Windows – How to select a random file in a folder

batch filecommand linewindows

I am trying to select a random file of a specific type like *.mp4
from a folder (and optionally subfolders) using the Windows command line batch script (no PowerShell)

The file full path should be stored in a environment variable for further use

How can I achieve this?

Best Answer

How do I select a random file in a folder?

Use the following batch file:

@echo off
setlocal
setlocal EnableDelayedExpansion
rem store the matching file names in list
dir /b *.txt /s 2> nul > files
rem count the match files
type files | find "" /v /c > tmp & set /p _count=<tmp 
rem get a random number between 0 and count-1
set /a _random=%random%*(%_count%)/32768
rem we can't skip 0 lines
if %_random% equ 0 (
  for /f "tokens=*" %%i in ('type files') do (
    set _randomfile=%%i
    echo !_randomfile!
    goto :eof
    )
) else (
  for /f "tokens=* skip=%_random%" %%i in ('type files') do (
    set _randomfile=%%i
    echo !_randomfile!
    goto :eof
    )
)

The environment variable !_randomfile! will contain the filename of a random file.

Notes:

Remove /s if you don't want to match files in subfolders.
0 =< %RANDOM% < 32767 so it won't work if you have more than 32766 matching files.

An A-Z Index of the Windows CMD command line - An excellent reference for all things Windows cmd line related.
find - Search for a text string in a file & display all the lines where it is found.
for /f - Loop command against the results of another command.
random - The Windows CMD shell contains a dynamic variable called %RANDOM% that can be used to generate random numbers.

Related Solutions

Windows – Opening random file from folder AND subfolders with batch script (Windows 7)

Not only does this code randomly open a file anywhere within the folder hierarchy, it is also more efficient than the original:

@echo off
setlocal

:: Create numbered list of files in a temporary file
set "tempFile=%temp%\%~nx0_fileList_%time::=.%.txt"
dir /b /s /a-d %1 | findstr /n "^" >"%tempFile%"

:: Count the files
for /f %%N in ('type "%tempFile%" ^| find /c /v ""') do set cnt=%%N

call :openRandomFile

:: Delete the temp file
del "%tempFile%"

exit /b

:openRandomFile
set /a "randomNum=(%random% %% cnt) + 1"
for /f "tokens=1* delims=:" %%A in (
  'findstr "^%randomNum%:" "%tempFile%"'
) do start "" "%%B"
exit /b

By default the script will look for files under the current directory, but you can pass a root path as the first argument, and it will start looking there instead.

The code is more efficient when opening just one file, but it really shows improvement if you want to open multiple files, since it only needs to generate the list once. It is also more efficient to let FINDSTR find the selected file instead of looping through the entire list.

I structured the code to make it easy to open multiple random files. Below I randomly select 25, and print out the command to open them. Simply remove the ECHO to actually open the files:

@echo off
setlocal

:: Create numbered list of files in a temporary file
set "tempFile=%temp%\%~nx0_fileList_%time::=.%.txt"
dir /b /s /a-d %1 | findstr /n "^" >"%tempFile%"

:: Count the files
for /f %%N in ('type "%tempFile%" ^| find /c /v ""') do set cnt=%%N

:: Open 25 random files
for /l %%N in (1 1 25) do call :openRandomFile

:: Delete the temp file
del "%tempFile%"

exit /b

:openRandomFile
set /a "randomNum=(%random% %% cnt) + 1"
for /f "tokens=1* delims=:" %%A in (
  'findstr "^%randomNum%:" "%tempFile%"'
) do echo start "" "%%B"
exit /b

Windows – How to Count Files in a Folder and Subfolders from Command Line

set filesCount=0 & for %f in (*) do @(set /a filesCount+=1 > nul)

Windows – How to select a random file in a folder

Best Answer

How do I select a random file in a folder?

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