Thread: recieving pipes from bash

Hi!
I read that programs read input from pipes through stdin.
So that if I wrote this in bash:

Code:

echo "Hei" | cat

The cat-program finds the "Hei" text from reading stdin.


But how does for example the more-program work?
Because I can pipe things to more in bash, but if I write just "more" without anything else, it just says: " Illegal command blablab".

Shouldnt the "more"-program be waiting from input from stdin?

Or does it have some way of doing like this?

Code:

if(pipedInfoInStandardInput) {
 readFromStandardInput();
} else { // Normal programflow
 ...
}

Put simple: How can "more" know when to read stdin for piped info, and when not to?
(Because when I write "more" and nothing else, it doesent accept input from stdin...it's only when I use pipes that it reads stdin)

aah, thanks a lot!
So we could do something like this then?

Code:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main() {
  if(isatty(fileno(stdin)) != 1) {
    // Is it safe to assume that we can read info from stdin as if it was a piped, now?
  } else {
    // Programflow that does terminal things
  }
  return EXIT_SUCCESS;
}

fileno(stdin) can be abbreviated as STDIN_FILENO (in unistd.h), which is defined by POSIX to be 0.

I find STDIN_FILENO to be more readable than fileno(stdin).

Whether 0 is more readable is a question of how much you work with POSIX systems, I suppose. I wouldn't have any problems with it, but I suppose people less used to it would have.