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I have started using Mathematica to automate file backup operations. The logistics I need make Mathematica a good choice. I am testing and developing the Notebook manually, and Print outputs document the operational events. The saved Notebook provides a record of the process for that day.

I would like to have an easy way to direct Print output to a separate file, for example, by adding

DirectPrintOutputTo["filename "<>DateList["Year","Month","Day"]<>".txt"] 

at the beginning of a Notebook. The advantages of this approach seem obvious. I assume I would need to load my function from a package before calling it. Is something like this this possible? Any suggestions about how to do it?

Alternatively, maybe I could write a print function called PrintTo that would, on first use, open an output file with the name assigned to printToFileName, and I could use the editor to change all the Print functions to PrintTo functions. Not so fancy, but I think I know how to how to do it.

I think that I answered my own question, in the sense that my alternative idea seems straightforward and pretty implementable.

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My current scripting approach used Print statements to print results to the scripting notebook. I don't think PutAppend would work (but I may not understand your suggestion). –  George Wolfe Jul 26 '13 at 19:34
I have missunderstood the question, I thought you are going to write this not to change Print. So it is not good suggestion then :) –  Kuba Jul 26 '13 at 19:54
Possible duplicates: (5040), (7081), (22584) –  Mr.Wizard Jul 26 '13 at 20:31
I wonder if one could use 'TagSet' to make extend the functionality of 'Print' to your specific needs. I don't have much time to think through this now (heading away for a week). May I can work something up later tonight. –  Jagra Jul 27 '13 at 17:30
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3 Answers

up vote 3 down vote accepted

From the documentation for Print: "Print sends its output to the channel $Output."

Therefore you can just do this:

$Output = OpenWrite["filename " <> DateList["Year", "Month", "Day"] <> ".txt"];

before you run the file.

Something more structured would be like this:

stream = OpenWrite["c:/users/gre/desktop/derp.txt"];
AppendTo[$Output, stream];

(* run your file here *)

$Output = Most[$Output];

Since Print by default outputs to stdout, you could also pipe the output by running the notebook as a script.

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I tried using Block[{$Output = ..., but this didn't seem to work for $Output. –  amr Jul 26 '13 at 20:20
Beat me to the punch about $Output. However, I was working on a generic environment. –  rcollyer Jul 26 '13 at 20:21
@rcollyer Well you both beat me, so... –  Mr.Wizard Jul 26 '13 at 20:23
@rcollyer agreed. usually i go the lazy route :D –  amr Jul 26 '13 at 20:23
@Mr.Wizard I beat both of you a long time ago, and Leonid. :) –  rcollyer Jul 26 '13 at 20:24
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I don't recommend modifying Print to redirect the output when it is not necessary. Instead, you can build up a custom solution that suits your needs. I use a variant of the following for logging in my projects and you should be able to tweak them to your application.

Clear[logFile, writeLog, showLog];
logFile := "project-" <> DateString[##, {"Year", "Month", "Day"}] <> ".log" &;

writeLog[message_String] := 
    Module[{str = OpenAppend[logFile[]], date = DateString[]}, 
        WriteString[str, StringJoin @@ Riffle[{date, message, "\n"}, "\t"]];

showLog := FilePrint@logFile@## &;
clearLog := Module[{str = OpenWrite[logFile@##]}, Close@str;] &

Note the use of OpenAppend and OpenWrite in the above functions. OpenAppend creates a new file if none exists, but appends to an already existing file. OpenWrite, on the other hand, simply creates a new empty file regardless of whether a file already exists with that name (thus erasing data, which is what we want here).

Since writeLog just takes a string as input, you can simply replace your Print commands with writeLog and it should work fine.

(* "project-20130726.log" *)

writeLog["Hello world"];
(* Fri 26 Jul 2013 12:51:31 Hello world *)

(* crickets *)
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This is already built-in; Print directs all output to the $Output channel where a channel is just a list of open streams. So, to redirect it, simply

oldout = $Output;
    strm = OpenAppend["filename "<>DateString[]<>".txt"];    
    $Output = {strm};

and to switch it back

$Output = oldout;

Assuming you just want an environment where Print is re-directed, you could do something like this

SetAttributes[RedirectPrint, HoldRest];
Options[RedirectPrint] = {OpenMethod -> OpenAppend};
RedirectPrint[filename_String, body_, opts:OptionsPattern[]] :=
 Block[{old, strm, open},
  open = OptionValue[OpenMethod]; (* needs error checking here *)
  old = $Output;
        strm = open[filename];
        $Output = {strm},
    $Output = old;
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@Mike the change is good, I was taking the OP's code directly without looking at it ... –  rcollyer Jul 28 '13 at 3:09
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