Compare lines of two files, print lines unique to first file

Try this

$ join file1.txt file2.txt -v 1

$ man join

-a FILENUM
   print unpairable lines coming from file FILENUM, where FILENUM is 1 or 2, corresponding to FILE1 or FILE2
-v FILENUM
   like -a FILENUM, but suppress joined output lines

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

You can use grep:

grep -f file1.txt -vFx file2.txt

Notice the usage of the flags F, --fixed-strings and x, --line-regexp, to force the comparison to be performed considering the entire line.