Question

Is it better to comment a batch file with :: or REM?

Asked by: BillDL

Hi Experts

I've asked this as the result of a specific suggestion made by our expert leew in another DOS-related question in which I was contributing:

"I don't know if BillDL and I have had this debate - but I strongly advise AGAINST using :: as a comment sign - it's not what it was intended for and in certain circumstances, it can have unexpected results.  If you want to comment in your batch files, then use the appropriate comment command - REM"

I am not intending to discredit or cross swords with leew, it's just that this has me curious simply because I've read conflicting explanations of which is better and I would like to hear the reasons.

I read (on the pages of other very well qualified IT gurus like leew) that lines preceded with REM actually have to be parsed and THEN disregarded, whereas the :: symbol is completely ignored, and thereby speeds up the processing of very long and heavily commented batch files.

I can see that an accidental space between the 2 colons could play havoc because the batch file would consider it to be a Label and then probably consider the 2nd colon to be an illegal Label name, being a reserved character.

What I'm most intrigued about is what kind of circumstances the :: may cause unwanted results.  I've used :: to comment batch files for a long time now and it's entirely possible that some of them didn't work because of such a circumstance arising.

Does anyone know of any good test batch files that would demonstrate unwanted results?

I hope this doesn't cause a riot of differing opinions from other experts who like using ::  :-)

Thanks
Bill

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Asked On
2008-12-09 at 02:58:02ID23968693
Topic

MS DOS

Participating Experts
1
Points
250
Comments
3

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Answers

 

by: leewPosted on 2008-12-09 at 03:06:56ID: 23128236

I believe SteveGTR did a test - and yes, :: is faster then REM in a 10K line batch file.  But, in the grand scheme of things, is 2 seconds going to make that much of a difference in your batch file?  The issue can come up if using them to comment within a loop - if I remember correctly.  Please see:
http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/MS_DOS/Q_21498411.html (not just the answer - this entire issue was hashed out as the question went on...)

 

by: BillDLPosted on 2008-12-09 at 04:07:07ID: 23128445

Thank you for that link Lee.  It led me to a host of other links, and each page contained not only information relating to :: vs REM, but loads of other useful info that was new to me.

Mark's comment in this question:
http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/MS_DOS/Q_21045653.html
explains quite well where the issues arise and what they result in:

So the rules about :: comments in multi-line blocks seem to be:

1. Don't use two :: lines in a row - "The system cannot find the drive specified."
2. Don't use a :: as the last line in a block - ") was unexpected at this time."
3. Don't use a :: followed by an empty line - "The syntax of the command is incorrect."

Thanks again
Bill

 

by: BillDLPosted on 2008-12-09 at 04:13:37ID: 31524134

Don't know why I didn't find the linked pages when I searched E-E before.  Your advice is appreciated.

20120131-EE-VQP-002

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