divider

#!/bin/bash

function divider {
  local w b="$1" m="${2:--}" e="$3"
  w=$(tput cols 2>/dev/null)
  if [[ ! "$w" =~ ^[0-9]+$ ]]; then w=80; fi # just in case
  if [[ "${#b}" -ge "$w" || "${#e}" -ge "$w" || "$((${#b}+${#e}))" -ge "$w" ]]; then
    printf '%s%s\n' "$b" "$e"
    return
  fi
  until [[ "${#m}" -ge "$w" ]]; do m+="$m"; done
  printf '%s%s%s\n' "$b" "${m:0:$(( w - ${#b} - ${#e} ))}" "$e"
}

##### usage and testing ########################################################

# NOTE: for testing, adjust your terminal width to 80 (normally the default)

# testing boundaries
divider 1234567890123456789012345678901234567890123456789012345678901234567890123456789
divider 12345678901234567890123456789012345678901234567890123456789012345678901234567890
divider 123456789012345678901234567890123456789012345678901234567890123456789012345678901

# more examples
divider '-----[ file: /this/is/a/test.cfg ]-----' # force minimum header
divider 'Once more with feeling!'
divider \
  "_.-'\"\"'-._[ Weeeeee! ]_.-'\"\"'-._" \
  "_.-'\"\"'-._"

# right aligned
divider - - '[ test ]-----'

# both
divider '--[ left ]' =- '[ right ]--'

# fancy multiline examples
divider '' ' .-.-.  '
divider '' '/ / \ \ '
divider '' "-'   '-'"

divider '' "     .-."
divider '' "'._.'   "

# plain divider
divider