Exploring Expect

Mailing From Expect

In the previous section, I suggested using mail as a last ditch attempt to contact the user or perhaps just a way of letting someone know what happened. Most versions of cron automatically mail logs back to users by default, so it seems appropriate to cover mail here. Sending mail from a background process is not the most flexible way of communicating with a user, but it is clean, easy, and convenient.

People usually send mail interactively, but most mail programs do not need to be run with spawn. For example, /bin/mail can be run using exec with a string containing the message. The lines of the message should be separated by newlines. The variable to names the recipient in this example:

exec /bin/mail $to << "this is a message\nof two lines\n"

There are no mandatory headers, but a few basic ones are customary. Using variables and physical newlines makes the next example easier to read:

exec /bin/mail $to << "From: $from
To: $to
From: $from
Subject: $subject
$body"

To send a file instead of a string, use the < redirection.

You can also create a mail process and write to it:

set mailfile [open "|/bin/mail $to" w]
puts $mailfile "To: $to"
puts $mailfile "From: $from"
puts $mailfile "Subject: $subject"

This approach is useful when you are generating lines one at a time, such as in a loop:

while {....} {
    ...   ;# compute line
    puts $mailfile "computed another line: $line"
}

To send the message, just close the file.

close $mailfile