Does running cron by itself have the same behavior as via the init.d script?

Question

I'm working with some code that fires up cron on a server (which doesn't have it running at boot time). The script which starts cron sets up some logging stuff and then simply invokes cron. It doesn't use /etc/init.d/cron or service cron start.

After starting cron this way, service cron status and service cron stop seem to be happily able to work, and the PIDFILE specified in /etc/init.d/cron is present.

I put a log line into /etc/init.d/cron, and it looks like running cron standalone does not invoke the script.

# service cron status
script is running
 * cron is running
# service cron stop
script is running
 * Stopping periodic command scheduler cron                                                                                                                                                        [ OK ]
# cron
#

What's going on here? Is this simply because the cron binary and the /etc/init.d/cron script use the same convention for the location of the pidfile?

Answer 1

Your hypothesis is correct: Vixie Cron has a fixed pidfile location (/var/run/crond.pid), which also prevents running it twice.

The init.d script, which is also called by service uses the standard /lib/lsb/init-functions, which sum up to:

1. The start action just calls /usr/sbin/cron (through the /sbin/start-stop-daemon helper),
2. The stop action just sends SIGTERM to the PID in the pidfile (through /sbin/start-stop-daemon),
3. The status action amounts to kill -0 $(cat /var/run/crond.pid).

However, if you have systemd installed, the standard init functions are rewritten in /lib/lsb/init-functions.d/40-systemd and running cron directly is not detected any more:

piotr@bialykiel:~$ sudo /usr/sbin/cron 
piotr@bialykiel:~$ sudo /etc/init.d/cron status
● cron.service - Regular background program processing daemon
   Loaded: loaded (/lib/systemd/system/cron.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Sun 2020-02-02 09:12:43 CET; 5min ago
     Docs: man:cron(8)
  Process: 734 ExecStart=/usr/sbin/cron -f $EXTRA_OPTS (code=killed, signal=TERM)
 Main PID: 734 (code=killed, signal=TERM)

since systemd would like to have a CGroup per service. What is worse start and restart won't work, since cron will fail on the already existing pidfile and stop will not kill the cron instance you created.