Print a nice ASCII-style section header / separator

sep.sh

#!/usr/bin/env bash
set -o errexit
set -o nounset
set -o pipefail

print_usage() {
  printf 'Usage: %s [-p PREFIX] [-w WIDTH] [WORDS...]\n' "$(basename "$0")" >&2
}

width=80
prefix=''

#               .- causes opt to be set to ':' in case an opt arg is missing
#              / .- help (no arg)
#              |/ .- width w/ arg
#              ||/| .- prefix w/ arg
#              ||||/
while getopts ':hw:p:' opt; do
  case "$opt" in 
    h)
      print_usage
      exit
      ;;
    p)
      prefix="$OPTARG"
      ;;
    w)
      if [[ "$OPTARG" =~ ^[[:digit:]]+$ ]]; then
        width="$OPTARG"
      else
        printf 'Width must be a natural number.\n' >&2
        exit 1
      fi
      ;;
    :)
      printf 'Option -%s requires an argument.\n' "$OPTARG" >&2
      print_usage
      exit 1
      ;;
    \?)
      printf 'Invalid option: -%s\n' "$OPTARG" >&2
      print_usage
      exit 1
  esac    
done
shift $((OPTIND - 1))

heading="$*"
len="${#heading}"
prefix_len="${#prefix}"
horiz=$((width - prefix_len - 2))

if (( prefix_len > width )); then
  printf 'Error: Prefix is too long.\n' >&2
  exit 1
elif (( len > horiz )); then
  printf 'Error: Text is too long.\n' >&2
  exit 1
fi

# Upper line
printf '%s╔' "$prefix"
printf '═%.0s' $(seq $horiz)
printf '╗\n'

# Mid line
right=$(((horiz - len + 1) / 2))
left=$(((horiz - len) / 2 + len))
printf "%s║%0${left}s%0${right}s║\n" "$prefix" "${heading}" ''

# Lower line
printf '%s╚' "$prefix"
printf '═%.0s' $(seq $horiz)
printf '╝\n'