ls
file1 file2 file3

Add the extension “.txt”

for F in $(ls);do mv $F $F.txt;done

Check the result:

ls
file1.rm file2.rm file3.rm

Change the extension from “.txt” to “.mp3”:

for F in $(ls);do mv $F $(echo $F|sed -e 's,\.rm,,').mp3 ;done

Check the result:

ls
file1.mp3 file2.mp3 file3.mp3

for F in $(ls);do mv $F $(echo $F|sed -e 's,\.mp3,,') ;done

Check the result:

ls
file1 file2 file3