Run tasks on a schedule

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$ als gewone gebruiker# als root (of met sudo)

Examendoelen in dit hoofdstuk

Taken plannen met at, cron en systemd-timers

Choose the right scheduler

Pick the tool by the kind of task.

Tool	For	Example
systemd timer	Repeating tasks, with logs in the journal and dependencies	Clean temp files every day
cron	Repeating tasks, simple one-line format	Backup every night at 02:00
anacron	Daily/weekly jobs on machines that are not always on	Laptop: run once a day when it is on
at	One task, once, at a later time	Restart a service tonight at 23:00

12.2

Use systemd timers

A timer unit starts a service unit with the same name at the times you set. Logs go to the journal.

$systemctl list-timers --all
$systemctl cat fstrim.timer
$systemctl cat logrotate.timer

Create the service that does the work.

/etc/systemd/system/backup-etc.service

[Unit]
Description=Back up /etc
 
[Service]
Type=oneshot
ExecStart=/usr/bin/tar -czf /var/backup/etc-%H.tgz /etc

Create the timer with the same name.

/etc/systemd/system/backup-etc.timer

[Unit]
Description=Back up /etc every day at 02:30
 
[Timer]
OnCalendar=*-*-* 02:30:00
Persistent=true
 
[Install]
WantedBy=timers.target

Enable the timer (not the service) and check it.

#mkdir -p /var/backup
#systemctl daemon-reload
#systemctl enable --now backup-etc.timer
$systemctl list-timers backup-etc.timer

Timer setting	Meaning
`OnCalendar=daily`	Every day at 00:00 (also `hourly`, `weekly`, `Mon..Fri 08:00`)
`OnBootSec=15min`	15 minutes after boot
`OnUnitActiveSec=1h`	1 hour after the last run
`Persistent=true`	Run missed jobs after the machine was off
`RandomizedDelaySec=10m`	Start up to 10 minutes later at random

Onthoud

Test a time expression with systemd-analyze calendar 'Mon..Fri 08:00'.

12.3

Manage the cron service

The crond daemon checks every minute if a job must run. It is installed and enabled on RHEL by default.

$systemctl status crond
$ls /etc/cron.d/ /etc/cron.daily/ /var/spool/cron/

12.4

Read the cron time format

A cron line has 5 time fields, then the command.

crontab line

# min  hour  day-of-month  month  day-of-week   command
  30   2     *             *      *             /usr/local/bin/backup.sh

Field	Values
Minute	0–59
Hour	0–23
Day of month	1–31
Month	1–12 or jan–dec
Day of week	0–7 (0 and 7 = Sunday) or sun–sat

Example	Runs
`0 * * * *`	Every hour at minute 0
`/15 * * *`	Every 15 minutes
`0 8-17 * * 1-5`	Every hour from 8 to 17, Monday to Friday
`0 0 1 * *`	The first day of every month at midnight
`30 23 * * 0`	Sunday at 23:30
`@reboot`	Once at every boot

Let op

When you fill in both day of month and day of week, cron runs when either matches, not both.

12.5

Create cron jobs

Users edit their own job list with crontab -e. root can edit the list of any user. System jobs go in files in /etc/cron.d/.

Edit your own crontab.
```
$crontab -e
```

Add a line, save and leave the editor.

crontab -e

*/10 * * * * /usr/bin/df -h / >> /var/tmp/disk.log

List it, and as root manage the jobs of another user.

$crontab -l
#crontab -e -u lisa
#crontab -l -u lisa

For system jobs, use a file in /etc/cron.d with an extra user field.
/etc/cron.d/cleanup
```
0 3 * * * root find /var/tmp -type f -mtime +7 -delete
```

Place	Who writes it	User field
`crontab -e` (saved in `/var/spool/cron/USER`)	Each user	No
`/etc/crontab`, `/etc/cron.d/*`	root, packages	Yes
`/etc/cron.hourly/`, `daily/`, `weekly/`, `monthly/`	Scripts that run at that interval	—

Examentip

A task like "user X must run Y every day at 14:23" = crontab -e -u X and the line 23 14 * * * Y. Check with crontab -l -u X.

Op het werk

cron jobs run with a very small environment (short PATH, no aliases). Use full paths and send output to a log, or it goes to local mail.

12.6

Understand anacron

anacron runs the daily, weekly and monthly jobs even when the machine was off at the planned time. On RHEL, cron starts anacron every hour; anacron runs what is due.

$cat /etc/anacrontab
$ls /var/spool/anacron/

/etc/anacrontab (part)

#period in days   delay in minutes   job-identifier   command
1                 5                  cron.daily       nice run-parts /etc/cron.daily
7                 25                 cron.weekly      nice run-parts /etc/cron.weekly
@monthly          45                 cron.monthly     nice run-parts /etc/cron.monthly

Notitie

To run a script daily in a simple way, put it (executable, without a dot in the name) in /etc/cron.daily/.

12.7

Control who may use cron

Two files decide which users may use crontab: /etc/cron.allow and /etc/cron.deny.

Situation	Result
`cron.allow` exists	Only users listed in it (and root)
Only `cron.deny` exists	Everyone except the users in it (RHEL default: empty deny file)

#echo lisa >> /etc/cron.deny
$crontab -e
You (lisa) are not allowed to use this program (crontab)

at uses the same idea with /etc/at.allow and /etc/at.deny.

12.8

Run a task once with at

at TIME reads commands and runs them once at that time. The atd service must run.

Make sure atd runs.

#dnf install -y at
#systemctl enable --now atd

Plan a task: type the commands, then press Ctrl+D.

$at 23:00
at> systemctl restart httpd
at> <EOT>
job 3 at Sun Oct 25 23:00:00 2026

Plan with a pipe, list and remove jobs.

$echo "tar -czf /tmp/home.tgz /home/lisa" | at now + 2 hours
$atq
$at -c 3
$atrm 3

Time form	Example
Clock time	`at 16:30`, `at 4pm`
Relative	`at now + 30 minutes`, `at now + 2 days`
Date	`at 09:00 2026-12-31`, `at noon tomorrow`

batch runs a job when the system load is low (under 0.8 by default).

12.9

Remember the main points

The most important facts of this chapter, in short.

RHEL has three ways to plan jobs: systemd timers, cron (repeating jobs) and at (one time).
A cron line has 5 time fields: minute, hour, day of month, month, day of week — then the command.
crontab -e edits your own jobs; crontab -e -u USER edits jobs of another user (as root).
System cron jobs go in /etc/crontab, /etc/cron.d/ and the /etc/cron.hourly|daily|weekly|monthly folders.
anacron runs daily, weekly and monthly jobs that were missed while the system was off.
/etc/cron.allow and /etc/cron.deny (and the same for at) control who may plan jobs.
at 14:00 runs commands once; atq lists and atrm removes jobs.
A timer unit (NAME.timer) starts a service unit with OnCalendar= or OnBootSec=.

12.10

Learn the key terms

The words of this chapter in simple English.

Term	Meaning
cron / crond	The service that runs planned jobs.
crontab	The table of cron jobs of one user.
Cron fields	The 5 time fields of a cron line.
anacron	Runs periodic jobs that were missed.
at	Runs a job one time at a set time.
Timer unit	A systemd unit that starts a service on a schedule.
OnCalendar	The time rule in a timer, like `--* 02:00:00`.
cron.allow / cron.deny	Lists of users who may / may not use cron.