Adding multiple line at the end of the file

I have 2 files which contains the following lines

file1.txt

line4
line5
line6

file2.txt
line1
line2
line3

When i execute a script , I want my file2.txt will looks like this:

line1
line2
line3
line4
line5
line6

please help.

Thanks

You can run this through command line

cat file1 >> file2

thanks a lot its works!