How to find a word in all files of type .html (or .txt)

[kwatts59]

How do I find a word in a file?
I want to search only files of a certain extension such as .html or .txt.
I want to search recursively since the files can be anywhere.

I tried the following command but it gave an error
$ grep -r "cell differentiation" *.html
The error was "*.html: No such file or directory".

If I leave off the *.html it scans every fracking file on the system.

[nikwasi]

Use --include option for grep.

grep -r "cell differentiation" . --include "*.html"

[alf55]

How do I find a word in a file?
I want to search only files of a certain extension such as .html or .txt.
I want to search recursively since the files can be anywhere.

I tried the following command but it gave an error
$ grep -r "cell differentiation" *.html
The error was "*.html: No such file or directory".

If I leave off the *.html it scans every fracking file on the system.

find . -type f -name '*.html' -print0 | xargs -0 grep "cell differentiation"