Check out example codes for "ruby fork self service". It will help you in understanding the concepts better.

Code Example 1

env: hash
  name => val : set the environment variable
  name => nil : unset the environment variable

  the keys and the values except for +nil+ must be strings.
command...:
  commandline                 : command line string which is passed to the standard shell
  cmdname, arg1, ...          : command name and one or more arguments (This form does not use the shell. See below for caveats.)
  [cmdname, argv0], arg1, ... : command name, argv[0] and zero or more arguments (no shell)
options: hash
  clearing environment variables:
    :unsetenv_others => true   : clear environment variables except specified by env
    :unsetenv_others => false  : don't clear (default)
  process group:
    :pgroup => true or 0 : make a new process group
    :pgroup => pgid      : join the specified process group
    :pgroup => nil       : don't change the process group (default)
  create new process group: Windows only
    :new_pgroup => true  : the new process is the root process of a new process group
    :new_pgroup => false : don't create a new process group (default)
  resource limit: resourcename is core, cpu, data, etc.  See Process.setrlimit.
    :rlimit_resourcename => limit
    :rlimit_resourcename => [cur_limit, max_limit]
  umask:
    :umask => int
  redirection:
    key:
      FD              : single file descriptor in child process
      [FD, FD, ...]   : multiple file descriptor in child process
    value:
      FD                        : redirect to the file descriptor in parent process
      string                    : redirect to file with open(string, "r" or "w")
      [string]                  : redirect to file with open(string, File::RDONLY)
      [string, open_mode]       : redirect to file with open(string, open_mode, 0644)
      [string, open_mode, perm] : redirect to file with open(string, open_mode, perm)
      [:child, FD]              : redirect to the redirected file descriptor
      :close                    : close the file descriptor in child process
    FD is one of follows
      :in     : the file descriptor 0 which is the standard input
      :out    : the file descriptor 1 which is the standard output
      :err    : the file descriptor 2 which is the standard error
      integer : the file descriptor of specified the integer
      io      : the file descriptor specified as io.fileno
  file descriptor inheritance: close non-redirected non-standard fds (3, 4, 5, ...) or not
    :close_others => false  : inherit
  current directory:
    :chdir => str

  The 'cmdname, arg1, ...' form does not use the shell. However,
  on different OSes, different things are provided as built-in
  commands. An example of this is 'echo', which is a built-in
  on Windows, but is a normal program on Linux and Mac OS X.
  This means that `Process.spawn 'echo', '%Path%'` will display
  the contents of the `%Path%` environment variable on Windows,
  but `Process.spawn 'echo', '$PATH'` prints the literal '$PATH'.

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