shell_spawn
if ((err = shell_spawn(cmd, -1, -1, -1, &pid)) != 0) {
if ((err = shell_spawn(cmd, pfds[0], stdout_fd, pfds[1], &pid)) != 0) {
if ((err = shell_spawn(cmd, -1, fd, -1, &pid)) != 0) {
err = shell_spawn(cp->c_shcmd, nullfd, fd, -1, &pid);