Add another TIL on AWK: curl flags

1 files changed, 142 insertions, 0 deletions

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/