Question

How can we trigger a shell script on an unix server through an email with particular subject?

Answer 1

procmail allows you to act on incoming mails, including filtering and starting external commands.

Some useful links:

• general procmail documentation: http://pm-doc.sourceforge.net/doc/
• start a shell command as a procmail rule: http://porkmail.org/era/procmail/mini-faq.html#rtfm

Just in case the link goes down, this is the link from the second point from above:

Q: How can I run an arbitrary Perl or shell script on all or selected
incoming mail?

A: Install Procmail. Read the manual pages (there are several). Thank
you.

:0 * conditions, if any | your-script-here

The conditions, in their simplest form, are regular expressions to
match against the header of each incoming mail message. Correction:
Even simpler, you can leave out the condition lines completely if you
want to do your action (in this case, run a shell script)
unconditionally.

More-complicated conditions can also be exit codes of other shell
scripts or programs, or tests against the full body of the message, or
against Procmail variables (Procmail’s variables are also exported to
the environment of subprocesses, so they are essentially environment
variables. There are details about this later in this FAQ.)

Actions can also be to save the message to a folder (appended to a
Unix mailbox file, or written to a new file in a directory) or to
forward the message to one or more other addresses. Finally, the
action can be a nested block of more “recipes,” as these
condition-action mappings are called in Procmail jargon, to try if the
outer condition is met. The procmailrc(5) manual page has the full
scoop.

Obviously, you are not restricted to Perl or shell scripts. Anything
you can run from a Unix command prompt can be run from Procmail, in
principle, although running interactive programs doesn’t usually make
much sense.