random-password

#!/bin/sh
set -euf

##
# Generate a random password for high security needs.
#
# Usage:
#
#     random-password [n] [characters]
#
# Options:
#
#     * n is an optional length; default is 24.
#     * characters is a string; default is "abcdefghijklmnop"
#
# Examples:
#
#     $ random-password
#     eoejnagpilnajfcmagffheie
#
#     $ random-password 8
#     midbpjkc
#
#     $ random-password 8 abc
#     bccabbcb
#
#
# ## Implementation
#
# This uses /dev/urandom to generate random characters.
# This is high security and suitable for cryptography.
#
# This script filters the stream of characters to pick just
# the ones that match the given characters.
#
#
# ## Configuration
#
# The default configuration is to match the first 16 letters
# in the English alphabet, namely "abcdefghijklmnop".
# 
# We choose to use exactly 16 characters because this
# means each character has 2^4 bits of random information,
# which is equivalent to one hexadecimal character.
#
# This has valuable mathematical properties for generating
# security values at high speed, and for calculating password
# strength, and for transforming among bits and characters.
#
# Some password-checker tools may report these passwords
# are insecure because they only have lowercase letters.
# This is not true for these passwords with the settings
# that we use i.e. secure random 24 picks of 16 characters.
#
# Mathematically, using 24 picks of 16 different characters
# is more computationally secure than 12 mixed characters,
# such as typical for passwords generated by Amazon.com for
# high security server sign in and system administration.
#
# We choose lowercase letters, rather than hexadecimal,
# because it's easier for most users to type many lowercase
# letters on most keyboards, especially on mobile devices.
#
#
# ## Customization
#
# If you prefer to use different security, use more picks,
# or different characters, or both.
#
# For example if you prefer to use 12 picks of hexadecimals:
#
#     random-password 12 "0123456789ABCDEF"
#     D22DCF30358B
#
# For example if you prefer to use 12 picks including symbols:
#
#     random-password 12 "\!\@\#\$ABCD"
#     B!$!ADB@$!!A$
#
#
# ## Tracking
#
#   * Command: random-password
#   * Version: 4.1.0
#   * Created: 2010-05-20
#   * Updated: 2021-10-14T17:58:40Z
#   * License: BSD or MIT or GPL or contact us for custom license
#   * Contact: Joel Parker Henderson (joel@joelparkerhenderson.com)
##

len=${1-24}
chars=${2-"abcdefghijklmnop"} 
LC_ALL=C < /dev/urandom tr -dc "$chars" | head -c"$len"