Next: sendmail connector, Previous: sendmail message, Up: sendmail [Contents][Index]
The script below can be found in examples/sendmail-plain.sh. It just sends a hard–coded email message, from a hard–coded address to a hard–coded address. It makes no use of MBFL.
Drive the script controlling the SMTP protocol. It should be obvious what it does once we understand the following functions.
Open a connection to the SMTP server using a fake device that Bash gives us as interface to the
network. For the localhost, it ends up being:
/dev/tcp/localhost/25
where 25 is the TCP port which is officially assigned to the SMTP service. To open the connection we use the idiom:
exec 3<>/dev/tcp/localhost/25
which means: open a read and write connection to the selected hostname, using file descriptor number 3. There is nothing special in number 3, it is just the first free file descriptor number after 0 (standard input), 1 (standard output) and 2 (standard error).
The line:
trap 'exec 3<&-' EXIT
means: close file descriptor 3 whenever the script terminates. This is redundant in such a simple script, it is there for completeness.
Send a string to the SMTP server. Use printf() to format the string template with
the optional arguments, then write the resulting string to file descriptor 3. The string written
out is terminated with the sequence \r\n as mandated by the SMTP protocol.
Read an email message from stdin line by line (newline terminator), and rewrite it to file descriptor 3 terminating each line with the sequence carriage return/line feed. With the exception of the terminating sequence, the lines are left unchanged.
Read a line (a sequence of characters up until the first \n) from file descriptor 3. The
line is interpreted as a message from the SMTP server: the first three characters are a numeric
code. If the code is different from expected_code, raise an error.
#! /bin/bash
#
# Part of: Marco's Bash Functions Library
# Contents: example script to send email
# Date: Thu Apr 23, 2009
#
# Abstract
#
# This script just sends a hardcoded email message from a
# hardcoded address to a hardcoded address. It makes no use of
# MBFL.
#
# The purpose of this script is to understand how to handle the
# SMTP protocol.
#
# Copyright (c) 2009, 2010, 2018 Marco Maggi <marco.maggi-ipsu@poste.it>
#
# The author hereby grants permission to use, copy, modify, distribute,
# and license this software and its documentation for any purpose,
# provided that existing copyright notices are retained in all copies
# and that this notice is included verbatim in any distributions. No
# written agreement, license, or royalty fee is required for any of the
# authorized uses. Modifications to this software may be copyrighted by
# their authors and need not follow the licensing terms described here,
# provided that the new terms are clearly indicated on the first page of
# each file where they apply.
#
# IN NO EVENT SHALL THE AUTHOR OR DISTRIBUTORS BE LIABLE TO ANY PARTY
# FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
# ARISING OUT OF THE USE OF THIS SOFTWARE, ITS DOCUMENTATION, OR ANY
# DERIVATIVES THEREOF, EVEN IF THE AUTHOR HAVE BEEN ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.
#
# THE AUTHOR AND DISTRIBUTORS SPECIFICALLY DISCLAIM ANY WARRANTIES,
# INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND
# NON-INFRINGEMENT. THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS,
# AND THE AUTHOR AND DISTRIBUTORS HAVE NO OBLIGATION TO PROVIDE
# MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
#
PROGNAME=${0##*/}
function main () {
local HOSTNAME=localhost
local SMTP_PORT=25
local FROM_ADDRESS=marco@localhost
local TO_ADDRESS=root@localhost
local LOGGING_TO_STDERR=yes
open_session "$HOSTNAME"
recv 220
send 'HELO %s' 127.0.0.1
recv 250
send 'MAIL FROM:<%s>' "$FROM_ADDRESS"
recv 250
send 'RCPT TO:<%s>' "$TO_ADDRESS"
recv 250
send %s DATA
recv 354
print_message | read_and_send_message
send %s .
recv 250
send %s QUIT
recv 221
}
function print_message () {
local LOCAL_HOSTNAME DATE MESSAGE_ID MESSAGE
LOCAL_HOSTNAME=$(hostname --fqdn) || exit 2
DATE=$(date --rfc-2822) || exit 2
printf -v MESSAGE_ID '%d-%d-%d@%s' \
$RANDOM $RANDOM $RANDOM "$LOCAL_HOSTNAME"
MESSAGE="Sender: $FROM_ADDRESS
From: $FROM_ADDRESS
To: $TO_ADDRESS
Subject: demo from $PROGNAME
Message-ID: <$MESSAGE_ID>
Date: $DATE
This is a text demo from the $PROGNAME script.
--\x20
Marco
"
printf "$MESSAGE"
}
function open_session () {
local HOSTNAME=${1:?}
local DEVICE
printf -v DEVICE '/dev/tcp/%s/%d' "$HOSTNAME" $SMTP_PORT
exec 3<>"$DEVICE"
trap 'exec 3<&-' EXIT
}
function recv () {
local EXPECTED_CODE=${1:?}
local line=
IFS= read -t 5 line <&3
if ((127 < $?))
then
printf '%s: connection timed out\n' "$PROGNAME" >&2
exit 2
fi
if test "$LOGGING_TO_STDERR" = yes
then printf '%s log: recv: %s\n' "$PROGNAME" "$line"
fi
if test "${line:0:3}" != "$EXPECTED_CODE"
then
send '%s' QUIT
# It is cleaner to wait for the reply from the server.
IFS= read -t 5 line <&3
if ((127 < $?))
then
printf '%s: connection timed out\n' "$PROGNAME" >&2
exit 2
fi
if test "$LOGGING_TO_STDERR" = yes
then printf '%s log: recv: %s\n' "$PROGNAME" "$line"
fi
exit 2
fi
}
function send () {
local template=${1:?}
shift
local line
printf -v line "$template" "$@"
printf '%s\r\n' "$line" >&3
if test "$LOGGING_TO_STDERR" = yes
then printf '%s log: sent: %s\n' "$PROGNAME" "$line"
fi
}
function read_and_send_message () {
local line
local -i count=0
while IFS= read line
do
if test "${line:0:1}" = '.'
then printf '.%s\r\n' "$line" >&3
else printf '%s\r\n' "$line" >&3
fi
let ++count
done
if test "$LOGGING_TO_STDERR" = yes
then printf '%s log: sent message (%d lines)\n' "$PROGNAME" $count
fi
}
main
### end of file
Next: sendmail connector, Previous: sendmail message, Up: sendmail [Contents][Index]