BiblioScript is a project mainly written in Shell, it's free.
A command line script to get a BibTex file out of a plain text bilbiography. Separate calls to ParsCit, an XSLT transformation and Bibutils are piped into a single command line script.
(For the installation of dependencies see project's websites)
Once cloned the git repo just change the path to executables in file biblio_script.sh accordingly to the local settings:
# paths to executables
PARSCIT_PATH="/Applications/ParsCit/bin/"
BIBUTILS_PATH="/Applications/bibutils_4.8/"
SAXON_PATH="/Applications/saxonhe9-2-1-2j/saxon9he.jar"
You can try out the script using the provieded input_sample.txt file:
./biblio_script.sh
The degree of accuracy of the resulting .bib file is depending (and it might vary from version to version) on the ParsCit engine used to parse the unstructured (plain text) bibliography.