How to join two text files, removing duplicates, in Windows

Question

file 1

A
B
C

file 2

B
C
D

file1 + file2 =

A
B
C
D

Is it possible to do

Answer 1

The solution below assume that both input files are sorted in ascending order using the same order of IF command's comparison operators and that does not contain empty lines.

@echo off
setlocal EnableDelayedExpansion

set "lastLine=ÿ"
for /L %%i in (1,1,10) do set "lastLine=!lastLine!!lastLine!"

< file1.txt (
   for /F "delims=" %%a in (file2.txt) do (
      set "line2=%%a"
      if not defined line1 set /P line1=
      if "!line1!" lss "!line2!" call :advanceLine1
      if "!line1!" equ "!line2!" (
         echo !line1!
         set "line1="
      ) else (
         echo !line2!
      )
   )
)
if "!line1!" neq "%lastLine%" echo !line1!
goto :EOF


:advanceLine1
echo !line1!
set "line1="
set /P line1=
if not defined line1 set "line1=%lastLine%"
if "!line1!" lss "!line2!" goto advanceLine1
exit /B