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How to Generate a Random Number in Batch Script

Generating random numbers is a key feature for scripts that need unpredictability. You might need a random number to select a file from a list, create a unique temporary ID, or to add a random delay to a network task. While batch scripting doesn't have a formal RAND() function, it provides a special, built-in dynamic variable, %RANDOM%, that makes this task simple and easy.

This guide will teach you how to use the %RANDOM% variable to get a random number, and more importantly, how to use the modulo operator to control the output and generate a random number within a specific, custom range.

The Core Variable: %RANDOM%​

The %RANDOM% variable is a special, dynamic variable provided by the cmd.exe interpreter. It is not a stored environment variable (it won't show up in the SET command's output).

Each time you access %RANDOM%, it expands to a different pseudo-random decimal integer between 0 and 32767 (inclusive).

Basic Example: Getting a Random Number​

This script simply demonstrates how %RANDOM% produces a different value each time it is called.

@ECHO OFF
ECHO Generating three random numbers...
ECHO.
ECHO Random Number 1: %RANDOM%
ECHO Random Number 2: %RANDOM%
ECHO Random Number 3: %RANDOM%

Output (will be different every time)

Generating three random numbers...

Random Number 1: 14572
Random Number 2: 3109
Random Number 3: 29881

The Key to Control: Generating a Random Number in a Range​

Getting a number between 0 and 32767 is rarely what you need. More often, you'll need a number within a specific range, like 1 to 100, or 0 to 5. This is achieved using the modulo operator (%%) inside a SET /A arithmetic expression.

The modulo operator gives you the remainder of a division. For example, 13 %% 5 is 3 (because 13 divided by 5 is 2 with a remainder of 3).

The Formula to Get a Number from 0 to N-1: SET /A "RandomNumber = %RANDOM% %% N" where N is the number of possible values you want.

The Formula to Get a Number from Min to Max: SET /A "RandomNumber = (%RANDOM% %% (Max - Min + 1)) + Min"

Example: Get a Random Number between 1 and 100

  • Range: We need 100 possible values (100 - 1 + 1).
  • Formula: (%RANDOM% %% 100) + 1

Example:

@ECHO OFF
SETLOCAL

ECHO --- Generating a random number between 1 and 100 ---
SET /A "RandomValue = (%RANDOM% %% 100) + 1"
ECHO The random number is: %RandomValue%

ECHO.
ECHO --- Generating a random number between 50 and 60 ---
SET /A "Min=50"
SET /A "Max=60"
SET /A "RandomValue = (%RANDOM% %% (Max - Min + 1)) + Min"
ECHO The random number is: %RandomValue%

ENDLOCAL

Let's see how the modulo formula works by breaking down (%RANDOM% %% 100) + 1:

  1. %RANDOM%: Generates a large random number (e.g., 29881).
  2. %% 100: The modulo operator. The remainder of 29881 / 100 is 81. The result of the modulo operation %RANDOM% %% 100 will always be a number between 0 and 99.
  3. + 1: We add 1 to the result to shift the range from 0-99 to the desired 1-100.

This simple formula is the standard and most reliable way to constrain the output of %RANDOM%.

Common Pitfalls and How to Solve Them​

  • Using Single % for Modulo: The modulo operator is %. However, inside a batch script, a single percent sign is interpreted as the start of a variable name. To use a literal percent sign in a batch file, you must escape it by doubling it (%%).

    • SET /A "Result = %RANDOM% % 10" -> WRONG
    • SET /A "Result = %RANDOM% %% 10" -> CORRECT

  • Modulo Bias (Advanced Topic): The %RANDOM% variable is not perfectly random, and using the modulo operator can introduce a slight bias. For example, in a 0-32767 range, some numbers in your smaller range might appear slightly more often than others. For scripting tasks like picking a random server or adding a delay, this bias is completely insignificant and can be ignored.

Practical Example: A Script with a Random Delay​

This script simulates a task that needs to pause for a random amount of time between 3 and 10 seconds. This is a common technique to make automated network requests look less robotic.

@ECHO OFF
SETLOCAL
SET "MinDelay=3"
SET "MaxDelay=10"

ECHO --- Automated Task Runner ---
ECHO.

FOR /L %%i IN (1,1,5) DO (
    ECHO Starting task %%i...
    
    REM --- Calculate the random delay ---
    SET /A "Delay = (%RANDOM% %% (MaxDelay - MinDelay + 1)) + MinDelay"
    
    ECHO   -> Task will run for a bit, then pause for %Delay% seconds.
    REM (Simulate some work)
    PING -n 2 127.0.0.1 > NUL
    
    TIMEOUT /T %Delay% /NOBREAK > NUL
    
    ECHO   -> Task %%i complete.
    ECHO.
)s

ECHO --- All tasks finished ---
ENDLOCAL

Conclusion​

The built-in %RANDOM% variable is the simple and effective tool for introducing unpredictability into your batch scripts.

Key takeaways for its successful use:

  • %RANDOM% generates a number between 0 and 32767.
  • To get a number in a specific range, you must use the modulo operator (%%) inside a SET /A command.
  • The standard formula for a range from Min to Max is: SET /A "Var = (%RANDOM% %% (Max - Min + 1)) + Min"
  • Always remember to double the percent sign (%%) for the modulo operator inside a batch file.

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