SED or AWK?: Add character to each line after found pattern

Question

I'm using Jenkins multi-line string parameter to append new entries to a yaml file. But then I want to format those entries to match the yaml syntax.

The desired end result is:

var:
  - subvar1
  - subvar2
  - subvar3
  ...

What I have done so far is:

I'm running a shell script which takes that multi-line string parameter (an environment variable) which looks like subvar1\nsubvar2\nsubvar3 and appends it into the yaml file. So, my line code looks like this:

printf "var:\n${HOSTNAMES}" >> ${WORKSPACE}/group_vars/all

The yaml file looks like this after that:

var:
subvar1
subvar2
subvar3

I am seeking assistance from you guys on how to add two spaces and a hyphen then a space before each line that comes after var:

EDIT: If SED or AWK is not a good idea to do this, what is a better way to do it?

Answer 1

If those are literal newlines in that variable, you can do:

HOSTNAMES='subvar1
subvar2
subvar3'
(IFS=$'\n'; printf -- "- %s\n" $HOSTNAMES)   # variable is unquoted

- subvar1
- subvar2
- subvar3

If you have the literal \ and n characters, then this has the same result:

HOSTNAMES='subvar1\nsubvar2\nsubvar3'
printf "$HOSTNAMES\n" | sed 's/^/- /'

Answer 2

With a bash array:

$ HostNames=( host1 "host two" host3 )
$ { printf 'var:\n' ; printf -- '  - %s\n' "${HostNames[@]}"; } > somefile
$ 
$ cat somefile
var:
  - host1
  - host two
  - host3