How to set variables in a DOS for loop

I unexpectedly ran into this issue. Thinking about how most modern programming languages work in relation to setting of variables, I forgot how older languages work.

In today’s languages, you can simply do something like:

   int myVar = 1;
   for(int i; i<4; i++)
      myVar = myVar + i;

However, that requires the variable of “myVar” to be evaluated/expanded during the assignment. This is something we take for granted. If “myVar” is not evaluated/expanded during the execution, then the result will not take the current value of “myVar” into consideration during the assignment/execution.

In DOS, this requires that we delay the expansion until after the execution (instead of before the execution which is default). Take these commands as an example:

   set FILES=
   for /f %%a IN (‘dir /b *.txt’) do set FILES=%FILES% %%a
   echo %FILES%

In a directory with 3 files that match the *.txt pattern, you’d expect the %FILES% variable would be set to “file1.txt file2.txt file3.txt”. However, the result will be ” file3.txt”.

We need to delay the execution/expansion of “%FILES%” until we assign the FILES variable, so our syntax changes to:

   SETLOCAL ENABLEDELAYEDEXPANSION
   set FILES=
   for /f %%a IN (‘dir /b *.txt’) do set FILES=!FILES! %%a
   echo %FILES%

You’ll noticed a new declaration in our DOS batch file and that we’ve changed the variable to use exclamation marks instead of the standard percent signs. This will result in our expected output of “file1.txt file2.txt file3.txt”.

Resources:

• http://www.robvanderwoude.com/variableexpansion.html

 

My Related Posts: Technology, Programming, DOS