Start a mock smtp server which can be useful for local dev environment and may also be bundled into a Docker image

start-mock-smtp.sh

#!/usr/bin/env bash

# Authors: Marcos Santos, 2017
# License: MIT

# This script starts a mock smtp server and stores the emails "sent" in a local
# directory (i.e. /tmp/smtp-sink ).
# This comes in handy when running tests on a dev environment.
#
# This script requires the binary smtp-sink which is normally shipped within
# Postfix package. See http://www.postfix.org/smtp-sink.1.html
# Alternatively, the Python onliner below can be used to start a simple mock
# stmp server and write the result to stdout
#   sudo python -m smtpd -n -c DebuggingServer 127.0.0.1:25

# Immediately exit this script if ..
#  any internal command exits with non-zero status
set -o errexit
#  any unbound variables are used
set -o nounset
#  any pipe fails
set -o pipefail

set_cmd_full_path() {
	# Prefix the command names with the string CMD_
	local cmd_name="CMD_${1}"
	local bin_name="$2"
	for P in /bin /usr/bin /usr/sbin;
	do
		if [[ -x "$P/$bin_name" ]]; then
			export "${cmd_name}"="$P/$bin_name"
			break
		fi
	done

	# The exclamation mark preceding the variable name expands the check to the
	# variable's value instead of the variable string name itself. And the ":-"
	# sets the initiate the variable and avoid triggering 'nonunset' early exit.
	if [[ -z "${!cmd_name:-}" ]]; then
		/bin/echo "Error: The binary '${bin_name}' was not found in any of the pre-defined paths."
		exit 128
	fi
}

set_cmd_full_path BIN_WHOAMI   whoami
set_cmd_full_path BIN_MKDIR    mkdir
set_cmd_full_path BIN_CHOWN    chown
set_cmd_full_path BIN_FIND     find
set_cmd_full_path BIN_SMTPSINK smtp-sink 

if [[ "$($CMD_BIN_WHOAMI)" != "root" ]]; then
	/bin/echo "Error: This script must be run by the 'root' user because it requires privilege changes."
	exit 128
fi

$CMD_BIN_MKDIR -p /tmp/smtp-sink
$CMD_BIN_CHOWN -R nobody:nobody /tmp/smtp-sink
$CMD_BIN_FIND /tmp/smtp-sink -type d -exec chmod 755 {} \;
$CMD_BIN_FIND /tmp/smtp-sink -type f -exec chmod 644 {} \;

# -u    Switch to the specified user privileges after opening the network socket
#       and optionally changing the process root directory.
# -R    Change the process root directory to the specified location. This option
#       requires super-user privileges (-u).
# -d    Dump each mail transaction to a single-message file whose name is 
#       created by expanding the dump-template via strftime(3) and appending a 
#       pseudo-random hexadecimal number.
#       (example: "%Y%m%d%H/%M." expands into "2006081203/05.809a62e3").

$CMD_BIN_SMTPSINK -u nobody -R /tmp/smtp-sink -d "maildir/%Y-%m-%d/mail.%H.%M." 127.0.0.1:25 1024