How to substitute variable contents in a Windows batch file

Question

I'm writing a simple script to substitute text in an environment variable with other text. The trouble I get is with the substituted or substituted text being pulled from other variables

SET a=The fat cat
ECHO %a%
REM Results in 'The fat cat'
ECHO %a:fat=thin%
REM Results in 'The thin cat'

Works fine (output is 'The fat cat' and 'The thin cat'

However, if 'fat' or 'thin' are in variables, it doesn't work

SET b=fat
ECHO %a:%c%=thin%
REM _Should_ give 'The thin cat'.
REM _Actually_ gives '%a:fat=thin%' (the %c% is evaluated, but no further).

REM using delayed evaluation doesn't make any difference either
ECHO !a:%c%=thin!
REM Actual output is now '!a:fat=thin!'

I know this can be done as I've seen it in blogs before, but I never saved the link to the blogs.

Anyone have any ideas?

PS. I'm running the scripts on Windows 7

PPS. I know this is easier in Perl/Python/other script language of choice, but I just want to know why something that should be easy is not immediately obvious.

PPPS. I've also tried the scripts with delayed expansion explicitly turned on

SETLOCAL enabledelayedexpansion

This makes no difference.

Answer 1

Please try the following:

Copy and paste the code into Notepad and save it as a batch file.

   @echo off
   setlocal enabledelayedexpansion

   set str=The fat cat
   set f=fat

   echo.
   echo          f = [%f%]

   echo.
   echo        str = [%str%]

   set str=!str:%f%=thin!

   echo str:f=thin = [%str%]

I hope you're convinced!

Answer 2

Use CALL. Put the following in a batch script and run it:

set a=The fat cat
set b=fat
set c=thin

REM To replace "%b%" with "%c%" in "%a%", we can do:
call set a=%%a:%b%^=%c%%%
echo %a%
pause

As stated here, we use the fact that:

CALL internal_cmd

...

internal_cmd Run an internal command, first expanding any variables in the argument.

In our case internal_cmd is initially set a=%%a:%b%^=%c%%%.

After expansion internal_cmd becomes set a=%a:fat=thin%.

Thus, in our case running

call set a=%%a:%b%^=%c%%%

is equivalent to running:

set a=%a:fat=thin%.