Move top 1000 lines from text file to a new file using Unix shell commands

Question

I wish to copy the top 1000 lines in a text file containing more than 50 million entries, to another new file, and also delete these lines from the original file.

Is there some way to do the same with a single shell command in Unix?

Answer 1

head -1000 input > output && sed -i '1,+999d' input

For example:

$ cat input 
1
2
3
4
5
6
$ head -3 input > output && sed -i '1,+2d' input
$ cat input 
4
5
6
$ cat output 
1
2
3

Answer 2

head -1000 file.txt > first100lines.txt
tail --lines=+1001 file.txt > restoffile.txt