FEATURE: add flag support to the `gpg-share` script

[amtoine]

This PR adds a bunch of flags to the gpg-share.sh script, to make its use both documented and easier. This PR will close #37.

the changes

• f55d62a..dfa0b31: add flag support using the getopt command :point_right: on dfa0b31, the script is equivalent to the one on 6d7a9f6, but with empty flag support
  • git diff f55d62a^..dfa0b31 to see the diff
• e83d952: allow the user to pass the file and the list of users with -f, -u and -U do not use USER_FILE anymore
  • git show e83d952 to see the diff
• da112f9..e30043d: dedicated to the help and the usage removes the echo -n lines thanks to the new better array system, see 9ec42b9 for details
  • git diff da112f9^..e30043d: to see the diff

details about the usage

one can see the following

> ./scripts/gpg-share.sh -h
gpg-share:
     a script to share a secret file with others using pgp public keys.
     it should guide you when you make mistakes, e.g. providing a non-existing file.

Usage:
     gpg-share [-h] -f FILE {-u USER} {-U USER_FILE}

Options:
     -h/--help               shows this help.
     -f/--file               the secret file to share.
     -u/--user               a single user id in the keyring.
                               can be used multiple times.
                               supports multi-word ids inside quotes, e.g. "John Smith".
     -U/--users              the path to a file containing pgp ids.
                               should contain exactly one valid pgp id per line.
                               can be used multiple times.

the -u and -U flags can be used multiple times, and they are meant to be used as such to avoid word splitting issues with bash, i chose to implement the flag parsing as follows:

• the user passes one user at the time with a separate -u each time. this allows to write ./scripts/gpg-share.sh -u "Alexandre Tullot" and the id won't be split :star_struck:
• to pass multiple users, simply write something like ./scripts/gpg-share.sh -u amtoine -u "Alexandre Tullot"

this is the meaning of the { ... } in the usage! they mean the -u and -U flags can be used any number of times :+1:

what i did not do for now

• -u and -U with multiple user ids: this is too complex in bash according to me, i think this would require way to much work and might even be ambiguous as bash does not have typed variables, e.g. -u amtoine "Alexandre Tullot" foo might be treated as "amtoine Alexandre Tullot foo" :thinking: in the end, i think the -u SINGLE_USER_ID syntax is as easy to write and far less ambiguous than the multi-id one
• the --check-keyring (-k) option: i do not know really what to check there :confused:
• exectute from ./scripts/: this was not the focus of this PR, so i did not change anything in that direction :+1:
• strong array checks: i did not implement strong checks on the array of users, for instance, an id might appear multiple times in the array, e.g. with -u amtoine -u amtoine and the script will encrypt the secret file twice and overwrite previous encryptions for the amtoine user

[amtoine]

Thanks for your work once again an amazing job rocket

glad you like it :relieved:

I think we're ready to merge, I however suggest that we wait some time for @vSt11 review/approval (in an educational will), but @amtoine feel free to merge this PR if @vSt11 doesn't answer in the coming days wink

no no, there's no rush, this PR can wait for @vSt11 :yum: