Awk snippet: send email to multiple recipients with cURL

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As I experiment with 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:

  1. store the email in a file, and send it later;

  2. 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:

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

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

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

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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:

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

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

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#!/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

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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:

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$ 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] }'

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(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:

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

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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.