Simple "Variable" Bash File

The first argument to the script will be in $1. You can use a bash string replacement to pull the extension; this removes everything from the last occurrence of . forward, and stores the result in $filename:

filename="${1%.*}"

Then you can use $filename in your script wherever you want:

nasm -f elf "$filename.asm"
ld -s -o "$filename" "$filename".o io.o