1. Catenate all the files together using DOS COPY (the "+" switch)
   
2. User DOS SORT to sort the catenated file.
   
3. Read each line in your sorted file. If it is the same as the last line ignore it, otherwise write it to your new file.
   

Quote:

---

Approximately how many unique lines in total will there be?

If it's no more than (say) 10,000 you can load each line into an array. Search the array for the string, if it's not present add it and write to your new file otherwise ignore it.

---

1. Dimension your array up front, say 1000 elements.
   
2. Read each file in turn, and for each line read...
   
3. If the line exists in the array then ignore it.
   
4. If the line does not exist, add it to the array and write it to your output file.
   

break on

$Out       = @scriptdir + "\result.txt"
$FileArray = @scriptdir + "\file1.txt",@scriptdir + "\file2.txt",@scriptdir + "\file3.txt"
Global $OutStrArray[0]

for $i = 0 to ubound($FileArray)
    $_ = execute("Global $$StringArray$i[0]")
next

for $i = 0 to ubound($FileArray)
    $_ = open(1,$FileArray[$i])
    $c = 0
    $l = readline(1)
    while not @error
        $_ = execute("redim preserve $$StringArray$i[$$c]")
        $_ = execute("$$StringArray$i[$$c] = $$l")
        $c = $c + 1
        $l = readline(1)
    loop
    $_ = close(1)
next

for $i = 0 to ubound($FileArray)
    $_ = execute("for $$j = 0 to ubound($$StringArray$i)
                      if ascan($$OutStrArray,$$StringArray$i[$$j]) = -1
                          $$OutStrArray[ubound($$OutStrArray)] = $$StringArray$i[$$j]
                          redim preserve $$OutStrArray[ubound($$OutStrArray)+1]
                      endif
                  next")
next

if ubound($OutStrArray) redim preserve $OutStrArray[ubound($OutStrArray)-1] endif

if exist($out) del $out endif

$_ = open(1,$out,5)

for each $e in $OutStrArray
    $_ = writeline(1,$e + @crlf)
next

$_ = close(1)

exit 1

break on

dim $Out, $FileArray, $file
dim $outArray, $line
$Out = @scriptdir + "\result.txt"
$FileArray = @scriptdir + "\file1.txt",@scriptdir + "\file2.txt",@scriptdir + "\file3.txt"

for each $file in $FileArray
   if open(1,$file) = 0
      $line = readline(1)
      while @error = 0
         if ubound($outArray) < 0
            redim $outArray[0]
            $outArray[0] = $line
         else
            if ascan($outArray,$line) < 0
               redim preserve $outArray[ubound($outArray)+1]
               $outArray[ubound($outArray)] = $line
            endif
         endif
         $line = readline(1)
      loop
      $ = close(1)
   endif
next

if open(1,$Out,5) = 0
   for each $line in $outArray
      $ = writeline(1, $line + @crlf)
   next
   $ = close(1)
endif

break on



$Out       = @scriptdir + "\result.txt"

$FileArray = @scriptdir + "\file1.txt",@scriptdir + "\file2.txt",@scriptdir + "\file3.txt"

Global $OutStrArray[0]

for $i = 0 to ubound($FileArray)

    $_ = open(1,$FileArray[$i])

    $l = readline(1)

    while not @error

        if ascan($OutStrArray,$l) = -1

            $OutStrArray[ubound($OutStrArray)] = $l

            redim preserve $OutStrArray[ubound($OutStrArray)+1]

        endif

        $l = readline(1)

    loop

    $_ = close(1)

next



if ubound($OutStrArray) redim preserve $OutStrArray[ubound($OutStrArray)-1] endif



if exist($out) del $out endif



$_ = open(1,$out,5)



for each $e in $OutStrArray

    $_ = writeline(1,$e + @crlf)

next



$_ = close(1)



exit 1

break on

dim $Out, $FileArray, $file

dim $outArray, $line

$Out = @scriptdir + "\result.txt"

$FileArray = @scriptdir + "\file1.txt",@scriptdir + "\file2.txt",@scriptdir + "\file3.txt"



if open(2,$Out,5) "failed to open output file, quitting" quit endif



for each $file in $FileArray

   if open(1,$file) = 0

      $line = readline(1)

      while @error = 0

            if ascan($outArray,$line) < 0

               redim preserve $outArray[ubound($outArray)+1]

               $outArray[ubound($outArray)] = $line

               $ = writeline(2, $line + @crlf)

            endif

         endif

         $line = readline(1)

      loop

      $ = close(1)

   endif

next

$ = close(2)