POSIX shell changes

[bewinsnw]

POSIX date does not support the %N format, also POSIX shell does not support the substring expansion format, or arrays.

Another problem with the original script is the use of dd, which may not return the number of bytes expected (see https://unix.stackexchange.com/questions/121865/create-random-data-with-dd-and-get-partial-read-warning-is-the-data-after-the#121888)

The original also did not format the resulting string like the other scripts in 8-4-4-4-6 format.

I think it's fine to change the original script to specify bash as the shell not sh, but just to point out the issues, here's a stab at a posixly correct version? (shellcheck at least does not complain)

[nalgeon]

This implementation yields incorrect results:

0190273c-00a0-7000-8000-

script.sh: line 10: xxd: command not found
script.sh: line 15: xxd: command not found
script.sh: line 19: xxd: command not found

https://onecompiler.com/bash/42ghbcfd4

[bewinsnw]

in that case the original implementation is also incorrect, because it also depends on xxd

[bewinsnw]

Figured out how to drop xxd from the script if I can use tr, which is also defined by POSIX. https://pubs.opengroup.org/onlinepubs/9699919799/nframe.html

this works, but is a bit of a hack... (inevitable if you stick to bourne shell). I tried it in that onecompiler environment as well as locally and it works just fine.

[nalgeon]

Awesome! Thank you so much!

[Summertime]

POSIX date does not support the %N format

It also does not support %s https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html

---

head does not support -c https://pubs.opengroup.org/onlinepubs/9699919799/utilities/head.html

---

dd should be safe due to

When calling read(2) for the device /dev/urandom, reads of up to 256 bytes will return as many bytes as are requested and will not be interrupted by a signal handler.

from urandom(4)

[bewinsnw]

fair points all. Any suggestions for fixing it? We can use dd to replace head, but date's more of an issue.

One possibility is awk: awk 'BEGIN{print srand(srand())"000"}' (from https://unix.stackexchange.com/a/703145)

[Summertime]

I wrote up a version here: https://posts.summerti.me/posix-compliant/

A fully POSIX approach is fairly painful and slow, so I'd recommend avoiding such considerations and just label it as Bash on GNU/Linux

---

Additionally: /dev/urandom isn't part of POSIX, so for a truely POSIX implementation, one would need to consider making an RNG in POSIX shell and somehow have that be a CSPRNG and somehow seed that in a cryptographically sane way