xmllint:tldr:a7cb7

The command xmllint --xpath "//${foo}" ${source_file-xml} uses the xmllint utility and its --xpath option to extract specific data from an XML file.

Let's break down the command:

1. xmllint is a command-line utility used for parsing and manipulating XML documents.
2. --xpath is an option provided by xmllint that allows us to specify an XPath expression to extract specific data from an XML document.
3. "//${foo}" is the XPath expression enclosed in double quotes. This expression will be expanded and replaced by the value of the variable ${foo}.
4. ${foo} is a variable that holds a value. Its actual value depends on what is set for foo in the current environment.
5. ${source_file-xml} is another variable that holds the name of the source XML file to be parsed. If the source_file variable is not set in the environment, the value xml will be used as a default.

In summary, this command instructs xmllint to parse the XML file specified by source_file, or xml if the variable is not set, and extracts the data based on the XPath expression //${foo}. The actual data to be extracted depends on the value of the variable ${foo}.