Add any number of whitespaces to file - sed

Question

I have a plain text file:
line1_text
line2_text
I need to add a number of whitespaces between the two lines.
Adding 10 whitespaces is easy.
But say I need to add 10000 whitespaces, how would I achieve that using sed?
P.S. This is for experimental purposes

Answer 1

There undoubtedly is a sed method to do this but, since sed does not have any natural understanding of arithmetic, it is not a natural choice for this problem. By contrast, awk understands arithmetic and can readily, for example, print an empty line n times for any integer value of n.
As an example, consider this input file:
$ cat infile
line1_text
line2_text
This code will add as many blank lines as you like before any line that contains the string line2_text:
$ awk -v n=5 '/line2_text/{for (i=1;i<=n;i++)print""} 1' infile
line1_text
line2_text
If you want 10,000 blank lines instead of 5, then replace n=5 with n=10000.
How it works
-v n=5
This defines an awk variable n with value 5.
/line2_text/{for (i=1;i<=n;i++)print""}
Every time that a line matches the regex line2_text, then a for loop is performed with prints an empty line n times.
1
This is awk's shorthand for print-the-line and it causes every line from input to be printed to the output.

Answer 2

This might work for you (GNU sed):
sed -r '/line1_text/{x;s/.*/ /;:a;ta;s/ /&\n/10000;tb;s/^[^\n]*/&&/;ta;:b;s/\n.*//;x;G}' file
This appends the hold space to the first line. The hold space is manipulated to hold the required number of spaces by a looping mechanism based on powers of 2. This may produce more than necessary and the remainder are chopped off using a linefeed as a delimiter.
To change spaces to newlines, use:
sed -r '/line1_text/{x;s/.*/ /;:a;ta;s/ /\n&/10000;tb;s/^[^\n]*/&&/;ta;:b;s/\n.*//;s/ /\n/g;x;G}' file
In essence the same can be achieved using this (however it is very slow for large numbers):
sed -r '/line1_text/{x;:a;/ {20}/bb;s/^/ /;ta;:b;x;G}' file