Why is the FOR /f loop in this batch script evaluating a blank line?

I just came over this topic. I've been using findstr /v to exclude empty lines:

FOR /f "usebackq skip=1 tokens=1 delims=:" %%a in (`WMIC logicaldisk WHERE "drivetype=3" GET deviceid ^| findstr /v /r "^$"`) do (

In this case the last iteration produces not an empty item, and you get your output of C|D|E|| only with echo %DISK_DATABASES%,
but echo !DISK_DATABASES! will output ||D|E|??

That's because the last element is a single <CR> character.
And <CR> characters are directly removed after the percent expansion, but not with delayed expansion.

You could avoid this, using the percent expansion to remove them

setlocal EnableDelayedExpansion
FOR /f "skip=1 tokens=1 delims=:" %%a in ('"WMIC logicaldisk WHERE drivetype=3 GET deviceid"') do (
  set "item=%%a"
  call :removeCR

  if not "!item!"=="" (
    SET "DISK_DATABASES=!DISK_DATABASES!!item!|"
    SET "DRIVES_TO_MONITOR=!DRIVES_TO_MONITOR!!item!:\\|"
  )
)
goto :eof
:removeCR

:removeCR
set "Item=%Item%"
exit /b

According to http://ss64.com/nt/for_f.html

Many of the newer commands and utilities (e.g. WMIC) output text files in unicode format, these cannot be read by the FOR command which expects ASCII. To convert the file format use the TYPE command.

So it appears that WMIC and FOR don't play nice together.

I discovered a more efficient and more reliable method to strip the unwanted <CR> from the end of each line. No temp file, and no CALL needed.

I don't understand the mechanism of how FOR /F converts the WMIC unicode output into ASCII. Normally FOR /F cannot read unicode. But however it works, each converted line ends with <CR><CR><LF>. FOR /F breaks lines at each <LF>, and then if the last character in the line is <CR> it strips that last <CR>, in this case leaving behind the unwanted <CR>.

The solution is to simply pass each line through one more FOR /F :-)

@echo off
setlocal enableDelayedExpansion
for /f "skip=1 delims=" %%A in (
  'wmic logicaldisk where "drivetype=3" get deviceid'
) do for /f "tokens=1 delims=:" %%B in ("%%A") do (
  set "disk_databases=!disk_databases!%%B|"
  set "drives_to_monitor=!drives_to_monitor!%%B:\\|"
)

This method is more reliable then using normal expansion because you don't have to worry about quoting or escaping special characters. For example, The CALL method that uses normal expansion cannot handle a string like "this & that" & the other. But this method has no problem with such a string.

Add ^| findstr . and you will get only not blank lines

    REM Build the list of disk drives to monitor
    SETLOCAL enabledelayedexpansion
    FOR /f "skip=1 tokens=1 delims=:" %%a in (
    '"WMIC logicaldisk WHERE drivetype=3 GET deviceid" ^| findstr .') do (
        SET "DISK_DATABASES=!DISK_DATABASES!%%a|"
        SET "DRIVES_TO_MONITOR=!DRIVES_TO_MONITOR!%%a:\|"
    )