How to insert a line using sed before a pattern and after a line number?

Question

How to insert a line into a file using sed before a pattern and after a line number? And how to use the same in shell script?

This inserts a line before

Answer 1

here is an example of how to insert a line before a line in a file:

example file test.txt :

hello line 1
hello line 2
hello line 3

script:

sed -n 'H;${x;s/^\n//;s/hello line 2/hello new line\n&/;p;}' test.txt > test.txt.2

output file test.txt.2

hello line 1
hello new line
hello line 2
hello line 3

NB! notice that the sed has as beginning a substitution of a newline to no space - this is necessary otherwise resulting file will have one empty line in the beginning

The script finds the line containing "hello line 2", it then inserts a new line above -- "hello new line"

explanation of sed commands:

sed -n:
suppress automatic printing of pattern space

H;${x;s/test/next/;p}

//  search for a 
${}  do this 'block' of code
H    put the pattern match in the hold space
s/   substitute test for next everywhere in the space
x    swap the hold with the pattern space
p    Print the current pattern hold space.