Re: Pipes and Redirection

WaltP · #1 02-Mar-2004, 14:24

I just posted a GID Community blog entitled: Piping and Redirection.

Quote:

There are three predefined devices:

stdin -- standard input read more...

Please let me know what you think.

This is an auto-generated message posted by GIDNetwork™ on behalf of the author.

dsmith · #2 11-Apr-2004, 08:01

Stderr Redirection

This is a small addition to this excellent piping and redirection tutorial. This is not so much a C/C++ theme as it is a scripting theme.

It is also possible to redirect the stderr stream as well as the stdin stream. This could be useful, if you want to create a "log" file of your errors/warnings, but not have them interfere with the "normal" output of your program.

Take this small, worthless example of a c code:

CPP / C++ / C Code:

#include <stdio.h>

int main(){
	fprintf(stderr,"This goes to the stderr location.");
	printf("This is the standard output.");
	
	return 0;
}

If you compile and run this, you will get:

Code:

This goes to the stderr location.
This is the standard output.

However, if you run this program with a redirection like:

Code:

pipetest 2>logfile.txt

You will get on the console only:

Code:

This is the standard output.

and you will also get a text file named logfile.txt that will contain the stderr output.

If you run it again with:

Code:

pipetest 2>>logfile.txt

It will add the stderr output to the end of the "log" file, so that you will now get two lines of the stderr output.

This is quite helpful in writing daemon routines. For *nix only, I could make any program a daemon by using:

Code:

mydaemon 1>>/var/log/messages 2>mydaemon.log &

The ampersand tells *nix to spawn the process in the background and return control to the calling program. This would throw all standard output to the end of the existing /var/log/messages file and throw all of the standard error stuff to a newly created file called mydaemon.log.

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