Find and replace in file and overwrite file doesn't work, it empties the file

When the shell sees > index.html in the command line it opens the file index.html for writing, wiping off all its previous contents.

To fix this you need to pass the -i option to sed to make the changes inline and create a backup of the original file before it does the changes in-place:

sed -i.bak s/STRING_TO_REPLACE/STRING_TO_REPLACE_IT/g index.html

Without the .bak the command will fail on some platforms, such as Mac OSX.

An alternative, useful, pattern is:

sed -e 'script script' index.html > index.html.tmp && mv index.html.tmp index.html

That has much the same effect, without using the -i option, and additionally means that, if the sed script fails for some reason, the input file isn't clobbered. Further, if the edit is successful, there's no backup file left lying around. This sort of idiom can be useful in Makefiles.

Quite a lot of seds have the -i option, but not all of them; the posix sed is one which doesn't. If you're aiming for portability, therefore, it's best avoided.

sed -i 's/STRING_TO_REPLACE/STRING_TO_REPLACE_IT/g' index.html

This does a global in-place substitution on the file index.html. Quoting the string prevents problems with whitespace in the query and replacement.