==============================================================================
                  uuid.sh -- a UUID Generator in POSIX Shell
==============================================================================

Usage:
    uuid [<options>] [<version> [<arguments>]]
        Generate a Universally Unique Identifier.
        The default version to use is "v4".

Options:
    -h, --help
        Show this help information.
    -b, --binary
        Output bytes in binary instead of plain-text format.
    -d, --decode
        Extract data from UUIDs generated by this program.
    -g, --guid
        Output variant-2 UUIDs by costing 1 bit of randomness.
    -n, --next
        Output the next UUID for version 1, 2, 6, or 7.
    -s, --seed <number>
        Use a deterministic pseudo-random number generator.
    -l, --lowercase
        Use lower-case letters for UUID output in plain text.
    -u, --uppercase
        Use upper-case letters for UUID output in plain text.

Summary of available versions:
    uuid v0 [<uuid>]
        ... Output key=value pairs for sourcing when -d flag is on ...
    uuid v1 [<period> [<sequence> [<node>]]]
        ... Gregorian timestamp plus privacy and security concerns ...
    uuid v2 <domain> <identifier> [<period> [<sequence> [<node>]]]
        ... v1 of less available IDs per clock tick but DCE Security ...
    uuid v3 <uuid> <text>...
        ... 16 bytes of 6 fixed bits from a 16-byte MD5 hash ...
    uuid v4 [<count>]
        ... 16 (pseudo-)random bytes of 6 predetermined bits ...
    uuid v5 <uuid> <text>...
        ... 16 bytes of 6 fixed bits from a 20-byte SHA-1 hash ...
    uuid v6 [<period> [<sequence> [<node>]]]
        ... Draft#2: lexicographically sortable by Gregorian timestamp ...
    uuid v7 [<second> [<subsecond> [<sequence> [<node>]]]]
        ... Draft#2: lexicographically sortable by Unix Epoch timestamp ...
    uuid v8 [<year> [<month> [<day> [<hour> [<minute> [<node>]]]]]]
        ... Draft#2: lexicographically sortable by custom timestamp ...

Notes:
    For time-based UUIDs, users must provide a system-wide global lock
    to avoid race conditions among generators runing at the same time.
    Please reference section 4.2 of RFC4122 for useful tips.
    https://www.rfc-editor.org/rfc/inline-errata/rfc4122.html

Dependencies:
    - for randomization in v1/v2/v4/v6/v7/v8: openssl or /dev/urandom
    - for timestamp in v1/v2/v6/v7/v8: date
    - for hashing in v3/v5: openssl