Is this an example of a simple and clean solution via shell script? I have some stylistic doubts:

1. What "exitcode" is set for:

  exitcode=1
  exit 1

2. I see a lot of "return $?". Why "$?" is returned if by default the shell returns the return value of the last command? Just ti name a few:

  lklfuse -o type=ext4 "${loop}" "$mnt"
  return $?
  ...
  veracrypt  --text --non-interactive -d "$file"
  return $?
  ...
  mount "$loop" "$mnt"
  return $?

3. Aren't =, != etc. used to compare strings and -eq, -ne, -gt etc. used to compare numbers? I see lot of numbers compared as strings, e.g.:

  [ $? = 0 ]
  [ $? != 0 ]
  [ $exitcode = 0 ]

4. There are lot of "cat <<EOF" blocks without indentation. I understand that this is made because the shell expects "EOF" on the line start, but there is a special syntax designed on purpose for this use case, simply put a dash between << and the token, e.g. "cat <<-EOF". In this case:

  tomb_init() {
   system="`uname -s`"
   case "$system" in
    FreeBSD)
     cat <<-EOF
     create=posix_create
     format=posix_format
     map=posix_map
     mount=freebsd_mount
     close=freebsd_close
     EOF
     ;;
    Linux)

5. Aren't backtick deprecated in favor of $()?

thanks for your review! tho you are referring to the tomb-portable unfinished experiment which is about to be dismissed since cross-platform experiments with veracrypt show very bad performance.

you are welcome to share a review of the tomb script, but be warned in that we use a lot of zsh specific features. It is a script that works since 15+ years so it has a discrete amount of patchwork to avoid regressions.