Scheduling Options

Updated Dec 27, 2021 ·

Cron

Cron is a scheduling tool used to execute commands or scripts at specific dates, times, or intervals.

It is stateless, meaning it doesn't remember the last time a job was run.
If the server is turned off at the scheduled time, the job will not run.
Different configuration files can be used to specify job schedule.
Cron does not have STDOUT.
Useful for re-occuring jobs, like backup jobs.

Scheduling options:

crontab -e - use as a specific user.
Can also create a cron file in /etc/cron.d

Examples:

To check all CRON jobs:
```
ll | grep cron 
```
To stop users from scheduling a cron, put them in /etc/cron.deny:
```
vim /etc/cron.deny
```
To specify users allowed and deny the rest, put them in /etc/cron.allow:
```
vim /etc/cron.allow
```
To schedule cron job (obsolete method):
```
vim /etc/crontab
```
New way to generate time-specific cron jobs:
```
vim /etc/cron.d
```

Crontab Format

The format for a cron job entry in the crontab file:

* * * * *  <user> <command or script>

First * - Minutes, 0-59
Second * - Hours, 0-23
Third * - Day of the month, 1-31
Fourth * - Month of the year, 1-12 or jan,feb,mar...
Fifth * - Day of the week, 0-6 or sun,mon,tue...

Examples:

Write "Hey!" to hello.txt every minute of every hour of every day:
```
* * * * * root echo "Hey!" >> /tmp/hello.txt
```
Run command every 5 minutes of every hour:
```
*/5 * * * * root <command>
```
Run command at 1am and then every 5 minutes afterwards:
```
*/5 1 * * * root <command>
```
Run command every 2 minutes of every 3 hours:
```
*/2 */3 * * * root <command>
```
Run every 2 minutes between 12am to 3am:
```
*/2 0-3 * * * root <command>
```
Run command every 2 minutes starting 1am, on the first 3 days of the month:
```
*/2 1 1-3 * * root <command>
```
Run command every 2 minutes starting 1am on 1st and 15th day of the month:
```
*/2 1 1,15 * * root <command>
```

Cron - Simplified

For simplified scheduling, place scripts in the appropriate directory:

To run script daily, put script here:

/etc/cron.daily

To run script hourly, put script here:

/etc/cron.hourly

To run script weekly, put script here:

/etc/cron.weekly

User Cron - Format

The format for a user-specific cron job entry:

* * * * *  <command or script>

To specify user-specific cron:

crontab -e

To view all my cron jobs:

crontab -l

To view all users with cron:

# NOTE: Files in this dir should not be edited directly
ll /var/spool/cron

To check logs of cron:

tail -f /var/log/cron

Anacron

Anacron is a periodic command scheduler, similar to cron, but it is used for running commands with a frequency of days instead of minutes or hours. Anacron ensures that jobs are run even if the machine was off during the scheduled time.

Execute jobs on a regular basis, but not at a specific time.
Takes care of the jobs in:
- /etc/cron.hourly
- /etc/cron.daily
- /etc/cron.weekly
- /etc/cron.monthly

To configure Anacron, edit the file /etc/anacrontab:

[root@localhost ~]# vim /etc/anacrontab

Example of an Anacron configuration file:

# /etc/anacrontab: configuration file for anacron
# See anacron(8) and anacrontab(5) for details.

SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

# 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

Systemd Timers

Systemd timers provide a more powerful and flexible way to schedule tasks compared to traditional cron jobs. Timers can be used to trigger systemd services at specific times or intervals.

Read man 7 systemd-timer for more information about systemd timers.
Read man 7 systemd-timer for specification of the time format to be used.

To view the manual for systemd timers:

man systemd.time

To list all timer files:

ll /usr/lib/systemd/system/*timer

In the examples above, we see timer files. All timer files have associated service files. For example, we see fstrim. To view the fstrim service and timer files:

cd /usr/lib/systemd/system
ll fstrim*

The fstrim.timer file specifies how often the fstrim.service is run:

cat fstrim.service

[Unit]
Description=Discard unused blocks

[Service]
Type=oneshot
ExecStart=/usr/sbin/fstrim -av

To view the fstrim.timer file:

cat fstrim.timer

[Unit]
Description=Discard unused blocks once a week
Documentation=man:fstrim

[Timer]
OnCalendar=weekly
AccuracySec=1h
Persistent=true

[Install]
WantedBy=timers.target

To check the status of the fstrim service:

systemctl status fstrim.service

To check the status of the fstrim timer:

systemctl status fstrim.timer

To enable and start fstrim, you enable the timer not the service:

systemctl enable fstrim.timer
systemctl start fstrim.timer
systemctl status fstrim.timer

