Question

I have two text files that contain a unique sorted list of words:

File 1:

a
b
c
d

File 2:

b
c

I need a new file that contains only the extraneous lines in File 1, so the result will be

a
d

Answer 1

This is what comm is for:

comm — select or reject lines common to two files

You want

comm -23 "File 1" "File 2"

which will suppress output of lines only in file 2 and lines in both files, leaving only lines in file 1.
More answers here on Greg Wooledge’s wiki