add a line to a file, on the first comment

addline

#!/usr/bin/env bash

newline="$1"
filein="$2"
fileout="$3"
counter=0

while read -r; do
    ((counter++))
    if ! [[ $REPLY =~ ^# ]]; then
        printf "%s\n" "$newline" "$REPLY" >> "$fileout"
        break
    fi
    printf "%s\n" "$REPLY" >> "$fileout"
done < "$filein"

sed -n "$counter,$(wc -l < "$filein")p" "$filein" >> "$fileout"

## or just use awk
#
# line="NEW STUFF HERE"
# awk -v text="$line" '!/^#/ && !p {print text; p=1} 1' file
#