Skip to main content

How to Create a Temporary File with a Unique Name in Batch Script

Many scripts need to store data temporarily. Whether you're processing the output of one command with another, staging data before writing a final report, or creating a file to pass to another program, temporary files are essential. However, hardcoding a filename like temp.txt is dangerous, as multiple instances of your script running at the same time could overwrite each other's data. The solution is to generate a unique filename for each run.

This guide will show you how to use the built-in %RANDOM% variable to create unique filenames, explain the best location for these files using the %TEMP% variable, and demonstrate the most robust method for guaranteeing uniqueness by checking for filename collisions.

The Core Method: Using the %RANDOM% Variable​

The simplest way to generate a unique filename is to use the dynamic system variable %RANDOM%. This variable returns a different pseudo-random integer between 0 and 32767 each time it is called. By incorporating this into your filename, you can significantly reduce the chance of two scripts creating a file with the same name.

@ECHO OFF
SET "TEMP_FILENAME=MyAppData_%RANDOM%.tmp"

ECHO Creating a temporary file named: %TEMP_FILENAME%
TYPE NUL > %TEMP_FILENAME%

DIR %TEMP_FILENAME%
DEL %TEMP_FILENAME%

Output:

Creating a temporary file named: MyAppData_14572.tmp
...
10/27/2023  02:15 PM                 0 MyAppData_14572.tmp
...

Creating a temporary file named: MyAppData_3109.tmp
...
10/27/2023  02:16 PM                 0 MyAppData_3109.tmp
...
note
  • Each time you run this, a new filename is generated.
  • This method is good enough for most simple, standalone scripts.

The Best Location: Using the %TEMP% Directory​

Creating temporary files in the current directory is bad practice. It clutters the workspace and can fail if the user doesn't have write permissions. The correct location for temporary data is the user's official temporary folder, which can be accessed via the %TEMP% environment variable.

This is the standard, correct way to define a temporary file path.

@ECHO OFF
SET "TEMP_FILE=%TEMP%\MyAppData_%RANDOM%.tmp"

ECHO Temporary file will be created at: %TEMP_FILE%
TYPE NUL > "%TEMP_FILE%"

ECHO.
DIR "%TEMP_FILE%"
DEL "%TEMP_FILE%"
note

This ensures your file is created in a location like C:\Users\YourUser\AppData\Local\Temp, which is writable and designed for this purpose.

A More Robust Method: Checking for Collisions​

While the chance of %RANDOM% generating the same number twice in a row is low, it's not zero. In critical applications where a filename collision would be catastrophic, the only truly safe method is to generate a name and then check if it already exists. If it does, you loop and generate another name until you find one that's free.

For example, the following script is used to guarantee unique file

@ECHO OFF
SETLOCAL

:GenerateFilename
SET "TEMP_FILE=%TEMP%\MyCriticalApp_%RANDOM%.lock"

REM Check if a file or folder with this name already exists.
IF EXIST "%TEMP_FILE%" (
    ECHO Filename "%TEMP_FILE%" is already in use. Retrying...
    GOTO :GenerateFilename
)

ECHO Unique filename selected: %TEMP_FILE%
TYPE NUL > "%TEMP_FILE%"

ECHO.
ECHO --- Doing work here ---
ECHO.

DEL "%TEMP_FILE%"
ENDLOCAL
note

This loop provides a 100% guarantee that you will not overwrite an existing file.

Cleaning Up: Deleting the Temporary File​

A well-behaved script should always clean up after itself. Leaving temporary files behind clutters the system and consumes disk space. The simplest way to do this is to add a DEL command at the end of your script.

For more complex scripts with multiple exit points, a common pattern is to use a GOTO to a cleanup routine.

@ECHO OFF
SET "TEMP_FILE=%TEMP%\MyAppData_%RANDOM%.tmp"
TYPE NUL > "%TEMP_FILE%"

ECHO Processing data using %TEMP_FILE%...
REM Do some work...

SET "ERROR_CONDITION=1"
IF %ERROR_CONDITION% EQU 1 (
    ECHO An error occurred.
    GOTO :Cleanup
)

ECHO Process finished successfully.

:Cleanup
ECHO Deleting temporary file...
IF EXIST "%TEMP_FILE%" DEL "%TEMP_FILE%"

:End
ECHO Script finished.

This ensures that DEL "%TEMP_FILE%" is always called, regardless of whether the script succeeded or failed.

Common Pitfalls and How to Solve Them​

Problem: Two Scripts Generate the Same Random Number​

As mentioned, this is rare but possible. The only foolproof solution is the check-and-loop method described above. Combining %RANDOM% with another variable, like the process ID (%PID%, available in newer Windows versions) or a timestamp, can also dramatically decrease the probability of a collision.

Problem: A Timestamp Is Needed for Sorting​

The %RANDOM% variable is not sequential. If you need temporary files that can be sorted by creation time, a timestamp is a better choice.

Solution: Use Date and Time​

You must sanitize the %TIME% variable, as it contains colons (:), which are invalid in filenames.

@ECHO OFF
REM Replace colons and periods with nothing.
SET "TIMESTAMP=%TIME::=%
SET "TIMESTAMP=%TIMESTAMP:.=%"
SET "TEMP_FILE=%TEMP%\Report_%DATE:~10,4%%DATE:~4,2%%DATE:~7,2%_%TIMESTAMP%.csv"

ECHO Creating timestamped file: %TEMP_FILE%

This creates a sortable filename like Report_20231027_14301512.csv.

Practical Example: Processing Data via a Temp File​

This script simulates a common task: it gets a list of running services, saves it to a temporary file, and then searches that file for a specific service. This demonstrates all the best practices.

@ECHO OFF
SETLOCAL
SET "SERVICE_TO_FIND=Winmgmt"

REM 1. Define a unique filename in the correct location.
SET "TEMP_FILE=%TEMP%\ServiceList_%RANDOM%.txt"

ECHO --- Service Checker ---
ECHO Storing service list in: %TEMP_FILE%

REM 2. Create the temporary file with data.
tasklist /SVC > "%TEMP_FILE%"

ECHO Searching for service '%SERVICE_TO_FIND%'...
FINDSTR /I "%SERVICE_TO_FIND%" "%TEMP_FILE%"

IF %ERRORLEVEL% EQU 0 (
    ECHO Found the service.
) ELSE (
    ECHO Did not find the service.
)

REM 3. Clean up the temporary file.
ECHO Deleting temporary file.
DEL "%TEMP_FILE%"

ENDLOCAL

Conclusion​

Creating temporary files is a standard scripting practice, but doing so safely requires a disciplined approach.

  • Always use the %TEMP% directory to avoid clutter and permission issues.
  • Use the %RANDOM% variable to generate a unique filename that is sufficient for most scenarios.
  • For critical applications where a file collision is unacceptable, use a loop with IF EXIST to guarantee a unique filename.
  • Always delete your temporary files before your script exits to be a good system citizen.

By following these simple rules, you can write robust and reliable batch scripts that manage temporary data cleanly and safely.

Table of Contents