There is a handy command-line tool to run XPath expressions and test them in the terminal on macOS.
The command is: xpath.
Assuming we have an XML file test.xml:
<books> <book id="123"> <title>Book A</title> </book> <book id="456"> <title>Book B</title> </book> </books>
To test an XPath statement on this file, we can use:
$ xpath test.xml "//book[@id=456]"
Found 1 nodes: -- NODE -- <book id="456"> <title>Book B</title> </book>
Note that the file argument is first and the XPath expression second, in double-quotes.