Hi. As I understand, you would like a bash shell script that will replace some of the TTGlyph and glyphVariations in [login to view URL] with the versions in [login to view URL] with the same name, and xmlstarlet (xml) is an acceptable command to use.
To make this replacement, I can deliver a bash shell script using an xsl transform. The script would take three parameters: the [login to view URL], the [login to view URL], and (optionally) a string containing a subset of the glyph names separated by commas with no spaces. (If the glyph names are not specified, then all glyphs that are in both [login to view URL] and [login to view URL] will be replaced.) The output will write to "[login to view URL]" if the first parameter is "[login to view URL]".
Have I understood correctly? Does this sound acceptable? Please let me know if you need changes.