Is stdin a Terminal or a Pipe?

Determining the Nature of Standard Input: Terminal or Pipe

In various scenarios, it can be beneficial to differentiate whether standard input (stdin) represents a terminal or a pipe. This distinction becomes evident in the divergence in behavior exhibited by Python when invoked without arguments from the terminal, compared to when piped input is provided. To replicate such detection, various programming languages, including C, C , and Qt, offer different approaches.

Unix Approach: isatty

For C and C , the Unix system call isatty() provides a means to ascertain the nature of stdin. This function takes an integer representing the file descriptor of the file being queried as its parameter. For stdin, this is typically obtained using the expression fileno(stdin). If the file descriptor corresponds to a terminal, isatty() returns a non-zero value, indicating that stdin is a terminal. Conversely, a zero value indicates that stdin is not a terminal, likely representing a pipe or file.

Example:

#include <stdio.h>
#include <io.h>
...
if (isatty(fileno(stdin)))
    printf("stdin is a terminal\n");
else
    printf("stdin is a file or a pipe\n");

Qt Approach: QTextStream

For Qt, a different method is required due to the framework's object-oriented approach. Qt employs a QTextStream to interact with stdin, which can be used to determine whether stdin originates from a terminal or not. A QTextStream instance is created by calling QTextStream(stdin) with stdin as the argument. The following code demonstrates this approach:

QTextStream qin(stdin);
bool isTerminal = qin.device()->isInteractive();

In this instance, isInteractive returns true if stdin is a terminal, and false if it is a pipe or file.

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