Bash - Stream line padding

str_pad.sh

#!/usr/bin/env bash

function str_pad() {

  local pad_length="$1" pad_string="$2" pad_type="$3"
  local pad length llength offset rlength

  pad="$(eval "printf '%0.${#pad_string}s' '${pad_string}'{1..$pad_length}")"
  pad="${pad:0:$pad_length}"

  if [[ "$pad_type" == "left" ]]; then

    while read line; do
      line="${line:0:$pad_length}"
      length="$(( pad_length - ${#line} ))"
      echo "${pad:0:$length}$line"
    done

  elif [[ "$pad_type" == "both" ]]; then

    while read line; do
      line="${line:0:$pad_length}"
      length="$(( pad_length - ${#line} ))"
      llength="$(( length / 2 ))"
      offset="$(( llength + ${#line} ))"
      rlength="$(( llength + (length % 2) ))"
      echo "${pad:0:$llength}$line${pad:$offset:$rlength}"
    done

  else

    while read line; do
      line="${line:0:$pad_length}"
      length="$(( pad_length - ${#line} ))"
      echo "$line${pad:${#line}:$length}"
    done

  fi

}

# Usage:
# str_pad 50 '-' right <<<'Hello world!'
# str_pad 50 '-' left  <<<'Hello world!'
# str_pad 50 '-' both  <<<'Hello world!'
[[ $_ == $0 ]] && str_pad "$@"