redirect the output of an executable

**lei_michelle** · 02-20-2008

Hi All,

I need to redirect the output of an executable to a file instead of the standard output. I wonder what the easiest way to achieve that. For example, I have an batch file, which prints out some information on screen when it is executed from command line. Now I want to write a C program, to put the output of the file to a log file (for both UNIX and Windows).

I know it can be achieved by first opening the file, fetching each line of the outputs and then writing each line to the log file. But my feeling is there should be some easier way to reach the same goal.

**matsp** · 02-20-2008

Do you mean like this:

Code:

C:\>dir > logfile
...
C:\>mybatch.bat > mybatch.log

Or where you after something else?
--
Mats

**robwhit** · 02-20-2008

system ( "program.exe > output.txt" ) ;

or

C:\> program.exe > output.txt

or

rename ( "old.txt" , "new.txt" ) ;

depending on what you want.

**lei_michelle** · 02-20-2008

Yes, it achieves the same goal as "C:\>mybatch.bat > mybatch.log" except for it needs to be done through programming.

**lei_michelle** · 02-20-2008

The system call should work. Thanks!

Can I use spawn?

**matsp** · 02-20-2008

In that case, I would write a program that does this:

Code:

#include <stdio.h>

int main(int argc, char **argv)
{
    if (argc < 2) return 1;
    else {
      FILE *f = fopen(argv[1], "w");
      char buffer[1000];
      if (f) {
         while(fgets(buffer, sizeof buffer, stdin))
             fwrite(buffer, 1, strlen(buffer), f);
          fclose(f);
          return 0;
      }
      return 2;
}

It's missing error checking, but roughly does what you want.

Use it like this: "mybatch | mylogger mybatch.log"

--
Mats

**lei_michelle** · 02-20-2008

Hi Mats,

I know your code can achieve the goal. But is your method better than the "system" call and why?

**matsp** · 02-20-2008

No, it's not better - but why do you want to do it from a C program in the first place, if the best solution (in my mind) is to redirect the batch file in the first place. If you want to avoid the user doing the redirection, you can either use a Windows shortcut, or use another batch-file to produce the desired result. This is the most efficient method, and also the most "simple" solution from a programming perspective - you just have to put the command(s) into a batch file and that's job done.

If you can do it using a batch-file, then that's as good as calling "system()" - there is no great deal of difference other than that you have to write some C-code that does the calling of a system command - not much point in that, in my opinion.

Perhaps you need to explain further what you are actually trying to achieve...

--
Mats

**dwks** · 02-20-2008

Another suggestion: the POSIX function popen() might do what you're looking for: http://opengroup.org/onlinepubs/0079...xsh/popen.html

popen() lets your program start another program and then take the output from that, rather than having to launch the other program externally for example with "program | program". You could, for example, launch the batch file from within the program, and grab its output.

Note: popen() is not ANSI standard. If your system doesn't have it, I'm sure there is a Win32 equivalent, though I'm not sure what it would be. Perhaps _popen() on MSVC.

**esbo** · 02-20-2008

Originally Posted by robwhit

system ( "program.exe > output.txt" ) ;

or

C:\> program.exe > output.txt

or

rename ( "old.txt" , "new.txt" ) ;

depending on what you want.

Hmm this is a bit of a confusing thread as it is a bit unclear what the question is.
Is it I want to call a batch file from a program and have the output of the batchfile redirected into a file? I guess that is what it seems to be.

I am just wondering if you can just call

Code:

system("batchfile");

And then just redirect the output of the program eg

Code:

program>results

Or whether the batchfile is 'unaware' of the redirection?

Anyway it seems it is aware of the redirection, so it may be better to do it that way
so that the redirection is not 'hard coded' into the program?
I guess it depends on what you want.

**lei_michelle** · 02-21-2008

Thanks all for the useful feedback.

I think the following method fits my goal:

Code:

system ( "program.exe > output.txt" ) ;

I don't want to add another batch file to my project, just to redirect the output. And I don't think it is necessary to write codes to manually put every output line to a file if the "system" call is as good.

Actually the output file will be used for later parsing, which is not supposed to be used by users.

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