Assume that, some files are stored in my user path with .txt suffix. If you want to rename them all to follow this pattern. For example filenamelinux.txt  to   filenamelinux.js

$ cd /home/thelinuxfaq/

$ ls


aSSb6EW7dfNUO52hQkae.txt  hE2uvdMIAJvEWpZaWxsJ.txt  nibzU9psJ4RICO88kofp.txt  TyhMgWml4au4zgDSivRc.txt  ZUweKr0Z12PSmVTFUdh3.txt

atoKLauqBT3qLNuehQXF.txt   filenamelinux.txt


Execute the below command to chnage file types,
 
for j in *.txt; do mv -v -- "$j" "${j%.}.dat"; done