> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/4.3-bsd-unix-cao-zuo-xi-tong-she-ji-yu-shi-xian/di-san-bu-fen-io-xi-tong/9.5.-jin-cheng-zu-yu-zhong-duan-kong-zhi.md).

# 9.5 进程组与终端控制

第 4 章描述的进程控制（作业控制）设施依靠终端 I/O 系统来控制对终端的访问。每个作业（一组受统一控制的进程）以一个进程组号标识；每个终端结构也含一个进程组号。终端初次打开时，shell 和终端被指派到一个进程组（shell 的 PID）。在终端上键入字符产生的信号，发给终端当前进程组中的全部进程；其中一些信号使进程组停止。shell 把每个作业的进程组号设为组内某一成员的 PID；每次把新作业放到前台，shell 就把终端进程组设为新进程组的组号。于是，终端进程组是当前控制终端的进程组——即运行在前台（foreground）的进程组——的标识；其他进程组在后台（background）运行。后台进程组中的进程试图从终端读时，其进程组会收到又一个信号而被停止。后台进程试图做终端输出时，也可以选择同样被停止。每个进程持有指向其控制终端（若有）的指针；上述输入输出操作的控制规则只适用于对控制终端的操作。

没有作业控制的早期系统对进程组的用法不同；4.3BSD 中使用不支持作业控制的登录 shell 时也会出现类似行为。shell 和终端起初被指派到同一进程组；这样的 shell 启动的新进程留在同一组里。因此，不用作业控制时，一个进程组标识着从同一登录 shell 发起的全部进程。


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