How to debug .htaccess RewriteRule not working

Enter some junk value into your .htaccess e.g. foo bar, sakjnaskljdnas any keyword not recognized by htaccess and visit your URL. If it is working, you should get a

500 Internal Server Error

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request....

I suggest you to put it soon after RewriteEngine on.

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Since you are on your machine. I presume you have access to apache .conf file.

open the .conf file, and look for a line similar to:

LoadModule rewrite_module modules/mod_rewrite.so

If it is commented(#), uncomment and restart apache.

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To log rewrite

RewriteEngine On
RewriteLog "/path/to/rewrite.log"
RewriteLogLevel 9

Put the above 3 lines in your virtualhost. restart the httpd.

RewriteLogLevel 9 Using a high value for Level will slow down your Apache server dramatically! Use the rewriting logfile at a Level greater than 2 only for debugging! Level 9 will log almost every rewritelog detail.

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UPDATE

Things have changed in Apache 2.4:

FROM Upgrading to 2.4 from 2.2

The RewriteLog and RewriteLogLevel directives have been removed. This functionality is now provided by configuring the appropriate level of logging for the mod_rewrite module using the LogLevel directive. See also the mod_rewrite logging section.

For more on LogLevel, refer LogLevel Directive

you can accomplish

RewriteLog "/path/to/rewrite.log"

in this manner now

LogLevel debug rewrite_module:debug

Perhaps a more logical method would be to create a file (e.g. test.html), add some content and then try to set it as the index page:

DirectoryIndex test.html

For the most part, the .htaccess rule will override the Apache configuration where working at the directory/file level

The 'Enter some junk value' answer didn't do the trick for me, my site was continuing to load despite the entered junk.

Instead I added the following line to the top of the .htaccess file:

deny from all

This will quickly let you know if .htaccess is being picked up or not. If the .htaccess is being used, the files in that folder won't load at all.

Generally any change in the .htaccess should have visible effects. If no effect, check your configuration apache files, something like:

<Directory ..>
    ...
    AllowOverride None
    ...
</Directory>

Should be changed to

AllowOverride All

And you'll be able to change directives in .htaccess files.