= Awk snippet: send email to multiple recipients with cURL

:neomutt: https://neomutt.org/
:found-out-article: https://blog.edmdesigner.com/send-email-from-linux-command-line/
:curl: https://curl.se/

As I experiment with {neomutt}[Neomutt], I wanted to keep being able to enqueue
emails for sending later like my previous setup, so that I didn't rely on having
an internet connection.

My requirements for the `sendmail` command were:

. store the email in a file, and send it later;
. send from different addresses, using different SMTP servers.

I couldn't find an MTA that could accomplish that, but I was able to quickly
write a solution.

The first part was the easiest: store the email in a file:

[source,sh]
----
# ~/.config/mutt/muttrc:
set sendmail=~/bin/enqueue-email.sh

# ~/bin/enqueue-email.sh:
#!/bin/sh -eu

cat - > "$HOME/mbsync/my-queued-emails/$(date -Is)"
----

Now that I had the email file store locally, I needed a program to send the
email from the file, so that I could create a cronjob like:

[source,sh]
----
for f in ~/mbsync/my-queued-emails/*; do
  ~/bin/dispatch-email.sh "$f" && rm "$f"
done
----

The `dispatch-email.sh` would have to look at the `From:` header and decide
which SMTP server to use.  As I {found-out-article}[found out] that {curl}[curl]
supports SMTP and is able to send emails, this is what I ended up with:

[source,sh]
----
#!/bin/sh -eu

F="$1"

rcpt="$(awk '
  match($0, /^(To|Cc|Bcc): (.*)$/, m) {
    split(m[2], tos, ",")
    for (i in tos) {
      print "--mail-rcpt " tos[i]
    }
  }
'  "$F")"

if grep -qE '^From: .*<addr@server1\.org>$' "$F"; then
  curl                                                      \
    -s                                                      \
    --url smtp://smtp.server1.org:587                       \
    --ssl-reqd                                              \
    --mail-from addr@server1.org                            \
    $rcpt                                                   \
    --user 'addr@server1.org:my-long-and-secure-passphrase' \
    --upload-file "$F"
elif grep -qE '^From: .*<addr@server2\.org>$' "$F"; then
  curl                                                      \
    -s                                                      \
    --url smtp://smtp.server2.org:587                       \
    --ssl-reqd                                              \
    --mail-from addr@server2.org                            \
    $rcpt                                                   \
    --user 'addr@server2.org:my-long-and-secure-passphrase' \
    --upload-file "$F"
else
  echo 'Bad "From: " address'
  exit 1
fi
----

Most of curl flags used are self-explanatory, except for `$rcpt`.

curl connects to the SMTP server, but doesn't set the recipient address by
looking at the message.  My solution was to generate the curl flags, store them
in `$rcpt` and use it unquoted to leverage shell word splitting.

To me, the most interesting part was building the `$rcpt` flags.  My first
instinct was to try grep, but it couldn't print only matches in a regex.  As I
started to turn towards sed, I envisioned needing something else to loop over
the sed output, and I then moved to Awk.

In the short Awk snippet, 3 things were new to me: the `match(...)`,
`split(...)` and `for () {}`.  The only other function I have ever used was
`gsub(...)`, but these new ones felt similar enough that I could almost guess
their behaviour and arguments.  `match(...)` stores the matches of a regex on
the given array positionally, and `split(...)` stores the chunks in the given
array.

I even did it incrementally:

[source,sh]
----
$ H='To: to@example.com, to2@example.com\nCc: cc@example.com, cc2@example.com\nBcc: bcc@example.com,bcc2@example.com\n'
$ printf "$H" | awk '/^To: .*$/ { print $0 }'
To: to@example.com, to2@example.com
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { print m }'
awk: ligne de commande:1: (FILENAME=- FNR=1) fatal : tentative d'utilisation du tableau « m » dans un contexte scalaire
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { print m[0] }'
To: to@example.com, to2@example.com
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { print m[1] }'
to@example.com, to2@example.com
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { split(m[1], tos, " "); print tos }'
awk: ligne de commande:1: (FILENAME=- FNR=1) fatal : tentative d'utilisation du tableau « tos » dans un contexte scalaire
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { split(m[1], tos, " "); print tos[0] }'

$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { split(m[1], tos, " "); print tos[1] }'
to@example.com,
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { split(m[1], tos, " "); print tos[2] }'
to2@example.com
$ printf "$H" | awk 'match($0, /^To: (.*)$/, m) { split(m[1], tos, " "); print tos[3] }'
----

(This isn't the verbatim interactive session, but a cleaned version to make it
more readable.)

At this point, I realized I needed a for loop over the `tos` array, and I moved
the Awk snippet into the `~/bin/dispatch-email.sh`.  I liked the final thing:

[source,awk]
----
match($0, /^(To|Cc|Bcc): (.*)$/, m) {
  split(m[2], tos, ",")
  for (i in tos) {
    print "--mail-rcpt " tos[i]
  }
}
----

As I learn more about Awk, I feel that it is too undervalued, as many people
turn to Perl or other programming languages when Awk suffices.  The advantage is
pretty clear: writing programs that run on any POSIX system, without extra
dependencies required.

Coding to the standards is underrated.
