biber complains about "junk" in what should be seen rather as commented-out lines in a BibTeX file. I was wondering if comments are discouraged in BibTeX at all.

\documentclass {article}

\usepackage {filecontents}

\begin {filecontents} {ref.bib}
 @book {book1,
 author={author 1},
 title={title 1}}

 book {book2,
 author={author 2},
 title={title 2}}

 @book {book3,
 author={author 3},
 title={title 3}}

\usepackage [backend=biber] {biblatex}
\addbibresource {ref.bib}

\begin {document} 
\end {document}

Here is biber's output:

INFO - This is Biber 2.1
INFO - Logfile is 'mwe.blg'
INFO - Reading 'mwe.bcf'
INFO - Using all citekeys in bib section 0
INFO - Processing section 0
INFO - Looking for bibtex format file 'ref.bib' for section 0
INFO - Decoding LaTeX character macros into UTF-8
INFO - Found BibTeX data source 'ref.bib'
WARN - BibTeX subsystem: /tmp/No7MbpTruX/ref.bib_31209.utf8, line 13, warning: 43 characters of junk seen at toplevel
INFO - Overriding locale 'en-US' defaults 'variable = shifted' with 'variable = non-ignorable'
INFO - Overriding locale 'en-US' defaults 'normalization = NFD' with 'normalization = prenormalized'
INFO - Sorting list 'nty' of type 'entry' with scheme 'nty' and locale 'en-US'
INFO - No sort tailoring available for locale 'en-US'
INFO - Writing 'mwe.bbl' with encoding 'ascii'
INFO - Output to mwe.bbl
  • You can ignore the warnings. You can use tex comments (%) to comment an entry if you don't like them. Imho is is rather useful that biber reports things like this, as the "junk" is often due to some faulty brace. Commented Aug 13, 2015 at 15:35
  • Are TeX comments % in BibTeX files a biber's extension or does it belong to the BibTeX format originally?
    – n.r.
    Commented Aug 13, 2015 at 15:41
  • Try it out. You can also look at some of the older bib files in your texmf. xampl.bib e.g. starts with % Copyright (C) 1988, 2010 Oren Patashnik. . Commented Aug 13, 2015 at 16:07
  • 1
    Technically you are right, according to btxdoc, p. 13: "BibTeX allows in the database files any comment that’s not within an entry. If you want to comment out an entry, simply remove the @ character preceding the entry type." Which seems to quite clearly indicate that everything that is not a proper entry starting with @ has to be seen as comment. Remember, though, that Biber does not give you an error, but only a warning. See it as a solicitous service by Biber. ...
    – moewe
    Commented Aug 14, 2015 at 11:41
  • 4
    You could also annotate the entry in the entry itself. Both Biber and BibTeX ignore entry fields for which they have received no instructions, so you can simply add a field like specann = {} and fill it up with whatever you like. I often, though not systematically, add library call numbers to my entries for this reason.
    – jon
    Commented Aug 14, 2015 at 14:54

1 Answer 1


It is in no way discouraged to comment any files you might have written. What you came across is a slight discrepancy in the handling of comments, or if you will a disagreement about what constitutes a comment.

According to btxdoc, §4, item 7, p. 13

BibTeX allows in the database files any comment that's not within an entry. If you want to comment out an entry, simply remove the @ character preceding the entry type.

That means that anything that is not within a @<entrytype>{...} entry block has to be considered a comment by the parser.

Furthermore, in §4, item 14, p. 14, we find

LaTeX's comment character % is not a comment character in the database files.

So technically (and legally, if you will) your block with book2 is indeed a comment.

Still, Biber warns about it mentioning "characters of junk". You can safely ignore this warning just as Biber ignores these characters. Instead you can see these warning as an additional service by Biber to warn you that something might be wrong. Sometimes people genuinely forget a @ or have some real junk in their files.

Contrary to the excerpt quoted above, Biber considers lines starting with % to be real comments and does not issue junk character warnings in these cases. So if you are OK with using % at the beginnings of comments in your .bib file, Biber will not bother you about them again, even though technically the lines outside a @ block should be considered comments without the % as well.

As biber considers % a real comment sign, it handles commented entries differently to BibTeX. An entry starting with


will be ignored by biber (and the rest will junk), while bibtex will still see it.

Since the % is not considered a comment character by BibTeX, but is passed through verbatim to LaTeX (where it will be recognized as a comment with possibly catastrophic results), you should better not use the % inside an entry. (Outside it doesn't matter as it is ignored along with all the rest).

The entry

  title = {Beispielaufsatz},
  journal = {Zeitschrift},
  year = {2001},
  %volume = {7},
  pages = {1--35, 99--291},
  annotation = {lorem},

will compile just fine with Biber (you will not get a volume field in the output of course), but BibTeX will throw an error. So a safer way to exclude a field is to rename it:

 xxxvolume = {7},


    author  = {Uthor, A.}, % her full name is Anne Uthor
    journal = {Journal 1},
    title   = {Title},
    year    = {2015},

will compile fine with Biber, but will throw the error

You're missing a field name---line 6 of file biblio.bib
 :         author  = {Uthor, A.}, 
 :                                % her full name is Anne Uthor
I'm skipping whatever remains of this entry

with BibTeX.

On the other hand

    author = {Author, A.},
    journal = {Journal 1},
    title = {Title %1
    year = {2014},

can work with BibTeX, but with biber you would get this in the bbl:

\field{title}{Title %1}

and this leads to a runaway argument.

If your aim is an annotated bibliography, you can also annotate an entry inside the entry in a field unknown to Biber/BibTeX, this leaves the option of – if need be – allowing an output in the document. biblatex even has an annotation field that is optionally shown by the reading style, all other standard styles ignore it, it is passed to the .bbl file, though. A new field such as myann will be completely ignored by Biber and BibTeX and thus can be filled with your comments as well.

  • 1
    I'd only add that using % inside an entry should be avoided.
    – egreg
    Commented Aug 20, 2015 at 8:01
  • @egreg That is a good point that should have been made more clear, thanks. Feel free to edit the answer if you feel that something should be made more explicit.
    – moewe
    Commented Aug 20, 2015 at 8:13
  • @moewe: I added some remarks regarding % inside entries. Commented Aug 20, 2015 at 8:25

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