Shell script with separate function to read and write out while appending to given multi-line section

replace_rest_extension.sh

#INPUT_FILE=/Users/abrahma/tmp/missing.properties
#OUTPUT_FILE=/Users/abrahma/tmp/output1.properties
#EXISTING_FILE=/Users/abrahma/tmp/existing.properties
EXISTING_FILE=/Users/abrahma/tmp/spaces.properties
OUTPUT_FILE=/Users/abrahma/tmp/output.properties
KEY="^\s*rest.extension.classes"
NEWVAL="io.confluent.connect.replicator.monitoring.ReplicatorMonitoringExtension"


replace_rest_extensions() {
    #echo "Replacing ..."

    # Modes
    # 0 -> looking for match
    # 1 -> within match
    # 2 -> done with match
    mode=0


    while IFS= read -r line; do
        #echo "Debug: read *$line*, mode is *$mode*"
        if [ $mode -eq 0 ]; then
            # Check for match
            if ! [[ -z $(grep $KEY  <<< $line) ]]
            then
                mode=1
            else
                # Passthrough line
                echo "$line"
            fi
        fi

        # If we were looking at single- or multi-line match earlier, or just got triggered ...
        if [ $mode -eq 1 ]; then
            # If we ever encounter our value, then we don't need to do anything here.
            has_value_already=$(grep $NEWVAL <<< $line)
            if ! [[ -z $has_value_already ]]; then
                echo "$line"
                mode=2
            else
                # If line terminates with backslash, passthrough
                # Otherwise, add on our value to the end (and we're done with this section)

                has_backslash=$(grep '\\$' <<< $line)

                if ! [[ -z $has_backslash ]]
                then
                    #echo "Debug: backslash, continue..."
                    echo "$line"
                else
                    #echo "Debug: No backslash, done with match..."
                    echo "$line,$NEWVAL"
                    mode=2
                fi
            fi
        else
            # Done with matching section: Passthrough output, until end of file
            echo $line
        fi
    done < $EXISTING_FILE > $OUTPUT_FILE
}

# Get existing line
existing_line=$(grep "$KEY" $EXISTING_FILE)
echo "Existing: [$existing_line]"

if [[ -z $existing_line ]]; then
    echo "Nothing found!"
    modifiedline="$KEY=$NEWVAL"

    cat < $INPUT_FILE > $OUTPUT_FILE
    echo $modifiedline >> $OUTPUT_FILE
else
    replace_rest_extensions
fi 

Updated to skip existing multi-line entries with the desired value.