grep -A <num> until a string - linux

Question

assuming that we have a file containing the following:
chapter 1 blah blah
blah num blah num
num blah num blah
...
blah num
chapter 2 blah blah
and we want to grep this file so we take the lines
from chapter 1 blah blah to blah num
(the line before the next chapter).
The only things we know are
the stating string chapter 1 blah blah
somewhere after that there is another line starting with chapter
a dummy way to do this is
grep -A <num> -i "chapter 1" <file>
with large enough <num> so the whole chapter will be in it.

Answer 1

sed -ne '/^chapter 1/,/^chapter/{/^chapter/d;p}' file

Answer 2

This is easy to do with awk
awk '/chapter/ {f=0} /chapter 1/ {f=1} f' file
chapter 1 blah blah
blah num blah num
num blah num blah
...
blah num
It will print the line if flag f is true.
The chapter 1 and next chapter to changes the flag.
You can use range with awk but its less flexible if you have other stuff to test.
awk '/chapter 1/,/chapter [^1]/ {if (!/chapter [^1]/) print}' file
chapter 1 blah blah
blah num blah num
num blah num blah
...
blah num

Answer 3

You could do this through grep itself also but you need to enable Perl-regexp parameter P and z.
$ grep -oPz '^chapter 1[\s\S]*?(?=\nchapter)' file
chapter 1 blah blah
blah num blah num
num blah num blah
...
blah num
[\s\S]*? will do a non-greedy match of zero or more characters until the line which has the string chapter at the start is reached.
From man grep
-z, --null-data a data line ends in 0 byte, not newline
-P, --perl-regexp PATTERN is a Perl regular expression
-o, --only-matching show only the part of a line matching PATTERN