A simple way to send emails using docker and bash

Introduction

I have a home server that I run a bunch of Docker containers on.

Very often I want to run some automated job on the machine, but I don't want to have to check when it is done. One example being periodic backups that I want to know the status of.

Sending an instant message or even better (for me at least) an email to say when something is done would be very nice.

I thought this would be hard but it wasn't really.

Are there better ways of doing this? I am sure there are - but this was quick, easy and I can reuse it.

The Components

Gmail account with an App Password
Docker compose running bytemark/smtp
Simple bash script called "send_mail.sh" in /opt (or anywhere you want really)
Cron

Docker Compose running bytemark/smtp

I use Docker Compose for everything because I find it easy to use. This compose file sets up a SMTP server that can handle sending emails.

Here is the image I am using: https://hub.docker.com/r/bytemark/smtp/

version: '3'
services:
  mail:
    image: bytemark/smtp
    restart: always
    environment:
      RELAY_HOST: smtp.gmail.com
      RELAY_PORT: 587
      RELAY_USERNAME: <EMAIL_USERNAME@host.com>
      RELAY_PASSWORD: "<APP_PASSWORD>" # Add secret however you want
    ports:
      - "127.0.0.1:2525:25"

Simple Bash Script

So I want to send email from a bunch of places.

I've got emails that:

Show me how much disk is left on the machine.
What new errors have cropped up in journalctl.
Send me summary of logs (I pass the logs to gpt-oss-20b and then send that as the subject)

Anyway, I expected I would want a simple reusable way to send emails by just detailing the to and from email addresses, subject line and body

#!/bin/bash

# Script to send an email using the local Docker SMTP container.
# This version reads the email body from standard input (stdin).
#
# Usage:
# ./send-email.sh "from@example.com" "to@recipient.com" "Test Subject"
# (and then pipe the body to it, e.g. echo "This is the body." | ./send-email.sh ...)

# --- Config ---
SMTP_URL="smtp://127.0.0.1:2525"

# --- Check for curl ---
if ! command -v curl &> /dev/null
then
    echo "Error: 'curl' is not installed. Please install it to use this script."
    exit 1
fi

if [ "$#" -ne 3 ]; then
    echo "Usage: $0 \"from@address.com\" \"to@address.com\" \"Subject Line\""
    echo "       (Email body must be piped via stdin)"
    exit 1
fi

FROM_ADDRESS="$1"
TO_ADDRESS="$2"
SUBJECT_LINE="$3"

# --- Send the email ---
echo "Connecting to $SMTP_URL..."
echo "From: $FROM_ADDRESS"
echo "To: $TO_ADDRESS"
echo "---"

# We create a subshell to prepend the "Subject:" line to the
# incoming stdin (represented by 'cat -') and pipe it all directly to curl.
(
    echo "Subject: $SUBJECT_LINE"
    echo "" # Required blank line between headers and body
    cat -  # This command reads from stdin and passes it on
) | curl -v \
    --url "$SMTP_URL" \
    --mail-from "$FROM_ADDRESS" \
    --mail-rcpt "$TO_ADDRESS" \
    --upload-file -

echo "---"
echo "Script finished."

Calling it

Here is the most basic use. I have a big bash script on my server called "run" and cron periodically calls functions in it like so:

30 21 * * * /bin/bash /opt/run disk_usage

And that function looks like this:

function disk_usage {
  disk_usage=$(df -h / | awk 'END{print $4}')
  now=$(date +%Y-%m-%d_%H-%M-%S)
  echo "$disk_usage" | /opt/mail/send_email.sh \
        "adam+alerts@adamfallon.com" \
        "adam@adamfallon.com" \
        "Disk Usage: $now"
}

And that sends me an email like this

Figure 1: Disk Space Email

But I've got more complicated things like backups going to tarsnap that have conditional subject lines:

function create_backup {
    COMPUTER_NAME=$(uname -n)
    NOW=$(date +%Y-%m-%d_%H-%M-%S)
    ARCHIVE_NAME=${COMPUTER_NAME}-${NOW}
    SOURCE_FOLDERS=(
        "/home/user/Shared/docs"
        "/home/user/Shared/books"
    )
    FROM_EMAIL="adam+alerts@adamfallon.com"
    TO_EMAIL="adam@adamfallon.com"
    TARSNAP_OUTPUT_FILENAME=/tmp/tarsnap-output-temporary.log

    echo "Starting tarsnap backup for: ${SOURCE_FOLDERS[*]}"
    echo "Creating tarsnap archive: $ARCHIVE_NAME"

    sudo tarsnap -c -v -f "$ARCHIVE_NAME" \
        --keyfile /root/tarsnap.key \
        "${SOURCE_FOLDERS[@]}" >${TARSNAP_OUTPUT_FILENAME} 2>&1

    if [ $? -eq 0 ]; then
        subject="✅ Tarsnap backup success: $ARCHIVE_NAME"
    else
        subject="❌ Tarsnap backup FAILED: $ARCHIVE_NAME"
    fi
    echo "$subject"

    # Send the email
    /opt/mail/send_email.sh \
        "$FROM_EMAIL" \
        "$TO_EMAIL" \
        "$subject" < "${TARSNAP_OUTPUT_FILENAME}"

    # Clean up the temporary log file
    rm "${TARSNAP_OUTPUT_FILENAME}"
}

Which looks like this

Figure 2: TarSnap Log

Conclusion

That's it! Just wanted to share because I thought it was neat. I don't know why I thought it would be really hard to do this, but it was fine.