how to copy lines 10 to 15 of a file into another file, in unix?

In complement to the previous answer, you can use one of the following 3 solutions.

sed

Print only the lines in the range and redirect it to the output file

sed -n '10,15p' file1.txt > file2.txt

head/tail combination

Use head and tail to cut the file and to get only the range you need before redirecting the output to a file

head -n 15 file1.txt | tail -n 6 > file2.txt

awk

Print only the lines in the range and redirect it to the output file

awk 'NR>=10 && NR<=15' file1.txt > file2.txt

Open a terminal with a shell then

sed -n '10,15p' file1.txt > file2.txt

Simple & easy.

If you want to append to the end instead of wiping file2.txt, use >> for redirection.

sed -n '10,15p' file1.txt >> file2.txt
                          ^^

AWK is also a powerful command line text manipulator:

awk 'NR>=10 && NR<=15' file1.txt > file2.txt