How to redirect a command that contains a variable to be outputed to a variable on the Windows commandline

There are generally two ways to store the output of a command into a variable. I'm going to assume there is only one line of output to be captured. I'm also assuming the output is on stdout, and not stderr. The command could be a call to a batch file, a call to a subroutine within a batch file, an internal command, a standard external command (executable), or a 3rd party executable - the process for capturing the output into a variable is the same.

1) Redirect the output to a temporary file, and then read the value into a variable using SET /P

call dateMath.bat %Date_Pattern% - 2 >output.tmp
<output.tmp set /p "result="
del output.tmp

Typically the temporary file is created in the %TEMP% folder, but I kept everything in the current directory just for simplicity.

People generally like to avoid temporary files if possible, although sometimes that is the best option.

2) Use FOR /F to capture the output directly, without using a temporary file. FOR /F is a complicated command with many options. It is designed to parse input into delimited tokens. The default delimiter is a space; but you want the entire string to be captured in one variable. So you need to set the delimiter to nothing. FOR /F can read file contents, a string, or command output. You want command output, so the IN() clause should be enclosed in single quotes. The @ symbol is used to suppress echoing of the SET command line.

for /f "delims=" %A in ('dateMath.bat %Date_Pattern% - 2') do @set "result=%A"

If you want to use the above command in a batch script, then the FOR variable percents need to be doubled. Your script probably has echo off, so @ is not needed.

for /f "delims=" %%A in ('dateMath.bat %Date_Pattern% - 2') do set "result=%%A"

There is an entirely different approach you could take: modify your batch script to optionally store the result directly in the variable. You should read the following excellent tutorial from DosTips.com DOS Batch - Function Tutorial The site constantly refers to DOS, but in reality it is a site that specializes in all things dealing with Windows batch scripting.