Batch file include external file for variables

If the external configuration file is also valid batch file, you can just use:

call externalconfig.bat

inside your script. Try creating following a.bat:

@echo off
call b.bat
echo %MYVAR%

and b.bat:

set MYVAR=test

Running a.bat should generate output:

test

Note: I'm assuming Windows batch files as most people seem to be unaware that there are significant differences and just blindly call everything with grey text on black background DOS. Nevertheless, the first variant should work in DOS as well.

Executable configuration

The easiest way to do this is to just put the variables in a batch file themselves, each with its own set statement:

set var1=value1
set var2=value2
...

and in your main batch:

call config.cmd

Of course, that also enables variables to be created conditionally or depending on aspects of the system, so it's pretty versatile. However, arbitrary code can run there and if there is a syntax error, then your main batch will exit too. In the UNIX world this seems to be fairly common, especially for shells. And if you think about it, autoexec.bat is nothing else.

Key/value pairs

Another way would be some kind of var=value pairs in the configuration file:

var1=value1
var2=value2
...

You can then use the following snippet to load them:

for /f "delims=" %%x in (config.txt) do (set "%%x")

This utilizes a similar trick as before, namely just using set on each line. The quotes are there to escape things like <, >, &, |. However, they will themselves break when quotes are used in the input. Also you always need to be careful when further processing data in variables stored with such characters.

Generally, automatically escaping arbitrary input to cause no headaches or problems in batch files seems pretty impossible to me. At least I didn't find a way to do so yet. Of course, with the first solution you're pushing that responsibility to the one writing the config file.