[notaddicted]

As long as we're bashing, it could easily be written loopless like so:

    basename -s .jpg -a *jpg | 
      xargs -I{} convert "{}.jpg" \
       -resize "900x" -unsharp 2x0.5+0.5+0 -quality 98 "{}_s.jpg"

EDIT: weird line breaking is to prevent sidescroll box

[rcfox]

You could also easily parallelize this with xargs' -P option.