`fork` and `exec` in Python.

exec.py

#!/usr/bin/env python3

import os

# `fork` current process. On success, the child's PID is returned to the parent
# and `0` is returned to the child. On failure, `-1` is returned and no process
# is created.
pid = os.fork()

if pid == -1:
    os.exit(1);

# At this point there are two processes running. The parent will not enter the
# `if` statement and the child will.
if pid == 0:
    # In the `exec` family of functions, functions with `l` are variadic (i.e.
    # take `argv` in multiple arguments) and functions with `p` search for the
    # executable file in the `PATH` environment variable.
    #
    # Note that `'echo'` is repeated twice because the first one refers to the
    # process to execute and the second one to the first argument passed to the
    # process, i.e. `argv[0]`.
    #
    # Since only the child enters the `if` statement, only the child becomes an
    # `echo` process.
    os.execlp('echo', 'echo', 'hello world')

# Suspend the execution of the parent until one of its children terminates. In
# this case, the only child is bound to execute `echo`.
os.wait()