JennieInMI has asked for the wisdom of the Perl Monks concerning the following question:

Hi Folks, My code is actually doing what I want it to, but I'm curious about some unexpected behavior so I am here to ask those who might know. :-)

My script sends a system command to TFS to attempt to extract a build label in order to assess if that build has been baselined. This is my mock-up to test the code: 

$tf="C:\\Program Files (x86)\\Microsoft Visual Studio 10.0\\Common7\\I
+DE\\tf.exe";
$buildLabel = "the build label";
$/ = undef;

BuildCheckedInToTFS();

sub BuildCheckedInToTFS(){
    my $checkedIn = 0;
    print "Verifying build label exists in TFS...\n";
    `\"$tf\" labels /owner:theOwner \"$buildLabel\"  > "TFSLabelInfo.t
+xt"`;
    open TFSLABEL, "TFSLabelInfo.txt" or die $!;
    my $TFSLabel = <TFSLABEL>;
    close TFSLABEL;
    if ($TFSLabel =~ /$buildLabel/){
        $checkedIn = 1;
        print "Build $buildLabel has been checked-in to TFS.";
    }
    else {
        print "Baselining failed. \n:" +
            "Build $buildLabel has not been checked-in to TFS.";
    }
    return $checkedIn;
}

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For those who are curious, I use the returned value in another portion of the script. Anywho, it is doing just what I want- it reports whether or not the build label in question is present. This is just an audit script so that's all I need. However, when I was just testing the tf command and all I had was this 

$tf="C:\\Program Files (x86)\\Microsoft Visual Studio 10.0\\Common7\\I
+DE\\tf.exe";
$buildLabel = "the build label";
`\"$tf\" labels /owner:theOwner \"$buildLabel\"`;

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in my script it produced no apparent result. When I would run the script there would be a pause as though some work were taking place, then the command prompt would simply reappear and there would be no output on the screen. I figured it was checking the mapped drive so I added the redirection to see if something would come of that. As soon as I did, the text file appeared and I got the expected result (a listing of the labels matching the specs). What I don't understand is why I get no output in the command window when I run the tf command via my Perl script. When I simply run the command at the command prompt I get the expected output (a listing of the labels matching the specs) in the window, but when I run it via my script I get nada in the command window. I even tried putting in an echo command and still the same thing - a pause followed by the command prompt. I would expect nothing to show in the window if the output is being redirected, but I don't get why it is happening when the output is not redirected.

This doesn't matter in terms of the function of this script, but I'm curious to know the reason why. I am new to Perl and perhaps what you might call an advanced beginner at programming, so if you need to get very technical a link to a detailed explanation would really be welcome. Thanks in advance!

Replies are listed 'Best First'.
Re: No command window output when running script
by syphilis (Archbishop) on Dec 29, 2012 at 09:11 UTC
    `\"$tf\" labels /owner:theOwner \"$buildLabel\"`;

    I think it's just that backticks returns the output, rather than prints the output ... and since you haven't collected what was returned, it just gets lost.
    By way of demonstration: 
    C:\_32\C>cd
C:\_32\C

C:\_32\C>perl -e "`cd`"

C:\_32\C>perl -e "$x=`cd`;print $x"
C:\_32\C

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    Cheers,
    Rob
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      Hi Rob,

      Thank you for this reply. It makes perfect sense. I wasn't storing what was returned and printing it, so nothing was output to the screen. Returning and outputting are not inherently equivalent.

      I just tried out your example. Cd doesn't return a value so nothing is output with this example, but when I used dir instead I got a nice screenful of info. :-D I'm not on my work computer right now so I can't try out my tf command yet, but I will give it a try later. Thank you again for the answer.

      Jennie

[reply]
        Cd doesn't return a value so nothing is output with this example

        Interesting ... and there's probably a good reason for that, but I don't know what it is.
        Certainly, the behaviour you're seeing doesn't conform to the documentation I get for cd, which begins with: 
        C:\_32\C>cd /?
Displays the name of or changes the current directory.

CHDIR [/D] [drive:][path]
CHDIR [..]
CD [/D] [drive:][path]
CD [..]

  ..   Specifies that you want to change to the parent directory.

Type CD drive: to display the current directory in the specified drive
+.
Type CD without parameters to display the current drive and directory.
.....

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        Perhaps you're running in a different shell (I use cmd.exe), or perhaps your cmd.exe shell finds a different 'cd' implementation to run.
        To force your cmd.exe shell to use its own implementation, I think you just need to involve a shell metacharacter in the command - eg: 
        cd <nul

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        Does that command print out the current directory for you ?

        (Admittedly this has nothing to do with the subject you initially raised ... but I find it a little bit interesting.)

        Cheers,
        Rob
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