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diff --git a/_tils/2021-01-12-awk-snippet-send-email-to-multiple-recipients-with-curl.md b/_tils/2021-01-12-awk-snippet-send-email-to-multiple-recipients-with-curl.md new file mode 100644 index 0000000..3ac5b97 --- /dev/null +++ b/_tils/2021-01-12-awk-snippet-send-email-to-multiple-recipients-with-curl.md @@ -0,0 +1,142 @@ +--- + +title: 'Awk snippet: send email to multiple recipients with cURL' + +date: 2021-01-12 + +layout: post + +lang: en + +ref: awk-snippet-send-email-to-multiple-recipients-with-curl + +--- + +As I experimented 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: +1. store the email in a file, and send it later. +1. 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: + +```shell +# ~/.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: + +```shell +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][curl-email] that [curl][curl] supports SMTP and is able to send emails, this is what I ended up with: + +```shell +#!/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 `$rcpt` flags. +My first instinct was to try grep, but it couldn't print only matches in a regex. +As I started to turn towards sed, I envisioned needing something else to loop over the sed output, and I then moved to Awk. + +In the short Awk snippet, 3 things were new to me: the `match(...)`, `split(...)` and `for () {}`. +The only other function I have ever used was `gsub(...)`, but these new ones felt similar enough that I could almost guess their behaviour and arguments. +`match(...)` stores the matches of a regex on the given array positionally, and `split(...)` stores the chunks in the given array. + +I even did it incrementally: + +```shell +$ 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: + +```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. + +[neomutt]: https://neomutt.org/ +[curl-email]: https://blog.edmdesigner.com/send-email-from-linux-command-line/ +[curl]: https://curl.se/ |