If you want lines X to Y inclusive (starting the numbering at 1), use
tail -n +$X /path/to/file | head -n $((Y-X+1))
tail will read and discard the first X-1 lines (there's no way around that), then read and print the following lines.
head will read and print the requested number of lines, then exit. When
tail receives a SIGPIPE signal and dies, so it won't have read more than a buffer size's worth (typically a few kilobytes) of lines from the input file.
Alternatively, as gorkypl suggested, use sed:
sed -n -e "$X,$Y p" -e "$Y q" /path/to/file
The sed solution is significantly slower though (at least for GNU utilities and Busybox utilities; sed might be more competitive if you extract a large part of the file on an OS where piping is slow and sed is fast). Here are quick benchmarks under Linux; the data was generated by
seq 100000000 >/tmp/a, the environment is Linux/amd64,
/tmp is tmpfs and the machine is otherwise idle and not swapping.
real user sys command
0.47 0.32 0.12 </tmp/a tail -n +50000001 | head -n 10 #GNU
0.86 0.64 0.21 </tmp/a tail -n +50000001 | head -n 10 #BusyBox
3.57 3.41 0.14 sed -n -e '50000000,50000010 p' -e '50000010q' /tmp/a #GNU
11.91 11.68 0.14 sed -n -e '50000000,50000010 p' -e '50000010q' /tmp/a #BusyBox
1.04 0.60 0.46 </tmp/a tail -n +50000001 | head -n 40000001 >/dev/null #GNU
7.12 6.58 0.55 </tmp/a tail -n +50000001 | head -n 40000001 >/dev/null #BusyBox
9.95 9.54 0.28 sed -n -e '50000000,90000000 p' -e '90000000q' /tmp/a >/dev/null #GNU
23.76 23.13 0.31 sed -n -e '50000000,90000000 p' -e '90000000q' /tmp/a >/dev/null #BusyBox
If you know the byte range you want to work with, you can extract it faster by skipping directly to the start position. But for lines, you have to read from the beginning and count newlines. To extract blocks from x inclusive to y exclusive starting at 0, with a block size of b:
dd bs=$b seek=$x count=$((y-x)) </path/to/file