Windows – Why does this batch file fail on a “REM” line

batchbatch filecommand linecommentswindows

I refer to question Add text to end of filename (but before extension) using batch file as I have the same problem. Using Windows 7 32-bit Enterprise (I know, I know …) with all updates I wrote a tiny batch file pdfrename.bat with only three lines:

  1. a comment, starting with REM and no "hidden" continuation sign more to the right of the line

  2. the proposed command, copy-pasted from the source as provided by @Karan, commented out with REM

  3. the adopted command for batch (doubling the %):

    REM Rework 2020-12-16
    REM for %a in (*.txt) do ren "%~a" "%~na version 1%~xa"
    for %%F in (*.pdf) do ren "%%~F" "%%~nF OdB%%~xF"

Running the command (3.) from the command-prompt,

for %F in (*.pdf) do ren "%~F" "%~nF OdB%~xF"

works fine.

But executing the whole batch-file pdfrename.bat from Windows explorer fails. Running the batch file from the command prompt pdfrename.bat produces an error message syntax error:

Die folgende Verwendung des Pfadoperators zur Ersetzung eines Batchparameters
ist ungültig: %~na version 1%~xa"

You need not to understand German. The important point is that the error message refers to the commented out second line (2.), not to the third line!

I tried to retype REM, inserted a tabulator after REM, inserted a second REM after the first (REM REM ....), inserted a third line after the second one with the same content and deleted the second line afterwards – nothing changed: the batch file terminates at the commented out second line with a syntax error. As soon as the incriminated second line is eliminated from the batch, the batch works fine.

I searched for "REM is ignored", but no luck, so I post the matter here. I never heard about or experienced before that the command processor tries at least to analyze a commented out line – and in the case that there is something wrong with the code after the comment-sign that it terminates the batch script.

Best Answer

The error-msg refers to the commented out second line

This is due to the very complex parsing that is used by cmd to process scripts.

In short the parser processes % before pretty much everything else (phase 1) and throws an error as some of the %s need to be doubled as %% when used in a batch file.

So in a batch file:

for %%F in (*.pdf) do ren "%%~F" "%%~nF OdB%%~xF"

is a valid command and:

REM for %a in (*.txt) do ren "%~a" "%~na version 1%~xa"

is a broken command (the %a should be %%a, etc).

Note that:

REM for %a in (*.txt) do ren "%~a" "%~na version 1%~xa"

is a valid command when run from the command line as then the % does not need to be doubled.

REM would be processed in phase 2 of the parser, but it never gets there as the % processing in phase 1 has already generated an error and terminated the parsing.

For all the gory details of the cmd parser please read parsing - How does the Windows Command Interpreter (CMD.EXE) parse scripts? - Stack Overflow

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