sigchld_handler:
  waitpid
  WIFSTOPPED / WSTOPSIG
  WIFSIGNALED / WTERMSIG
  WIFEXITED

sigint_handler:
  kill

sigtstp_handler:
  kill

main:
  fgets
  eval

"eval":
  sigemptyset
  sigaddset
  sigprocmask
  setpgid
  fork
  execve

"builtin_command":
  exit
  kill

"waitfg":
  sigemptyset
  Sigsuspend