jon-hanson jon-hanson - 1 year ago 66
Bash Question

How expand a CMD shell variable twice (recursively)

Using the Windows XP CMD command-line I can expand a variable twice as follows:

set AAA=BBB
set BBB=CCC
for /F "usebackq tokens=*" %i in (`echo %%AAA%%`) do echo %i


will echo
CCC
. I.e.
AAA
has been expanded to the string
BBB
, and then the variable
BBB
has been expanded to
CCC
.

This doesn't work from inside a batch script (i.e. a .cmd file). Changing the
%%AAA%%
to either
%%%AAA%%%
or
%%%%AAA%%%%
doesn't work either.

Any idea how i can achieve this from within a script, namely to take expand the variable AAA to the string CCC?

Late Edit

The answers posted work for my reduced example however the non-tortuous answer doesn't work for the real case. Here's an extended example (which doesn't work), which illustrates what I was actually trying to do:

setlocal enabledelayedexpansion

set LIST=BBB CCC DDD
set BBB=111
set CCC=222
set DDD=333

for %%i in (%LIST%) do (

for /F %%a in ('echo %%%i%') do echo !%%a!

)


I would like to see

111
222
333


output.

akf akf
Answer Source

Thinking in terms of a less tortuous solution, this, too, produces the CCC you desire.

setlocal enabledelayedexpansion
set AAA=BBB
set BBB=CCC
for /F  %%a in ('echo %AAA%') do  echo !%%a!

edit:

to dissect this answer:

setlocal enabledelayedexpansion - this will allow for any environment variable setting during your bat to be used as modified during the process of your for loop.

set AAA=BBB, set BBB=CCC - your data population set statements

for /F %%a in ('echo %AAA%') do echo !%%a! - This tells the processor to loop, albeit only once, and take out the first token that is returned (default delimiter of space and tab apply) from the running of the command in the parens and put it in the var %%a (outside of a batch, a single % will do). If you specify that var as %%a, you need to use %%a in your do block. Likewise, if you specify %%i, use %%i in your do block. Note that to get your environment variable to be resolved within the do block of the for loop, you need surround it in !'s. (you don't need to in the in block, as I originally posted - I have made that change in my edit).

edit:

You were very close with your updated example. Try it like this:

@echo off
setlocal enabledelayedexpansion
set LIST=BBB CCC DDD
set BBB=111
set CCC=222
set DDD=333

for %%i in (%LIST%) do (
    for /F %%a in ('echo %%i') do echo !%%a!
)

The difference between your update and this is that you were trying to echo the environment variable in the in set with in ('echo %%%i%'), but without the !'s for the delayed expansion of set variables. Were you to use in ('echo !%%i!'), you would see your BBB, CCC, and DDD variables resolved, but then the do block of your inner loop wouldnt have anything to resolve - you dont have any 111 environment variables. With that in mind, you could simplify your loop with the following:

@echo off
setlocal enabledelayedexpansion
set LIST=BBB CCC DDD
set BBB=111
set CCC=222
set DDD=333

for %%i in (%LIST%) do (echo !%%i!)
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