Generate a random password of length N meeting complexity requirements in pure Bash 4+

generate-passwd

#!/usr/bin/env bash
set -Eeuo pipefail

die() {
  echo >&2 "$@"
  echo >&2
  echo >&2 "Usage: $(basename "$0") {{length>=1}} {{char_range[:min] ...}}"
  echo >&2
  echo >&2
  exit 1
}

random_chars() {
  head /dev/random | LC_ALL=C tr -dc "$1" | head -c "$2"
}

length="${1-}" total_min=0 rest_charset='' r=
if (($# < 2)) || [[ ! "$length" =~ ^[1-9][0-9]*$ ]]; then
  die 'Must specify length >= 1 and at least one character range'
fi
shift

# Parse ranges and minimum requirements
for arg in "$@"; do
  if [[ "$arg" =~ .+:[0-9]+$ ]]; then
    min=${arg##*:} charset=${arg%:*}
    if ((min == 0)); then
      rest_charset+="$charset"
    else
      r+="$(random_chars "$charset" "$min")"
      ((total_min += min))
    fi
  else
    rest_charset+="$arg"
  fi
done

rest_length=$((length - total_min))
if ((rest_length < 0)); then
  die "Sum of minimum requirements ($total_min) exceeds password length ($length)"
elif ((rest_length > 0)); then
  [[ "$rest_charset" ]] || die 'One or more character ranges must be specified without :N to generate remaining characters'
  r+=$(random_chars "$rest_charset" "$rest_length")
fi

# shuffle the result
for ((i = ${#r}; i > 0; i--)); do
  j=$((SRANDOM % i))
  echo -n "${r:$j:1}"
  r="${r:0:j}${r:j+1}"
done
echo