How do I put basename into a variable?
Use the command substitution $(...) mechanism:
test=$(basename "$file" .deb)You can also use backquotes, but these are not recommended in modern scripts (mainly because they don't nest as well as the $(...) notation).
test=`basename "$file" .deb`You need to know about backquotes in order to interpret other people's scripts; you shouldn't be using them in your own.
Note the use of quotes around "$file"; this ensures that spaces in filenames are handled correctly.