bash, Linux: Set difference between two text files

The comm command does that.

Somebody showed me how to do exactly this in sh a couple months ago, and then I couldn't find it for a while... and while looking I stumbled onto your question. Here it is :

set_union () {
   sort $1 $2 | uniq
}

set_difference () {
   sort $1 $2 $2 | uniq -u
}

set_symmetric_difference() {
   sort $1 $2 | uniq -u
}