cron

Crontab

Print crontabs for all users:

find /var/spool/cron -type f -printf 'USER: %u \n' -exec cat {} \;

[root@rocky8 ~]# find /var/spool/cron -type f -printf 'USER: %u \n' -exec cat {} \;
USER: root 
* * * * * sleep 2
USER: vagrant 
* * * * * sleep 3

crontab syntax

[root@rhel9-a ~]# cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed

Helpful crontab syntax editor: crontab guru

Useful links/references

• ArchWiki - cron
• Crontab - Quck Reference
• crontab guru - "The quick and simple editor for cron schedule expressions..."
• Cron cheatsheet
• LinuxUpskillChallenge: Day 10 - Scheduling Tasks - I rewrote/edited this
• Arabesque: Cron best practices

Monitoring cronjobs

Monitoring cronjobs with Prometheus...

• Arabesque: Cron best practices. See section "Don't throw away useful output"
• Monitor cron jobs with Prometheus, Grafana and Node exporter - Example use-case that pipes cronjob status to a Bash script that then writes to a Prometheus textfile collector.
• Prometheus textfile-collector: Cronjob Monitoring Script - My implementation submitted as a PR to prometheus-textfile-collector-scripts community repo.