You may not always want to modify the contents of existing libraries, even although you may be referring to their documents. For example, you may have an established set of documentation in one or more libraries and be developing a new document that will eventually form part of that set. Your new document will probably need to refer to the others while you are working on it, but you may not want to (or have permission to) modify any of the existing documents.
In this case, you can specify a search path, on which HTX will look for other documents to which you may be referring, but it will not attempt to link (i.e. modify) those other documents. The search path is specified as a colon-separated list of directories via the HTX_PATH environment variable, for instance:
setenv HTX_PATH $HOME/mydocs:/docman/newdocs:/docman/olddocs
This search path is used by all HTX commands when they need to find documents for which no explicit location has been given. Note that if two documents with the same name occur at different points on your HTX_PATH, only the first one will be used.
You can set HTX_PATH to search anywhere you like, but if you do not specify it yourself, it defaults to:
$INSTALL/docs:$INSTALL/help:$STARLINK/docs:$STARLINK/help
where INSTALL in turn defaults to $HOME/star and STARLINK defaults to /star. The values of these environment variables are evaluated when the HTX software is installed (not when you later make use of it). By default, therefore, HTX will search for the standard set of Starlink documents and on-line hypertext help, plus any others you may have installed locally under your own user name (in the location identified by the INSTALL environment variable).