Bash numerando incluso el nombre de archivo
i=0; for f in [folder]/*; do echo mv "$f" "${f%/*}/$((i+=2)).[ext_file]"; done
Testy Tortoise
i=0; for f in [folder]/*; do echo mv "$f" "${f%/*}/$((i+=2)).[ext_file]"; done
i=-1; for f in [folder]/*; do echo mv "$f" "${f%/*}/$((i+=2)).[ext_file]"; done