Lovie Rydstrom

If you are starting from scratch we recommend using biblatex because that package provides localization in several languages, it’s actively developed and makes bibliography management easier and more flexible.

Many tutorials have been written about what \(\mathrm{Bib\TeX}\) is and how to use it. However, based on our experience of providing support to Overleaf’s users, it’s still one of the topics that many newcomers to \(\mathrm{\LaTeX}\) find complicated—especially when things don’t go quite right; for example: citations aren’t appearing; problems with authors’ names; not sorted to a required order; URLs not displayed in the references list, and so forth.

In this article we’ll pull together all the threads relating to citations, references and bibliographies, as well as how Overleaf and related tools can help users manage these.

We’ll start with a quick recap of how \(\mathrm{Bib\TeX}\) and bibliography database (.bib) files work and look at some ways to prepare .bib files. This is, of course, running the risk of repeating some of the material contained in many online tutorials, but future articles will expand our coverage to include bibliography styles and biblatex—the alternative package and bibliography processor.

Let’s first take a quick look “under the hood” to see what a \(\mathrm{\LaTeX}\) reference list is comprised of—please don’t start coding your reference list like this because later in this article we’ll look at other, more convenient, ways to do this.

A reference list really just a thebibliography list of \bibitems:

By default, this thebibliography environment is a numbered list with labels [1], [2] and so forth. If the document class used is article, \begin{thebibliography} automatically inserts a numberless section heading with \refname (default value: References). If the document class is book or report, then a numberless chapter heading with \bibname (default value: Bibliography) is inserted instead. Each \bibitem takes a cite key as its parameter, which you can use with \cite commands, followed by information about the reference entry itself. So if you now write

together with the thebibliography block from before, this is what gets rendered into your PDF when you run a \(\mathrm{\LaTeX}\) processor (i.e. any of latex, pdflatex, xelatex or lualatex) on your source file:

Figure 1: Citing entries from a thebibliography list.

Notice how each \bibitem is automatically numbered, and how \cite then inserts the corresponding numerical label.

\begin{thebibliography} takes a numerical argument: the widest label expected in the list. In this example we only have two entries, so 9 is enough. If you have more than ten entries, though, you may notice that the numerical labels in the list start to get misaligned:

Figure 2: thebibliography with a label that’s too short.

We’ll have to make it \begin{thebibliography}{99} instead, so that the longest label is wide enough to accommodate the longer labels, like this:

Figure 3: thebibliography with a longer label width.

If you compile this example code snippet on a local computer you may notice that after the first time you run pdflatex (or another \(\mathrm{\LaTeX}\) processor), the reference list appears in the PDF as expected, but the \cite commands just show up as question marks [?].

This is because after the first \(\mathrm{\LaTeX}\) run the cite keys from each \bibitem (texbook, lamport94) are written to the .aux file and are not yet available for reading by the \cite commands. Only on the second run of pdflatex are the \cite commands able to look up each cite key from the .aux file and insert the corresponding labels ([1], [2]) into the output.

On Overleaf, though, you don’t have to worry about re-running pdflatex yourself. This is because Overleaf uses the latexmk build tool, which automatically re-runs pdflatex (and some other processors) for the requisite number of times needed to resolve \cite outputs. This also accounts for other cross-referencing commands, such as \ref and \tableofcontents.

Processing \(\mathrm{\LaTeX}\) reference lists or other forms of cross-referencing, such as indexes, requires multiple runs of software—including the \(\mathrm{\TeX}\) engine (e.g., pdflatex) and associated programs such as \(\mathrm{Bib\TeX}\), makeindex, etc. As mentioned above, Overleaf handles all of these mulitple runs automatically, so you don’t have to worry about them. As a consequence, when the preview on Overleaf is refreshing for documents with bibliographies (or other cross-referencing), or for documents with large image files (as discussed separately here), these essential compilation steps may sometimes make the preview refresh appear to take longer than on your own machine. We do, of course, aim to keep it as short as possible! If you feel your document is taking longer to compile than you’d expect, here are some further tips that may help.

There are, of course, some inconveniences with manually preparing the thebibliography list:

This is where \(\mathrm{Bib\TeX}\) and bibliography database files (.bib files) are extremely useful, and this is the recommended approach to manage citations and references in most journals and theses. The biblatex approach, which is slightly different and gaining popularity, also requires a .bib file but we’ll talk about biblatex in a future post.

Instead of formatting cited reference entries in a thebibliography list, we maintain a bibliography database file (let’s name it refs.bib for our example) which contains format-independent information about our references. So our refs.bib file may look like this:

You can find more information about other \(\mathrm{Bib\TeX}\) reference entry types and fields here—there’s a huge table showing which fields are supported for which entry types. We’ll talk more about how to prepare .bib files in a later section.

Now we can use \cite with the cite keys as before, but now we replace thebibliography with a \bibliographystyle{...} to choose the reference style, as well as \bibliography{...} to point \(\mathrm{Bib\TeX}\) at the .bib file where the cited references should be looked-up.

