sed replace line with multiline-variable

Question

I'm trying to replace a single line in a file with a multiline string stored in a variable.

I am able to get the correct result when printing to screen, but not if I want to do an in-place replacement.

The file has form:

*some code*
*some code*
string_to_replace
*some code*

I want the resulting file to be:

*some code*
*some code*
line number 1
line number 2
line number 3
*some code*

The code I tried was:

new_string="line number 1\nline number 2\nline number 3"

# Correct output on screen
sed -e s/"string_to_replace"/"${new_string}"/g $file

# Single-line output in file: "line number 1line number 2line number 3"
sed -i s/"string_to_replace"/"${new_string}"/g $file

When trying to combine -i and -e options, the result is the same as when only using -i.

I'm using GNU sed version 4.1.5 on CentOS (connected to it through ssh from Mac).

Answer 1

In sed you can double quote the command string and let the shell do the expansion for you, like this:

new_string="line number 1\nline number 2\nline number 3"
sed -i "s/string_to_replace/$new_string/" file