Windows – How to set/use an empty string value in a variable in Windows XP CMD.EXE command-line

cmd.execommand linewindowswindows xp

Using SET to set an empty value to a variable works nicely in a Windows XP .CMD script. However, the same SET command behaves differently in the same CMD interpreter when used directly from command-line:

Trying to test it in a CMD.EXE command-line:

C:\>set Q=

C:\>echo %Q%
%Q%

C:\>echo "%Q%"
"%Q%"

C:\>set Q=/Q

C:\>echo %Q%
/Q

C:\>echo "%Q%"
"/Q"

Trying to test it with a .CMD script like this:

set Q=
echo %Q%
echo "%Q%"
set Q=/Q
echo %Q%
echo "%Q%"

Running it:

C:\>c.cmd

C:\>set Q=

C:\>echo
ECHO is on.

C:\>echo ""
""

C:\>set Q=/Q

C:\>echo /Q
/Q

C:\>echo "/Q"
"/Q"

What am I missing?

Is that a nice joke from Redmond, or is there any sane explanation for this difference?

How am I supposed to test lines from a .CMD script when they behave differently when used in a command-line?

How do I properly set a variable to an empty string value in a CMD command-line and how do I reference that variable so that I get that empty string?

Best Answer

tl;dr - Problem you got there is that echo %Q% expands to echo. Use echo.%Q%

Expanded Answer

echo command got 4 behaviors:

  • echo on - enables echoing commands.
  • echo off - disables echoing commands.
  • echo - shows state of echoing commands option.
  • echo ... - puts ... and newline on the screen.

If you pass variable to echo as an argument, and it's empty, it will expand to echo, and will show something like "ECHO is off."

Many people use echo. command to display empty string (read: output newline), but not everybody knows that echo. can be used to excplicitly specify that you want output behavior. e.g.:

  • echo.on - will output on and newline
  • echo.off - will output off and newline
  • echo. - will output newline
  • echo.something - will output something and newline
  • echo.%Q% - will output contents of %Q% whether or not it's ""/"on"/"off" or whatever else.

Keep in mind that there should not be space between . and arguments.

See https://technet.microsoft.com/en-us/library/bb490897.aspx?f=255&MSPPError=-2147217396

Related Question