how to replace a block of text with new block of text?

As the question title specifies , i have to replace a block to text in a file with a new block of text

I have searched all over for this thing but every solution i ever found was just too specific to the question. Isn't it possible to create a function which is flexible/reusable ?

To be very specific i need something which has options like

1) File ( where changes are to be done )
2) Exiting block of text
3) New block of text
( 2nd & 3 option could be either as manually pasted text or cat $somefile)

whereby i could change these 3 and use the script for all cases of text block replacement , i am sure it will help many other people too

As for an example , currently i need to replace the below block of text with one at bottom and say the file is $HOME/block.txt . Although i need the solution which is easily reusable/flexible as mentioned above

- name: Set default_volumes variable
  set_fact:
    default_volumes:
      - "/opt/lidarr:/config"
      - "/opt/scripts:/scripts"
      - "/mnt:/mnt"
      - "/mnt/unionfs/Media/Music:/music"

- name: Set default_volumes variable
  set_fact:
    default_volumes:
      - "/opt/lidarr:/config"
      - "/opt/scripts:/scripts"
      - "/mnt:/mnt"
      - "/mnt/unionfs/Media/Music:/music"
      - "/mnt/unionfs/downloads/lidarr:/downloads-amd"

PS / while replacement i need the spacing and indentation to be preserved.

Solution

Using GNU awk for multi-char RS and ARGIND, this will work for any chars in your old or new text including regexp metachars, delimiters, quotes, and backreferences as it's just doing literal string search and replace:

awk -v RS='^$' -v ORS= '
    ARGIND==1 { old=$0; next }
    ARGIND==2 { new=$0; next }
    s=index($0,old) {
        $0 = substr($0,1,s-1) new substr($0,s+length(old))
    }
1' old new file

or you can do the same using any awk in any shell on every Unix box with:

awk -v ORS= '
    { rec = (FNR>1 ? rec RS : "") $0 }
    FILENAME==ARGV[1] { old=rec; next }
    FILENAME==ARGV[2] { new=rec; next }
    END {
        $0 = rec
        if ( s=index($0,old) ) {
            $0 = substr($0,1,s-1) new substr($0,s+length(old))
        }
        print
    }
' old new file

For example:

$ head old new file
==> old <==
- name: Set default_volumes variable
  set_fact:
    default_volumes:
      - "/opt/lidarr:/config"
      - "/opt/scripts:/scripts"
      - "/mnt:/mnt"
      - "/mnt/unionfs/Media/Music:/music"

==> new <==
- name: Set default_volumes variable
  set_fact:
    default_volumes:
      - "/opt/lidarr:/config"
      - "/opt/scripts:/scripts"
      - "/mnt:/mnt"
      - "/mnt/unionfs/Media/Music:/music"
      - "/mnt/unionfs/downloads/lidarr:/downloads-amd"

==> file <==
foo
- name: Set default_volumes variable
  set_fact:
    default_volumes:
      - "/opt/lidarr:/config"
      - "/opt/scripts:/scripts"
      - "/mnt:/mnt"
      - "/mnt/unionfs/Media/Music:/music"
bar

$ awk -v RS='^$' -v ORS= 'ARGIND==1{old=$0; next} ARGIND==2{new=$0; next} s=index($0,old){ $0=substr($0,1,s-1) new substr($0,s+length(old))} 1' old new file
foo
- name: Set default_volumes variable
  set_fact:
    default_volumes:
      - "/opt/lidarr:/config"
      - "/opt/scripts:/scripts"
      - "/mnt:/mnt"
      - "/mnt/unionfs/Media/Music:/music"
      - "/mnt/unionfs/downloads/lidarr:/downloads-amd"
bar