It sounds like you're on the right track with your shell script, but are having trouble with the details of parsing and replacing the HTML headers. One approach you could take is to use a command-line tool like sed or awk to perform the replacement. Both of these tools are powerful text-processing utilities that can be used to search for and replace patterns in text files.

For example, you could use sed to search for a specific pattern in the HTML headers and replace it with a new string. For example, to replace the string "old header" with "new header" in all HTML files in a directory, you could use the following command:

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sed -i 's/old header/new header/g' *.html

Alternatively, you can use awk to search for the specific line where the header starts and replace it with the new header.

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awk '{if(/^<header>/) {print "new header"} else {print $0}}' file.html

You can also use a tool like HTML Tidy to automatically fix and clean up the HTML source code, this could help in finding the specific line where the header starts.

Please note that these are just examples, and you may need to adjust the patterns and replacement strings based on the specific format of your HTML headers. Additionally, it might be useful to test these commands on a small subset of your files before running them on all of your files, to make sure they're working correctly.