In Apache how to do an external redirect to the slashless version of a URL with a subfolder .htaccess file

On Apache 2.4 I have an .htaccess (in a subfolder) which rewrites slashless requests inside that folder to appropriate index files:

DirectorySlash Off

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} -d
RewriteCond %{REQUEST_URI} !/$
RewriteCond %{REQUEST_FILENAME}/index.html -f
RewriteRule (.*) $1/index.html [L]

This works for the slashless version exactly as expected. Now I want to redirect the slashed version externally to the slashless version. I tried adding the lines:

RewriteCond %{REQUEST_FILENAME} -d
RewriteCond %{REQUEST_URI} /$
RewriteRule ^(.*)/ $1 [R=302,L]

However this does not work: The redirect is issued, however it does not go to the slashless URL, but to a URL with a system specific part injected.

So, for a sample URL http://example.com/path/to/dir/ the redirected URL looks like this http://example.com/fs9e/username/sub/public/path/to/dir instead of just http://example.com/path/to/dir.

How can I fix this? Many thanks for any pointers!

PS: The real case is a little bit more complicated because I do a subdomain-to-folder rewrite in the root .htacces, but I assume this is not relevant here.

RewriteRule ^(.*)/ $1 [R=302,L]

You are missing the slash prefix (/) on the substitution string (2nd argument) - to make the substitution root-relative. Or rather, /subfolder/ (since this .htaccess file is located in a subfolder). Since this is a relative substitution string (not starting with a slash or scheme+hostname), the directory-prefix^*1 (which I assume is /fs9e/username/sub/public/path/) is added back (by default^*2), resulting in a malformed redirect. (This is correct for internal rewrites, but not external redirects.)

It should be like this:

RewriteRule ^(.*)/$ /subfolder/$1 [R=302,L]

Note you were also missing the end-of-string anchor ($) on the RewriteRule pattern. (This also negates the need for the preceding condition that checks that REQUEST_URI ends in a slash.)

Note also that this "redirect" should go before the earlier "rewrite".