Check if Standard Output is Terminal or Redirected using C++

In C++ programs, it's sometimes important to know whether the output is being displayed directly on the terminal or redirected to a file or pipe. This can help adjust formatting, enable or disable color output, or choose different logging behavior. For instance, progress bars or interactive prompts make sense only when writing to a terminal. This tutorial demonstrates how to check if standard output is terminal or redirected using C++.

C++ itself doesn't provide a direct way to detect this, but most operating systems offer a simple API to check. The key idea is to query if the standard output file descriptor points to a terminal device.

Different platforms provide different APIs:

• Unix systems (Linux, macOS, etc.) use the isatty function.
• Windows provides _isatty function.

We can write a cross-platform check using conditional compilation:

#include <iostream>

#ifdef _WIN32
#include <io.h>
#define isatty _isatty
#define fileno _fileno
#else
#include <unistd.h>
#endif

int main() {
    if (isatty(fileno(stdout))) {
        std::cout << "stdout is terminal" << std::endl;
    } else {
        std::cout << "stdout is redirected" << std::endl;;
    }

    return 0;
}

Run normally in a terminal:

./test

Output:

stdout is terminal

Redirect output to a file:

./test > test.txt

File content:

stdout is redirected

Pipe output to another command:

./test | grep redirected

This will also detect redirection.