How to mass add file extension?

Shell globs should work even with spaces in the names if used properly e.g.

rename -nv -- 's/$/.jpg/' *

or

for file in *; do echo mv -- "$file" "$file.jpg"; done

[NOTE: these are 'no ops' until the n switch or the echo are removed - so you can check the correct replacement before committing]

If you do want to automatically distinguish between jpg and avi files that would also be possible using a more complex loop and the file or mimetype command

If you have a more complex hierarchy, you'll need a slightly more sophisticated approach. You can either use bash's globstar option which lets ** match zero or more directories and subdirectories or you can use find.

1. globstar

   shopt -s globstar
   for f in **/*; do [ -f "$f" ] && mv "$f" "$f".jpg; done
   

   The [ -f "$f" ] & ensures that the mv command is only run on files and not directories.

2. find

   find . -type f -print0 | xargs -0 -I{} mv "{}" "{}".jpg
   

   The -type f ensures we only find files and no directories and the print0 causes find to print its output separated with the null character (\0) instead of newlines. This ensures that we deal correctly with file names containing spaces or newlines or other weirdness.

   xargs takes a list of input and runs the command given on it. The -0 flag tells it to expect null-separated input and the -I{} will make it replace each instance of {} with each of the input records (files in this case) passed.

I would advise you use a program called pyrenamer.

Open it, and in the Renamed file name pattern, enter {1}.jpg.