One-liner: how to change file extensions according to their type

What if you have PNG or WEBP files, saved with .jpg extension? Here’s a one liner to rename them accordingly. You need mediainfo package:

sudo apt-get install mediainfo

Imagine we have file1.jpg in the current directory:

% ls
file1.jpg
% file file1.jpg
file1.jpg: RIFF (little-endian) data, Web/P image, VP8 encoding, 800x800, Scaling: [none]x[none], YUV color, decoders should clamp

No problem just rename it! But what if we have a hundred?

for i in * 
do 
  oldext=`echo "$i" | sed -e 's/^.*\.\([A-Za-z][A-Za-z]*\)$/\1/'` 
  echo "Old ext=$oldext" 
  newext=`mediainfo "$i" | egrep "^Format[[:space:]]+:" | sort -u | tr '[A-Z]' '[a-z]' | sed -e 's/.* \([a-z][a-z]*\)/\1/' -e 's/jpeg/jpg/'` 
  echo "New ext=$newext" 

  oldname=`echo $i | sed -e 's/\.'$oldext'$/.'$oldext'/'` 
  newname=`echo $i | sed -e 's/\.'$oldext'$/.'$newext'/'` 

  echo old=$oldname 
  new=$newname 
  mv -i $oldname $newname 

done

The result:

Old ext=jpg
New ext=webp
old=file1.jpg new=file1.webp

% ls
file1.webp