recieving pipes from bash

Drogin · 04-12-2009, 06:46 AM

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)

draugr · 04-12-2009, 07:46 AM

Hei Drogin

It's actually simple and you pretty much guessed it: the "magic function" is isatty(int filedes). The function can tell you if stdin (file descriptor 0) comes from an interactive terminal or not, so you can do different things depending on what it tells you.

Drogin · 04-12-2009, 09:05 AM

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;
}

draugr · 04-12-2009, 09:21 AM

Yes, that should work.

**CornedBee** · 04-12-2009, 01:22 PM

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

Drogin · 04-12-2009, 04:07 PM

It can, but don't you think
isatty(fileno(stdin))
is more readable than a "magic" number 0?

EDIT:
Ah,nvm, understood what you meant now

**CornedBee** · 04-12-2009, 04:09 PM

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.

04-12-2009, 06:46 AM	#1
Drogin Shalom Join Date: Oct 2005 Location: Norway Posts: 92	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) Last edited by Drogin; 04-12-2009 at 07:20 AM.
Drogin is offline

04-12-2009, 07:46 AM	#2
draugr Registered User Join Date: Mar 2009 Posts: 9	Hei Drogin It's actually simple and you pretty much guessed it: the "magic function" is isatty(int filedes). The function can tell you if stdin (file descriptor 0) comes from an interactive terminal or not, so you can do different things depending on what it tells you.
draugr is offline

04-12-2009, 09:21 AM	#4
draugr Registered User Join Date: Mar 2009 Posts: 9	Yes, that should work.
draugr is offline

04-12-2009, 01:22 PM	#5
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,492	fileno(stdin) can be abbreviated as STDIN_FILENO (in unistd.h), which is defined by POSIX to be 0. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
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04-12-2009, 04:07 PM	#6
Drogin Shalom Join Date: Oct 2005 Location: Norway Posts: 92	It can, but don't you think isatty(fileno(stdin)) is more readable than a "magic" number 0? EDIT: Ah,nvm, understood what you meant now
Drogin is offline

04-12-2009, 04:09 PM	#7
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,492	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. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
CornedBee is offline

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