Controlling terminal
在类 Unix 操作系统中，控制终端是与进程组相关联的终端设备，它控制着进程与终端输入和输出的交互。控制终端通常是启动进程的终端。它负责处理进程的输入（如键盘输入）和输出（如显示文本）。
A controlling terminal is the terminal device associated with a process group in Unix-like operating systems, which controls the process's interaction with the terminal input and output. The controlling terminal is typically the terminal from which a process was started. It is responsible for handling input (like keyboard input) and output (like displaying text) for the process.
Controlling Terminal in Practice
- 在终端中启动进程：
- 在 Linux 系统上打开一个终端（如 `bash` 或 `sh`）并运行一个命令（如 `vim`、`cat` 或任何脚本）时，该终端就会成为 shell 进程和从该 shell 启动的任何进程的控制终端。
- Starting a Process in a Terminal:
- When you open a terminal (like `bash` or `sh`) on a Linux system and run a command (e.g., `vim`, `cat`, or any script), the terminal becomes the controlling terminal for the shell process and any processes started from that shell.
- 交互式程序：
- 假设你打开一个终端，运行 `vim` 来编辑一个文件：
vim example.txt - 在这里，你输入命令的终端是 `vim` 进程的控制终端。您键入的输入会发送到 `vim`，而 `vim` 的输出会显示在该终端上。
- Interactive Programs:
- Suppose you open a terminal and run `vim` to edit a file:
vim example.txt - Here, the terminal where you typed the command is the controlling terminal for the `vim` process. Input you type is sent to `vim`, and output from `vim` is displayed on that terminal.
- 无控制终端（守护进程示例）：
- 许多服务器进程或守护进程在启动时没有控制终端。典型的例子是启动时启动的服务或由系统管理器（如 `systemd`）启动的服务。这些进程通常与任何终端分离，以确保它们不会意外接收或需要终端输入。
- 例如，像 Apache 这样的网络服务器是作为后台进程启动的，没有控制终端：
apachectl start - apache进程在后台运行，与终端分离，这意味着它没有控制终端。
注意，使用"&"启用后台进程时，控制终端是仍然存在的。后台进程不直接接收当前终端的输入。
- No Controlling Terminal (Daemon Example):
- Many server processes or daemons are started without a controlling terminal. This is typical for services started at boot time or by a system manager like `systemd`. These processes are usually detached from any terminal to ensure they don't accidentally receive or require terminal input.
- For example, a web server like Apache is started as a background process with no controlling terminal:
apachectl start - The `apache` process runs in the background, detached from the terminal, meaning it has no controlling terminal.
- 使用 `O_NOCTTY`：
- 如果编写的程序要打开终端设备（如串行端口），但又不想让终端设备成为进程的控制终端，则可以使用 `O_NOCTTY` 标志。
- 例如
int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY)； - 这将确保打开串行端口 `/dev/ttyS0`，但它不会成为打开该端口的进程的控制终端。
- Using `O_NOCTTY`:
- If you write a program that opens a terminal device (like a serial port) but doesn't want the terminal device to become the controlling terminal for the process, you use the `O_NOCTTY` flag.
- For instance:
int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY); - This ensures that the serial port `/dev/ttyS0` is opened, but it does not become the controlling terminal for the process that opened it.
Why Is This Important?
控制终端的概念对于信号处理、作业控制以及与终端的交互都很重要。例如，"SIGINT"（通过按下 "Ctrl+C "产生）等信号会发送到控制终端的进程组。如果进程没有控制终端，它就不会从终端接收此类信号，而这些信号对于进程在后台或作为守护进程时的行为至关重要。
The concept of a controlling terminal is important for signal handling, job control, and interaction with the terminal. For example, signals like `SIGINT` (generated by pressing `Ctrl+C`) are sent to the process group of the controlling terminal. If a process doesn't have a controlling terminal, it won't receive such signals from the terminal, which can be crucial for how a process behaves in the background or as a daemon.