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I have a markdown file that consists of many different headings, I am writing a CI script that modifies one of the section with every push to the repo. My README.md is as follows,

README.md

# Title
....some text...
## Heading 1
...some text...

## Heading 2 
...some text....

## Structure
<pre>
┬  
 ├ first-dir
     ┬  first-sub-dir

 ├ second-dir
     ┬  second-sub-dir-1
     ├  second-sub-dir-2
</pre>

## Heading 3 
...some text....

I wanted to modify the Structure section with the output of the command tree -d -L 2 -n using sed. I tried using

var=$(tree -d -L 2 -n)
sed -i -E "s/## Structure\n<pre>\n(.|\n)*?<\/pre>/## Structure\n<pre>\n ${var} \n<\/pre>/g" README.md

but it was not working properly. I read about command substitution in sed but I wasn't able to understand it properly. Any help regarding this with sed or awk will be beneficial.

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awk '
/^<\/pre>$/             {flag=0}
/^## Structure$/        {print; getline; print
                        system("tree -d -L 2 -n")
                        flag=1}
!flag' <<<$(<file) >file

<<<$(<file) >file - a kind of custom buffer (does not work in all shells). It is possible to replace with a temporary file: file >tmp

A more complex way, I cite just for example:

sed -i '/^## Structure/!b
N;h
:1;N
/<\/pre>$/!b1
s/.*\n//
x;p
s/.*/tree -d -L 2 -n/e
G' file

/^## Structure/!b - All lines that do not match the pattern are ignored by the script. And are displayed unchanged. As soon as a pattern is encountered, the following script starts running.
N - Append the next line of input into the pattern space(working buffer). As a result, we have - ## Structure\n<pre>
h - Copy pattern space to hold space (spare buffer).
:1 - Put a mark for the jump.
N - Append the next line. We get in the working buffer - ## Structure\n<pre>\n┬
/<\/pre>$/!b1 - If the end of the line in the working buffer does not match the pattern, then we return to label :1 and add each cycle the next line until line </pre> is added.
s/.*\n// - Then we delete everything in the working buffer except the last line </pre>.
x - We change the contents of the buffers among themselves so that the line appears in the working buffer - ## Structure\n<pre>. And in the spare buffer there is a line - </pre>
p - Printing the contents of the working buffer ## Structure\n<pre>
s/.*/tree -d -L 2 -n/e - We replace the contents of the buffer with the shell command and execute it.
G - Append the contents </pre> of the spare buffer to the working one - append to the output shell command. The content is printed and the job returns to the beginning of the script. The next line after </pre> is read into the working buffer. And since there are no more lines in the file matching pattern /^## Structure/!b, the remaining lines are also displayed unchanged as first.

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  • Hello, can you explain the sed expression that you have wrote? Jul 7 at 17:12

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