I am using LibreOffice Writer’s built in Bibliography feature. I have defined various entry formats for the different types of citations, however there are some citations for which I don’t have all the information, e.g. I have set it to put a ‘p’ next to the page number yet some Journals are online and there are no page numbers. Hence for that entry there is a p. with no number after it. Is it possible to set it so that the various separators and text in between only comes out when there is data for that entry, in other words, so that an entry which does not have a page number does not come out with p. in it.
The easiest way would be to create a specific style for online Journals. I would even say this is the only correct way since you’ve to provide some specific fields for this kind of publication, such as the url and the date you retrieved the citation from the url. You can read the ISO 690 specs to know more about how to format your bibliography.
To be fair, I should say the built-in bibliography tool of LibreOffice is very limited. You should consider applications like Zotero (and its LibreOffice extension) to have a comprehensive, versatile and powerful tool. I’ve been using it for three years and I’m perfectly happy with it.