This command renames all files in your current working directory from uppercase to lowercase. Make sure to use -i with mv command so you do not accidentally overwrite some of your files. On a Linux command line File and file are to separate files. If you mv File to file you overwrite your current file.
for i in $( ls | grep [A-Z] ); do mv -i $i `echo $i | tr 'A-Z' 'a-z'`; done