crond runs /bin/sh not /bin/bash

Got caught by this today. Cron jobs normally run /bin/sh, not /bin/bash. It's probably required POSIX behaviour.

Normally most commands run out of cron are not affected because they are not dependent on bash, or the shell script starts with #!/bin/bash. However commands like vncserver which start up desktops (e.g. with the @reboot time spec) can be sensitive to the shell used because of environment scripts in /etc/profile.d.

Therefore if you want to ensure that the cron job works the same whether run interactively or from a crontab, specify the shell. E.g.

@reboot SHELL=/bin/bash /usr/bin/vncserver ...

How to send email that will expire (Outlook only)

I get notification email from Linux cron jobs. I wanted some way to automatically mark mail as no longer relevant after a week. I recently discovered that there are a couple of mail headers defined for this purpose in RFCs: Expires: and the older, superseded Expiry-date: which is what Outlook uses. They are not honoured by most mail readers though so this tip is specific to Outlook.

I discovered sample code from Internet search. Here is my version in Perl which does the job.


use Net::SMTP;
...
my $expire_time = strftime "%d %b %y %T %Z", localtime(time() + 86400 * 7);
...

$smtp = Net::SMTP->new($mailserver);

...

$smtp->datasend("Expires: $expire_time\n");

$smtp->datasend("Expiry-date: $expire_time\n");

The Net::SMTP module allows one to send arbitrary headers. You can see the required format of the date time string from the strftime() call.

Expired emails are not deleted, but are displayed in desktop Outlook with strikethrough. No effect that I know of in other mail readers, not even the Internet Office 365.