Variables in batch file not being set when inside IF?
Environment variables in batch files are expanded when a line is parsed. In the case of blocks delimited by parentheses (as your if defined
) the whole block counts as a "line" or command.
This means that all occurrences of %FOO% are replaces by their values before the block is run. In your case with nothing, as the variable doesn't have a value yet.
To solve this you can enable delayed expansion:
setlocal enabledelayedexpansion
Delayed expansion causes variables delimited by exclamation marks (!
) to be evaluated on execution instead of parsing which will ensure the correct behavior in your case:
if not defined BAR (
set FOO=1
echo Foo: !FOO!
)
help set
details this too:
Finally, support for delayed environment variable expansion has been added. This support is always disabled by default, but may be enabled/disabled via the
/V
command line switch toCMD.EXE
. SeeCMD /?
Delayed environment variable expansion is useful for getting around the limitations of the current expansion which happens when a line of text is read, not when it is executed. The following example demonstrates the problem with immediate variable expansion:
set VAR=before if "%VAR%" == "before" ( set VAR=after if "%VAR%" == "after" @echo If you see this, it worked )
would never display the message, since the
%VAR%
in bothIF
statements is substituted when the first IF statement is read, since it logically includes the body of theIF
, which is a compound statement. So the IF inside the compound statement is really comparing "before" with "after" which will never be equal. Similarly, the following example will not work as expected:set LIST= for %i in (*) do set LIST=%LIST% %i echo %LIST%
in that it will not build up a list of files in the current directory, but instead will just set the
LIST
variable to the last file found. Again, this is because the%LIST%
is expanded just once when theFOR
statement is read, and at that time theLIST
variable is empty. So the actual FOR loop we are executing is:for %i in (*) do set LIST= %i
which just keeps setting
LIST
to the last file found.Delayed environment variable expansion allows you to use a different character (the exclamation mark) to expand environment variables at execution time. If delayed variable expansion is enabled, the above examples could be written as follows to work as intended:
set VAR=before if "%VAR%" == "before" ( set VAR=after if "!VAR!" == "after" @echo If you see this, it worked ) set LIST= for %i in (*) do set LIST=!LIST! %i echo %LIST%
The same behavior also happens when the commands are on a single line (&
is the command separator):
if not defined BAR set FOO=1& echo FOO: %FOO%
Joey's explanation is my favorite. Note however that enabledelayedexpansion
does not work on Windows NT 4.0 (and I'm not sure about Windows 2000).
About your follow-up question, no, it is not possible to EnableDelayedExpansion
without setlocal
. However the original behavior that was going against you can be used to workaroud that second problem: the trick is to endlocal
on the same line where you set again the values of the variables you need.
Here is your test.bat
modified:
@echo off
setlocal EnableDelayedExpansion
IF NOT DEFINED BAR (
set FOO=1
echo FOO: !FOO!
)
endlocal & set FOO=%FOO%
But here is another workaround to that problem: use a procedure in the same file instead of an inline block or an external file.
@echo off
if not defined BAR call :NotDefined
pause
goto :EOF
:NotDefined
set FOO=1
echo FOO: %FOO%
goto :EOF
If it isn't working that way, you likely have delayed environment variable expansion on. You can either turn it off with cmd /V:OFF
or use exclamation marks inside your if:
@echo off
IF NOT DEFINED BAR (
set FOO=1
echo FOO: !FOO!
)
pause
echo on