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Performs conditional processing in batch programs.

IF [NOT] ERRORLEVEL number command
IF [NOT] string1==string2 command
IF [NOT] EXIST filename command

NOT               Specifies that Windows 2000 should carry out the command
only if the condition is false.

ERRORLEVEL number Specifies a true condition if the last program run
returned an exit code equal to or greater than the number
specified.

string1==string2  Specifies a true condition if the specified text strings
match.

EXIST filename    Specifies a true condition if the specified filename
exists.

command           Specifies the command to carry out if the condition is
met.  Command can be followed by ELSE command which
will execute the command after the ELSE keyword if the
specified condition is FALSE

The ELSE clause must occur on the same line as the command after the IF.  For
example:

IF EXIST filename. (
del filename.
) ELSE (
echo filename. missing.
)

The following would NOT work because the del command needs to be terminated
by a newline:

IF EXIST filename. del filename. ELSE echo filename. missing

Nor would the following work
since the ELSE command must be on the same line
as the end of the IF command:

IF EXIST filename. del filename.
ELSE echo filename. missing

The following would work if you want it all on one line:

IF EXIST filename. (del filename.) ELSE echo filename. missing

If Command Extensions are enabled IF changes as follows:

IF [/I] string1 compare-op string2 command
IF CMDEXTVERSION number command
IF DEFINED variable command

where compare-op may be one of:

EQU - equal
NEQ - not equal
LSS - less than
LEQ - less than or equal
GTR - greater than
GEQ - greater than or equal

and the /I switch
if specified
says to do case insensitive string
compares.  The /I switch can also be used on the string1==string2 form
of IF.  These comparisons are generic
in that if both string1 and
string2 are both comprised of all numeric digits
then the strings are
converted to numbers and a numeric comparison is performed.

The CMDEXTVERSION conditional works just like ERRORLEVEL
except it is
comparing against an internal version number associated with the Command
Extensions.  The first version is 1.  It will be incremented by one when
significant enhancements are added to the Command Extensions.
CMDEXTVERSION conditional is never true when Command Extensions are
disabled.

The DEFINED conditional works just like EXISTS except it takes an
environment variable name and returns true if the environment variable
is defined.

%ERRORLEVEL% will expand into a string representation of
the current value of ERRORLEVEL
provided that there is not already
an environment variable with the name ERRORLEVEL
in which case you
will get its value instead.  After running a program
the following
illustrates ERRORLEVEL use:

goto answer%ERRORLEVEL%
:answer0
echo Program had return code 0
:answer1
echo Program had return code 1

You can also using the numerical comparisons above:

IF %ERRORLEVEL% LEQ 1 goto okay

%CMDCMDLINE% will expand into the original command line passed to
CMD.EXE prior to any processing by CMD.EXE
provided that there is not
already an environment variable with the name CMDCMDLINE
in which case
you will get its value instead.

%CMDEXTVERSION% will expand into the a string representation of the
current value of CMDEXTVERSION
provided that there is not already
an environment variable with the name CMDEXTVERSION
in which case you
will get its value instead.