Assume your excel files are in some directory, move into that directory and execute the command below which uses LibreOffice in bash/Terminal to convert all your excel files into CSV files:
for i in *.xlsx; do libreoffice --headless --convert-to csv "$i" ; done
Or install
csvkit:
sudo apt-get install csvkit
and use it to convert your excel files into CSV files as
in2csv data.xlsx > data.csv
See more
excel_to_csv