cp.py

#!/usr/bin/python3.11
import os
import sys

move = os.SPLICE_F_MOVE if os.getenv("MOVE") else 0
more = os.SPLICE_F_MORE if os.getenv("MORE") else 0
flag = move | more


def copy(src_path, dst_path, buffer_size=1 << 16):
    src_fd = os.open(src_path, os.O_RDONLY)
    dst_fd = os.open(dst_path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC)
    # create a pipe for our buffer instead of using python memory
    r_fd, w_fd = os.pipe()
    while True:
        # transfer data from src to the write end of the pipe
        # it stays in the kernel
        bytes_in = os.splice(src_fd, w_fd, buffer_size, flags=flag)
        if bytes_in == 0:  # EOF
            break
        # transfer data from the read end of the pipe to dst
        # it doesn't go through userspace
        bytes_out = os.splice(r_fd, dest_fd, bytes_in, flags=flag)


if __name__ == "__main__":
    copy(sys.argv[1], sys.argv[2])