$MAILPATH
I've known for donkeys' ages that many Unix shells honour $MAIL: they
check the modification time of the file every $MAILCHECK seconds
(default: 600) and announce "You have mail". Back in the days, we then
used mail(1) or mailx(1) to read our messages, today it's more likely
mutt(1) or alpine(1).
[1]Tim's original tweet
What I wasn't aware of, and I thank Tim (of [2]@ed1conf fame) for
showing me, is that POSIX shells should honour $MAILPATH, a
colon-separated list of file names, for the shell to check for
incoming mail.
More importantly, $MAILPATH can contain optional notification messages
for each mailbox, and these are separated by ? from the filename (in
Bash) or, as POSIX mandates, by a % which Bash also supports. The
notification message itself is expanded by the shell.
$ MAILCHECK=1
$ MAILPATH=/var/mail/jpm%'Mail for $USER at $(date)':$HOME/logg%'$(tai
l -1 $HOME/logg)'
$ echo hello | mail jpm
$ pwd
/home/jpm
Mail for jpm at Fri Mar 6 16:49:56 UTC 2020
$ echo "Dinner time" >> logg
Dinner time
$
These two variables would cause a compliant shell to check for mail to
me every second, and to check whether the file $HOME/logg has been
modified, and if so, print its last line.
Quoting Tim and [3]POSIX:
POSIX requires parameter expansion in MAILPATH: "Each pathname can be
followed by `%' and a string that shall be subjected to parameter
expansion and written to standard error when the modification time
changes" which includes "Command Substitution"
I'm thinking of interesting things we could do with this - think
$MAILCHECK set to 0 which means the shell shall check before issuing
each primary prompt ($PS1).
We've been playing around with this for a bit, and to cut a long story
short: verify that this works in the shell you use.
And last but not least: [4]Tim is a person you should follow if you're
interested in Unix commands and odds and ends; he digs up the neatest
things.
References
1.
https://twitter.com/gumnos/status/1235598421366648832
2.
https://twitter.com/ed1conf
3.
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html
4.
https://twitter.com/gumnos
