Batch File input validation - Make sure user entered an integer

This is the same idea as that of Johannes.. SET /A sets a numeric value. If the input is not a number, it changes it to 0. That's what you can exploit here to do your check.

@ECHO OFF
SET /P UserInput=Please Enter a Number:
IF %UserInput% EQU 0 GOTO E_INVALIDINPUT

SET /A UserInputVal="%UserInput%"*1
IF %UserInputVal% GTR 0 ECHO UserInput "%UserInputVal%" is a number
IF %UserInputVal% EQU 0 ECHO UserInput "%UserInputVal%" is not a number
GOTO EOF

:E_INVALIDINPUT
ECHO Invalid user input
 
:EOF

---

As an alternative, you could always create a little javascript file and call it from your batchfile. With parseInt() you could force the input to be an integer, or you could roll your own function to test the input.

Writing the javascript is just as fast as the batchfile, but it's much more powerful. No IDE or compiler required; notepad will do. Runs on every windows box, just like your batchfiles. So why not make use of it?

You can even mix batchfiles and javascript. Example:

contents of sleep.js:

var SleepSecs=WScript.Arguments.Item(0);
WScript.Sleep(SleepSecs*1000)

contents of sleep.cmd:

cscript /nologo sleep.js %1

You can now call this from a batchfile to make your script sleep for 10 seconds. Something like that is difficult to do with just a plain batchfile.

sleep 10

You're probably not doing this in a DOS batch file. Or at least, support for set /p is unheard of for me in DOS :-)

You could use substrings. In fact I have written a parser for a specific regular language that way once, but it's cumbersome. The easiest way would probably be to assign the contents of %userinput% to another variable, using set /a. If the result comes out as 0 you need to check whether the input itself was 0, otherwise you can conclude it was a non-number:

@echo off
setlocal enableextensions enabledelayedexpansion
set /p UserInput=Enter a number: 
set /a Test=UserInput
if !Test! EQU 0 (
  if !UserInput! EQU 0 (
    echo Number
  ) else (
    echo Not a number
  )
) else (
  echo Number
)

However, this works only for numbers in the range of Int32. If you just care for any number (possibly floating-point as well) then you need to resort to the loop-based approach of dissecting it.

NOTE: Updated to solve the space issues. However, there is still a problem lurking: Entering 123/5 yields "number", since set /a can evaluate this ...

You can also use a quite simple trick:

echo %userinput%|findstr /r /c:"^[0-9][0-9]*$" >nul
if errorlevel 1 (echo not a number) else (echo number)

This uses findstr's regular expression matching capabilities. They aren't very impressive but useful at times.

Thanks all. I was trying to make it harder for myself looking at loops and string manipulation. I used your tips on math evaluation and comparison. Here's what I finally came up with as my concept script:

:Top
@ECHO OFF
ECHO.
ECHO ---------------------------------------
SET /P UserInput=Please Enter a Number: 
ECHO.
ECHO UserInput = %UserInput%
ECHO.
SET /A Evaluated=UserInput
ECHO Math-Evaluated UserInput = %Evaluated%
if %Evaluated% EQU %UserInput% (
    ECHO Integer
    IF %UserInput% GTR 0 ( ECHO Positive )
    IF %UserInput% LSS 0 ( ECHO Negative )
    IF %UserInput% EQU 0 ( ECHO Zero )
    REM - Other Comparison operators for numbers
    REM - LEQ - Less Than or Equal To
    REM - GEQ - Greater Than or Equal To
    REM - NEQ - Not Equal To
) ELSE (
    REM - Non-numbers and decimal numbers get kicked out here
    ECHO Non-Integer
)

GOTO Top

This method catches all numbers and can detect whether it's positive, negative, or zero. Any decimal or string will be detected as non-integers. The only edge case I've found is a string with spaces. For example, the text "Number 1" will cause the script to crash/close when the user input is evaluated as math. But in my situation, this is fine. I don't want my script to go on with invalid input.