Ian & Stuart's Australian Mac 1/Ian and Stuart's One (Australia).iso/Australasian Legends/Commercial/Rainbow Hill/MacDOS™ 2.0.0/batches/existVar.bat

DOS Batch File | 1994-08-26 | 2KB | 47 lines

@echo off
!  existVar.bat   Batch file to check/ensure that a variable exists
!
!  existVar name default
!
!  where 'name' is the name of the variable and default is the value to
!  which the variable should be set if it does not exists.
!  This batch is useful because MacDOS (like DOS) removes a user defined
!  variable when it gets assigned the empty string. As a consequence, a
!  check like   if %var% == "whatever" goto LBL   generates an error when
!  the variable does not exist.
!
!  The simplest way to check for existence of a variable is as follows:
!    if not "%varName%" == %%VARNAME%% goto EXISTS_LBL
!  This check works because a reference to a non-existing variable returns
!  a capitalised version of the reference itself. So, if the variable varName
!  does not exist, an attempt to de-reference it returns the string %VARNAME% .
!  The double percent signs are necessary to represent one '%'.
!
!  Unfortunately, you cannot derefrence twice a variable. That is, you cannot
!  extract the value from a variable whose name is stored in another variable.
!  Therefore, the simple 'if' shown above cannot be used in a batch like this
!  in which the name of a variable is passed as an argument. The way to do it
!  here is to use the internal checking of MacDOS.
!
!  This batch files lets you assign a default to a variable and it ensures
!  that the variable exists. This of course is only true if the second
!  parameter is defined!
!

    !
    ! append a character to the variable and then remove it. MacDOS will
    ! reports error 63 if the variable does not exist. To keep it simple,
    ! we assume that the variable does not exist if ANY error occurs.
    !
    onerror VAR_ERR_LBL
    incr %1 by "*"
    decr %1
    goto DONE_LBL

:VAR_ERR_LBL
    !
    ! The variable does not exist. Create it by setting it to the requested default.
    set %1=%2

:DONE_LBL