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 rerun @reboot crontab entries without a reboot

Vixie cron and its descendants have a feature where an entry with the special time specification @reboot in place of the first five date and time fields indicates a one-shot action to be run when the machine is first booted.

But how do you test such entries without actually having to reboot the machine? My thinking was that somewhere crond must note the information that it already has been run once at boot.

Indeed a quick search of the filesystem found the zero length file /var/run/cron.reboot used as a flag that @reboot jobs have already been done.

So, to rerun @reboot jobs:

rm -f /var/run/cron.reboot

followed by:

systemctl restart crond

for systemd systems or

/etc/init.d/crond restart

for SysVinit systems.