fd is a non-negative integer, an index value, pointing to the record table of the process's open files maintained by the kernel for each process .
A program opens an existing file or creates a new file , and the kernel returns a file descriptor to the process.
Each Unix process has three standard POSIX file descriptors, corresponding to three standard streams:
（ref：https://zh.wikipedia.org/wiki/%E6%96%87%E4%BB%B6%E6%8F%8F%E8%BF%B0%E7%AC%A6）
| integer value | name | < unistd.h > symbolic constants [1] | < stdio.h > file stream [2] |
|---|---|---|---|
| 0 | Standard input | STDIN_FILENO | stdin |
| 1 | Standard output | STDOUT_FILENO | stdout |
| 2 | Standard error | STDERR_FILENO | stderr |
Standard input : is the data (usually a file) input to the program. The program requires a read operation to transfer data.
C++ <iostream> variable is std::cin
Not all programs require input, such as ls
Unless redirected, input is expected to be obtained by the keyboard.
Standard output : C++ <iostream> variable is std::cout
Output to terminal (screen) unless redirected
Not all programs require output, like mv
Standard error : C++ <iostream> variables are std::cerr and std::clog
Default output to terminal (screen)