This is processed with the following sequence of commands, assuming our \(\mathrm{\LaTeX}\) document is in a file named main.tex (and that we are using pdflatex):

and we get the following output:

Figure 4: \(\mathrm{Bib\TeX}\) output using the plain bibliography style.

Whoah! What’s going on here and why are all those (repeated) processes required? Well, here’s what happens.

As before, the latexmk build tool takes care of triggering and re-running pdflatex and bibtex as necessary, so you don’t have to worry about this bit.

A few further things to note about using \(\mathrm{Bib\TeX}\) and .bib files:

Figure 5: IEEEtran bibliography style output.

We’ll talk more about different bibliography styles, including author–year citation schemes, in a future article. For now, let’s turn our attention to .bib file contents, and how we can make the task of preparing .bib files a bit easier.

As you may have noticed earlier, a .bib file contains \(\mathrm{Bib\TeX}\) bibliography entries that start with an entry type prefixed with an @. Each entry has a some key–value \(\mathrm{Bib\TeX}\) fields, placed within a pair of braces ({...}). The cite key is the first piece of information given within these braces, and every field in the entry must be separated by a comma:

As a general rule, every bibliography entry should have an author, year and title field, no matter what the type is. There are about a dozen entry types although some bibliography styles may recognise/define more; however, it is likely that you will most frequently use the following entry types:

In a .bib file, commas are only used to separate the last name from the first name of an author—if the last name is written first. Individual author names are separated by and. So these are correct:

or

But none of the following will work correctly—you’ll get weird output, or even error messages from \(\mathrm{Bib\TeX}\)! So take extra care if you are copying author names from a paper or from a web page.

If an author’s last name is made up of multiple words separated by spaces, or if it’s actually an organisation, place an extra pair of braces around the last name so that \(\mathrm{Bib\TeX}\) will recognise the grouped words as the last name:

Alternatively, you can use the Lastname, Firstname format; some users find that clearer and more readable:

Remember: Whether the first or last name appears first in the output (“John Doe” vs “Doe, John”), or whether the first name is automatically abbreviated “J. Doe” or “Doe, J.” vs “John Doe” “J. Doe”), all such details are controlled by the \bibliographystyle.

% is actually not a comment character in .bib files! So, inserting a % in .bib files not only fails to comment out the line, it also causes some \(\mathrm{Bib\TeX}\) errors. To get \(\mathrm{Bib\TeX}\) to ignore a particular field we just need to rename the field to something that \(\mathrm{Bib\TeX}\) doesn’t recognise. For example, if you want to keep a date field around but prefer that it’s ignored (perhaps because you want \(\mathrm{Bib\TeX}\) to use the year field instead) write Tdate = {...} or the more human-readable IGNOREdate = {...}.

To get \(\mathrm{Bib\TeX}\) to ignore an entire entry you can remove the @ before the entry type. A valid reference entry always starts with a @ followed by the entry type; without the @ character \(\mathrm{Bib\TeX}\) skips the lines until it encounters another @.

Because .bib files are plain text you can certainly write them by hand—once you’re familiar with \(\mathrm{Bib\TeX}\)’s required syntax. Just make sure that you save it with a .bib extension, and that your editor doesn’t surreptitiously add a .txt or some other suffix. On Overleaf you can click on the “Files…” link at the top of the file list panel, and then on “Add blank file” to create a fresh .bib file to work on.

Many users prefer to use a dedicated \(\mathrm{Bib\TeX}\) bibliography database editor/manager, such as JabRef or BibDesk to maintain, edit and add entries to their .bib files. Using a GUI can indeed help reduce syntax and spelling errors whilst creating bibliography entries in a \(\mathrm{Bib\TeX}\) file. If you prefer, you can prepare your .bib file on your own machine using JabRef, BibDesk or another utility, and then upload it to your Overleaf.

If you click on the Upload files button above the file list panel, you'll notice some options: Import from Mendeley, and Import from Zotero. If you’re already using one of those reference library management services, Overleaf can now hook into the Web exporter APIs provided by those services to import the .bib file (generated from your library) into your Overleaf project. For more information, see the Overleaf article How to link your Overleaf account to Mendeley and Zotero.

For other reference library services that don’t have a public API, or are not yet directly integrated with Overleaf, such as EndNote or Paperpile, look for an “export to .bib” option in the application or service. Once you have a .bib file, you can then add it to your Overleaf project.

It used to be that you would have to hand-code each line into a \bibitem or an @article{...} entry (or another entry type) in a .bib file. As you can imagine, it’s not exactly a task that many people look forward to. Fortunately, these days some tools are available to help. They typically take a plain text file, e.g.

and attempt to parse the lines, converting it into a structured bibliography as a \(\mathrm{Bib\TeX}\) .bib file. For example, have a look at text2bib or Edifix. Be sure to go through the options of these tools carefully, so that they work well with your existing unstructured bibliography in plain text.

We’ve had a quick look at how \(\mathrm{Bib\TeX}\) processes a .bib bibliography database file to resolve \cite commands and produce a formatted reference list, as well as how to prepare .bib files.

Happy \(\mathrm{Bib\TeX}\)ing!

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