adding text to filename before extension

Using standard POSIX parameter expansion:

for f in *.shp; do printf '%s\n' "${f%.shp}_poly.shp"; done

Sometimes there is a tool called "rename" installed.

rename 's/\.shp$/_poly.shp/' *shp

It might not be portable but it is easy to use.

Use this:

for file in *.shp; do echo $(basename $file .shp)_poly.shp; done