0) {
    // Mimicking the behavior of `>` in the shell, open a handle to the file
    // `hello.txt` for writing. If the file doesn't exist, it will be created,
    // and if it does exist, truncate the contents of the file.
    int handle = open(filename, O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR);

    // Map the standard output of the child process to the open file handle.
    // This means that whatever writes on the child's standard output stream
    // will instead go to the file. The parent's standard output stream will
    // remain unchanged.
    dup2(handle, STDOUT_FILENO);

    // Execute the program in the child as normal, now that the standard output
    // stream has been redirected to the file.
    execvp(prog[0], prog);
  } else {
    // Wait in the parent process for the child to finish running. Once it is
    // finished, write the child's process and exit information to its own
    // standard output stream, which was not changed by the child's `dup2`
    // call.
    int status = 0;
    wait(&status);
    printf("Process %d exited with status %d.\n", pid, status);
  }

  return 0;
}