latexCites - list bib keys cited in LaTeX files and not defined in bib files.
Input is read from stdin or from named files.
The keys are those found in \cite commands.
Each \cite command can contain several keys, and can span lines.
The keys are presented sorted lexicographically, with each key listed once
even if it appears more than once.
If any .bib files are named on the command line, these are scanned for
definitions of keys.  Then only cited keys that are not defined are listed.
  -help  Show this help.
  -1     Display keys one per line.
  -,     Display keys in a comma-separated list.
  -v     Verbose.

Here is an example use of latexCites with bibfile to collect all the bib
entries needed for a group of .tex files:
bibfile -keep=`latexCites *.tex` -expandCrossrefs -keepStandardFields -showEntry ~/alspaugh.bib >cited.bib