There are already several tools to achieve what you want, writing your own is probably needless.

A standard Unix-like solution (works under bash):

$ diff <( sort master ) <( sort completed ) | grep '^<' | cut -d ' ' -
+f2-
[download]

Depending on your needs you may want to use sort -u instead of a simple sort.

Or if you're under some Debian-derivative distro just install the moreutils package and use combine:

$ combine master not completed 
1ao8A
1jkxA
1juvA
1mejA
1meoA
1n0uA
1pjqA
[download]

Hope that helps.