Crond Demystified !!

In Linux Fedora Core 1, "crond" is started from the script "/etc/rc.d/init.d/crond". The manual `man cron` says that cron searches "/var/spool/cron" for crontab files which are named after accounts in "/etc/passwd"; crontabs found are loaded into memory. Cron also searches for "/etc/crontab" and the files in the "/etc/cron.d/" directory.

The "/etc/crontab" file contains the following.

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

The "/etc/cron.d" directory contains nothing. So clearly, all system cron are in the /etc/cron.* files. The above commands mean that the program "run-parts" will be run as user "root" for each of the scripts.

The command "run-parts" turns out to be the script "/usr/bin/run-parts", as follows.

#!/bin/bash
 
# run-parts - concept taken from Debian
 
# keep going when something fails
set +e
 
if [ $# -lt 1 ]; then
        echo "Usage: run-parts <dir>"
        exit 1
fi
 
if [ ! -d $1 ]; then
        echo "Not a directory: $1"
        exit 1
fi
 
# Ignore *~ and *, scripts
for i in $1/*[^~,] ; do
        [ -d $i ] && continue
        # Don't run *.{rpmsave,rpmorig,rpmnew,swp} scripts
        [ "${i%.rpmsave}" != "${i}" ] && continue
        [ "${i%.rpmorig}" != "${i}" ] && continue
        [ "${i%.rpmnew}" != "${i}" ] && continue
        [ "${i%.swp}" != "${i}" ] && continue
        [ "${i%,v}" != "${i}" ] && continue
 
        if [ -x $i ]; then
                $i 2>&1 | awk -v "progname=$i" \
                              'progname {
                                   print progname ":\n"
                                   progname="";
                               }
                               { print; }'
        fi
done
 
exit 0

In essence, this just runs all of the scripts in the specified directory. E.g. each hour, the executable plain files in /etc/cron.hourly are run.

See also crontab tutorial.