用途isatty()：

$ man isatty
ISATTY(3)                  Linux Programmer's Manual                 ISATTY(3)

NAME
       isatty - does this descriptor refer to a terminal

SYNOPSIS
       #include <unistd.h>

       int isatty(int desc);

DESCRIPTION
       returns  1  if  desc is an open file descriptor connected to a terminal
       and 0 otherwise.

由于stdout始终是文件描述符1，因此您可以执行以下操作：

if(isatty(1))
    // stdout is a terminal