Environment variable expansion occurs when the command is read
On the command line (and in batch files), environment variable expansion occurs when the command is read. This sounds obvious at first, but it has its own consequences.
In the online documentation for
SET, one such
consequence is spelled out:
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
in both “
is substituted when the first “
if” statement is read,
since it logically includes the body of the “
which is a compound statement.
In other words, the “
if” command is not complete
until the closing parenthesis is read.
You can see this if you type the commands interactively:
C:\>set VAR=before C:\>if "%VAR%" == "before" ( More? set VAR=after More? if "%VAR%" == "after" @echo If you see this, it worked More? ) C:\>
Notice that the “
if” command didn’t execute
until you closed the parenthesis;
the command interpreter kept prompting “More?” to collect
the body of the “
This means that everything you type as the body of the “
is evaluated before the “
if” condition or
any of the lines in the body are evaluated.
It’s as if you had typed
C:\>if "before" == "before" ( More? set VAR=after More? if "before" == "after" @echo If you see this, it worked More? )
Note that this is different from most UNIX shells, which do not expand environment variables until the enclosing command is executed. For example,
$ var=before $ var=after; echo $var after
Notice that the
$x is not expanded until the
command’s arguments are being computed.
The analogous commands in the Windows command interpreter
result in something quite different:
C:\>set VAR=before C:\>set VAR=after & echo %VAR% before
That’s because the command interpreter expanded the environment variables at the time the line was read (not at the time the line is executed), yielding
set VAR=after & echo before
As a result, the old value of
VAR is echoed.
Some people treat this as a feature, allowing them to “restore”
a variable without having to save it anywhere:
set VAR=newvalue & call helper.cmd & set VAR=%VAR%
This command sets the
VAR variable to a new value,
helper.cmd (which presumably uses the value
%VAR% variable to control its behavior),
then magically restores the variable to its original value
%VAR% is expanded early, producing the
But what if you want the variable to be expanded at execution
time rather than at parse time?
For that, you use “delayed expansion”, which is enabled by
/V command line option or by using the
SETLOCAL ENABLEDELAYEDEXPANSION command in
a batch file.
C:\> copy con "%TEMP%\helper.cmd" SETLOCAL ENABLEDELAYEDEXPANSION set VAR=before set VAR=after & echo immediate:%VAR%, delayed:!VAR! ENDLOCAL ^Z 1 file(s) copied. C:\> "%TEMP%\helper.cmd" C:\>SETLOCAL ENABLEDELAYEDEXPANSION C:\>set VAR=before C:\>set VAR=after & echo immediate:before, delayed:!VAR! immediate:before, delayed:after C:\>ENDLOCAL
Immediate expansion is performed with percent signs, whereas delayed expansion is performed with exclamation points.
Why is immediate expansion the default? Because prior to Windows NT, that was the only type of expansion supported by the command interpreter. Retaining immediate expansion as the default preserved backwards compatibility with existing batch files. (The original command interpreter was written in assembly language. You really didn’t want to be too clever or it would make your brain hurt trying to maintain the code. An interpreter loop of the form “Read a line, expand environment variables, evaluate” was therefore simple and effective.)
Armed with this understanding of immediate versus delayed
expansion, perhaps you can explain
what is really going on here.
(Hint: It has nothing to do with