Find BibTeX entries that are not referenced from the TeX document files.

check-bibtex.sh

#!/bin/sh

if [ $# -ge 1 ]; then
    bibtex_file=$1
else
    bibtex_file="bibliography.bib"
fi

awk -F "[{,]" '/^@/ { print $2 }' $bibtex_file \
| while read reference; do
    grep --quiet --no-messages --max-count=1 --fixed-strings "$reference" *.tex
    if [ $? -eq 1 ]; then
        echo $reference;
    fi;
done