Problems with For Loop - html

Question

I am using a for loop, and within it, I echo a variable, that updates as the loop goes on. For the format of my code, I have to append everytime it loops. Is there a way for it to overwrite the first time, then on the rest of the for loops, it appends? Here is my code:
#echo off
set file=file
cd %cd%
SETLOCAL ENABLEDELAYEDEXPANSION
for /d %%b in (*) do dir /ad /on /s /b "%%b" >> get_dirs.txt
for /F "tokens=*" %%A in (get_dirs.txt) do (
echo ^<a href="%file%:///%%A"^>%%A^</a^>^<br^> >>index.html
)
pause
So everytime I run this, even when I delete the output file, the html document gets bigger and bigger by duplicating the text inside it so I have the same thing up to over 100 times.
If you want some more backstory of the code head over to my other question that developed some of the code, Command Variable Outputs.

Answer 1

You could use a variable to track whether it is the first iteration or not, and then use an IF/ELSE statement with the same command in each branch, except one uses overwrite, and the other append. But that is needlessly complex.
The simplest thing to do is to delete any existing file before the start of the loop, and then use append in all cases. The append redirection will automatically create the file if it does not yet exist.
#echo off
setlocal
set file=file
del get_dirs.txt
for /d %%b in (*) do dir /ad /on /s /b "%%b" >>get_dirs.txt
del index.html
for /f "delims=" %%A in (get_dirs.txt) do (
echo ^<a href="%file%:///%%A"^>%%A^</a^>^<br^> >>index.html
)
pause
Unless you need "get_dirs.txt" later on, I don't see any need to create that file. you can use FOR /F to directly process the result of the DIR command.
Your code opens the output file, positions the file pointer to the end, and closes the output file once for each iteration. This slows the program down. It is much more efficient (faster) to put parentheses around the entire FOR statement and then use a single redirection. This avoids the need for append mode, so no DEL command is needed.
#echo off
setlocal
set file=file
>index.html (
for /d %%b in (*) do for /f "delims=" %%A in (
'dir /ad /on /s /b "%%b"'
) do echo ^<a href="%file%:///%%A"^>%%A^</a^>^<br^>
)
pause
Here are some additional notes regarding changes I made to your code that are unrelated to your question:
cd %cd% does absolutely nothing useful (change directory to the current directory). So I removed that line.
You enable delayed expansion, but never use it (all your expansion uses %var% instead of !var!). I don't see any need for delayed expansion, and enabled delayed expansion can corrupt FOR variables if they contain !. So I disabled delayed expansion.
I put the SETLOCAL before you define your file variable. No sense in cluttering up your command session environment variable space after your script runs.
Your use of "tokens=" will probably work in real world situations. But technically it is not what you want. It will strip leading spaces from the values. Instead you want to disable delims by using "delims=".