curl : send html email with embedded image and attachment

The solution I came up with is to base64 encode all the attachment (image and text file) and to include them into the uploaded file directly in the multipart/mixed body such as :

--MULTIPART-MIXED-BOUNDARY
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline
Content-Id: <admin.png>
iVBORw0KGgoAAAANSUhEUgAAAIAAAACgCAIAAABL8POqAAAACXBIWXMAAAsTAAALEwEAmpwYAAAA
B3RJTUUH4AQNDwEVouBdqAAAG2xJREFUeNrtfX9oHFe25jdDBU5BG25BG7pABhXEkDJjSIsYIs1m
WbfJA8ubhcjjgdiTQNJOYCInj0RKYGIl8CbyPF4iZSCxEkgsB5LIgWQlL2Pcfow3bdgw0mMzox6e
....


--MULTIPART-MIXED-BOUNDARY
Content-Type: text/plain
Content-Transfer-Encoding: base64
Content-Disposition: inline; filename=log.txt
c29tZSBsb2cgaW4gYSB0eHQgZmlsZSB0byBhdHRhY2ggdG8gdGhlIG1haWwK


--MULTIPART-MIXED-BOUNDARY--

The Content-Id header is used to identify the resource that can be referenced in the html with : cid: like :

<img src="cid:admin.png" width="150" height="50">

Here is a complete bash example to send an html email with and admin.png embedded image and a log.txt attached to it :

#!/bin/bash

rtmp_url="smtp://smtp.gmail.com:587"
rtmp_from="[email protected]"
rtmp_to="[email protected]"
rtmp_credentials="[email protected]:secretpassword"

file_upload="data.txt"

# html message to send
echo "<html>
<body>
    <div>
        <p>Hello, </p>
        <p>Please see the log file attached</p>
        <p>Admin Team</p>
        <img src=\"cid:admin.png\" width=\"150\" height=\"50\">
    </div>
</body>
</html>" > message.html

# log.txt file to attached to the mail
echo "some log in a txt file to attach to the mail" > log.txt

mail_from="Some Name <$rtmp_from>"
mail_to="Some Name <$rtmp_to>"
mail_subject="example of mail"
mail_reply_to="Some Name <$rtmp_from>"
mail_cc=""

# add an image to data.txt : 
# $1 : type (ex : image/png)
# $2 : image content id filename (match the cid:filename.png in html document)
# $3 : image content base64 encoded
# $4 : filename for the attached file if content id filename empty
function add_file {
    echo "--MULTIPART-MIXED-BOUNDARY
Content-Type: $1
Content-Transfer-Encoding: base64" >> "$file_upload"

    if [ ! -z "$2" ]; then
        echo "Content-Disposition: inline
Content-Id: <$2>" >> "$file_upload"
    else
        echo "Content-Disposition: attachment; filename=$4" >> "$file_upload"
    fi
    echo "$3

" >> "$file_upload"
}

message_base64=$(cat message.html | base64)

echo "From: $mail_from
To: $mail_to
Subject: $mail_subject
Reply-To: $mail_reply_to
Cc: $mail_cc
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=\"MULTIPART-MIXED-BOUNDARY\"

--MULTIPART-MIXED-BOUNDARY
Content-Type: multipart/alternative; boundary=\"MULTIPART-ALTERNATIVE-BOUNDARY\"

--MULTIPART-ALTERNATIVE-BOUNDARY
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: base64
Content-Disposition: inline

$message_base64
--MULTIPART-ALTERNATIVE-BOUNDARY--" > "$file_upload"

# add an image with corresponding content-id (here admin.png)
image_base64=$(curl -s "https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_116x41dp.png" | base64)
add_file "image/png" "admin.png" "$image_base64"

# add the log file
log_file=$(cat log.txt | base64)
add_file "text/plain" "" "$log_file" "log.txt"

# add another image 
#image_base64=$(curl -s "https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_116x41dp.png" | base64)
#add_file "image/png" "something.png" "$image_base64"

# end of uploaded file
echo "--MULTIPART-MIXED-BOUNDARY--" >> "$file_upload"

# send email
echo "sending ...."
curl -s "$rtmp_url" \
     --mail-from "$rtmp_from" \
     --mail-rcpt "$rtmp_to" \
     --ssl -u "$rtmp_credentials" \
     -T "$file_upload" -k --anyauth
res=$?
if test "$res" != "0"; then
   echo "sending failed with: $res"
else
    echo "OK"
fi

Here's a shell script you could use. I heavily derived from https://blog.ambor.com/2021/08/using-curl-to-send-e-mail-with.html

Passwords could be stored in a .netrc file as discussed on: https://everything.curl.dev/usingcurl/netrc but I haven't tried that.

Just make sure the image you need is embedded in the HTML using base64 encoding.

#!/bin/bash

declare -a VOPTS;
declare -a HOPTS;

sesAccess="[email protected]" ;
sesSecret="sender.account.passwordXXXXXX";
sesFromName="Sender Full Name";
sesFromAddress='<[email protected]>';
sesToName="Recipient Full Name";
sesToAddress="<[email protected]>"
sesSubject="Email Subject Line";
sesSMTP='mail.server.fqdn';
sesPort='465';
sesMessage=$'Test of line 1\nTest of line 2'
sesFile="$1"; # attachment is first argument
sesHTMLbody="/path/to/html/file.html"; # content of this file will be used to create HTML body

sesMIMEType=`file --mime-type "$sesFile" | sed 's/.*: //'`;
# sesMIMEType=`file -b --mime-type "$sesFile"`;

VOPTS=();
HOPTS=();

#Curl Options
VOPTS+=("-v");
VOPTS+=("--url"); VOPTS+=("smtps://$sesSMTP:$sesPort"); 
VOPTS+=("--ssl-reqd")
VOPTS+=("--user"); VOPTS+=("${sesAccess}:${sesSecret}"); 
VOPTS+=("--mail-from"); VOPTS+=("${sesFromAddress}");
VOPTS+=("--mail-rcpt"); VOPTS+=("${sesToAddress}");

#Header Options
HOPTS+=("-H"); HOPTS+=("Subject: ${sesSubject}");
HOPTS+=("-H"); HOPTS+=("From: ${sesFromName} ${sesFromAddress}"); 
HOPTS+=("-H"); HOPTS+=("To: ${sesToName} ${sesToAddress}"); 

curl "${VOPTS[@]}" -F '=(;type=multipart/mixed' -F "=<$sesHTMLbody;type=text/html;encoder=base64" -F "file=@$sesFile;type=$sesMIMEType;encoder=base64" -F '=)' "${HOPTS[@]}"

exit

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Email

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Curl

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