bash: how to extract thousands of non-sequential lines from a file

Question

I have a file with millions of lines. I also have a list of line numbers, a few hundred thousand which are non-sequential and without a pattern. I would like to extract the lines of the file with the list of line numbers. How can I achieve this efficiently in linux?

Answer 1

If you have the list of linenumbers in a file then with awk you can do something like this -

awk 'NR==FNR { a[$1];next } (FNR in a)' line_number_list.txt bigfile.txt

Test:

$ cat bigfile.txt
line1
line2
line3
line4
line5
line6
line7
line8
line9
line10

$ cat line_number_list.txt
1
2
4
5
8
9

$ awk 'NR==FNR { a[$1];next } (FNR in a)' line_number_list.txt bigfile.txt
line1
line2
line4
line5
line8
line9