passgen

#!/bin/sh
PASSTYPE="chars"
TARGET_BITS=80
DICT=/usr/share/dict/words
RANDOM_SOURCE=/dev/urandom
ALLOWED_CHARS="A-Za-z0-9" # tr format
ALLOWED_WORD_CHARS="a-z" # sed character class

unset LENGTH
for arg in "$@"; do
case "$arg" in
  -w|--word|--words)
    PASSTYPE="words"
    ;;
  *)
    LENGTH="${arg}"
    ;;
esac
done

case ${PASSTYPE} in
  chars)
    if [ -z "$LENGTH" ]; then
      BITS_PER_CHAR=6 # log_2 62
      LENGTH=$((TARGET_BITS/BITS_PER_CHAR))
    fi
    tr -dc "${ALLOWED_CHARS}" <"${RANDOM_SOURCE}" | head -c $LENGTH
    echo
    ;;
  words)
    if [ \! -e "${DICT}" ]; then
      echo "Missing word dictionary: ${DICT}"
      exit 1
    fi
    if [ -z "${LENGTH}" ]; then
      #AVAILABLE_WORDS=$(sed -e "/[^a-z]/d" </usr/share/dict/words | wc -l | cut -d' ' -f1)
      #BITS_PER_WORD=$(python -c "import math; print(math.log2(${AVAILABLE_WORDS}))")
      BITS_PER_WORD=16 # floor (log_2 77115)
      LENGTH=$((TARGET_BITS/BITS_PER_WORD))
    fi
    shuf -r "${DICT}" --random-source="${RANDOM_SOURCE}" | sed -e "/[^${ALLOWED_WORD_CHARS}]/d" | head -n "${LENGTH}" | tr "\n" " "
    echo
    ;;
esac