solution_1

To rename the files with the correct file extension, the following bash command can be used:

for f in *.csv; do mv ${f} ${f%.csv}.dat; done

Breaking this down, 'for f in *.csv; do ... done', is a for loop and generates a list of all the files in the directory with the file extension .csv, with the asterisk serving as a wildcard character.
This the loops through every element in the list and for each one executes the  command contained between 'do' and 'done'  in turn. 
When writing the command in between 'do ... done' the elements in the list are referred to by the variable name we specified, in this case 'f'.

The ${f} syntax tells the computer we are referring to the variable held in 'f', with the $ sign indicating that we are referring to a variable, and the curly braces specifying where the variable name starts and ends - this allows us to write commands that will be performed on each element in the list. 
The syntax ${f%.csv}.dat takes the variable we have stored in 'f' and the % character performs an operation on the variable that will expand it out until the characters after the % sign are seen and then append it with the text after the curly braces are closed.
In the first instance, for datafile100.csv, this command 'mv ${f} ${f%.csv}.dat' will expand out to 'mv datafile100 datafile100.dat'. 
This command uses the move command, mv, to rename the file to a .dat file - this will only work without the loop for renaming individual files, the for loop is required to rename more than one file.

More information on for loops can be found with the command 'help for', and more information on mv can be found with the command 'man mv'.