How should I extract and combine parts of files based on a common text string effectively in bash? - bash

Question

Suppose I have two similar files:
a.yaml
data:
- name: a1
args: ["cmd", "something"]
config:
- name: some
val: thing
- name: a2
args: ["cmd2", "else"]
[...other array values...]
tags: ["something-in-a"]
values: ["else-in-a"]
substitutions:
key1: a-value
key2: a-value
key3: a-value
b.yaml
data:
- name: b1
args: ["cmd", "something"]
config:
- name: some
val: thing
- name: b2
args: ["cmd2", "else"]
[...other array values...]
tags: ["something-in-b"]
values: ["else-in-b"]
substitutions:
key1: b-value
key2: b-value
key3: b-value
My goal is to combine parts of a and b file such that I have a new file which consists of file content before substitutions: from b.yaml and content including and after substitutions: from a.yaml
So in this case, my desired output would be like this:
c.yaml
data:
- name: b1
args: ["cmd", "something"]
config:
- name: some
val: thing
- name: b2
args: ["cmd2", "else"]
[...other array values...]
tags: ["something-in-b"]
values: ["else-in-b"]
substitutions:
key1: a-value
key2: a-value
key3: a-value
The parts before and after substitutions: in both file contents might have different lengths.
Currently, my method is like this:
head -q -n `awk '/substitution/ {print FNR-1}' b.yaml` b.yaml >! c.yaml ; \
tail -q -n `awk '/substitution/ {ROWNUM=FNR} END {print NR-ROWNUM+1}' a.yaml` a.yaml >> c.yaml; \
rm a.yaml b.yaml; mv c.yaml a.yaml; # optional newfile renaming to original
But I wonder if there's an alternative or better method for combining parts of different files based on a common text string in bash?

Answer 1

Use awk, you just need to flag the flow based on the string:
awk '$1 == "substitutions:"{skip = FNR==NR ? 1:0}!skip' b.yaml a.yaml
Explaination:
FNR==NR: if true, process lines in the first file b.yaml, otherwise the 2nd file a.yaml
!skip: if TRUE, print the line, otherwise skip the line.

Answer 2

{
head -B9999 'substitutions:' a.yaml | head -n -1
head -A9999 'substitutions:' b.yaml
} > c.yaml
A oneliner:
{ head -B9999 'substitutions:' a.yaml | head -n -1; head -A9999 'substitutions:' b.yaml; } > c.yaml
The -A9999 and -B9999 are a bit dirty, here's a solution with sed's:
{
sed '/substitutions:/,$d' a.yaml
echo substitutions:
sed '1,/substitutions:/d' b.yaml
} > c.yaml