● fstrim.timer - Discard unused blocks once a week
   Loaded: loaded (/usr/lib/systemd/system/fstrim.timer; enabled; vendor preset: disabled)
   Active: active (waiting) since Sun 2021-12-26 14:42:07 PST; 28s ago
  Trigger: Mon 2021-12-27 00:00:00 PST; 9h left
     Docs: man:fstrim

Dec 26 14:42:07 localhost systemd[1]: Started Discard unused blocks once a week.

At

The at command is used to schedule commands to be executed once at a particular time in the future. It uses the atd daemon to execute scheduled commands.

Unit atd.service could not be found.

To check the status of the atd service:

user1@localhost:system $ systemctl status atd
Unit atd.service could not be found.

user1@localhost:system $ systemctl status atd.service
Unit atd.service could not be found.

To locate atd:

user1@localhost:system $ which atd
/usr/bin/which: no atd in (/home/user1/.local/bin:/home/user1/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin)

Following this link online: How to install and use at job scheduling?

To find which package provides atd:

user1@localhost:system $ sudo yum whatprovides atd

Updating Subscription Management repositories.
Unable to read consumer identity

This system is not registered with an entitlement server. You can use subscription-manager to register.

Last metadata expiration check: 2:22:31 ago on Sun 26 Dec 2021 01:25:20 PM PST.
at-3.1.20-11.el8.x86_64 : Job spooling tools
Repo        : rhel-8-baseos-rhui-rpms
Matched from:
Filename    : /usr/sbin/atd

To get information about atd:

user1@localhost:system $ sudo yum info atd

Updating Subscription Management repositories.
Unable to read consumer identity

This system is not registered with an entitlement server. You can use subscription-manager to register.

Last metadata expiration check: 2:23:04 ago on Sun 26 Dec 2021 01:25:20 PM PST.
Error: No matching Packages to lis

To install at:

user1@localhost:system $ sudo yum install -y at

To enable and start atd:

user1@localhost:system $ sudo systemctl enable atd.service
user1@localhost:system $ sudo systemctl start atd.service

To check the status of atd:

user1@localhost:system $ systemctl status atd.service

● atd.service - Job spooling tools
   Loaded: loaded (/usr/lib/systemd/system/atd.service; enabled; vendor preset: enabled)
   Active: active (running) since Sun 2021-12-26 15:52:36 PST; 3s ago
 Main PID: 74416 (atd)
    Tasks: 1 (limit: 100840)
   Memory: 372.0K
   CGroup: /system.slice/atd.service
           └─74416 /usr/sbin/atd -f

Dec 26 15:52:36 tst-rhel systemd[1]: Started Job spooling tools.

Using `atd`

To schedule a job at 4 PM (teatime):

user1@localhost:system $ at teatime
warning: commands will be executed using /bin/sh
at> logger 'have a cup of tea'
at> mail -s 'hello root' < .
at>  <EOT>
job 1 at Sun Dec 26 16:00:00 2021

To query all scheduled jobs, run atq:

user1@localhost:system $ atq
1       Sun Dec 26 16:00:00 2021 a eden

To see the log:

$ sudo tail -f /var/log/messages

Dec 26 15:59:37 localhost systemd[1]: NetworkManager-dispatcher.service: Succeeded.
Dec 26 16:00:00 localhost systemd-logind[861]: New session 226 of user eden.
Dec 26 16:00:00 localhost systemd[1]: Started Session 226 of user eden.
Dec 26 16:00:00 localhost eden[74508]: have a cup of tea

To remove a job, use "atrm job-num":

$ atrm 1
Cannot find jobid 1
$ atq

Since the job has ran already, it won't appear anymore.

Managing Temporary Files

Systemd provides mechanisms for the creation, deletion, and cleaning of temporary files using the tmpfiles.d configuration.

/usr/lib/tmpfiles.d manages setting for creating, deleting, and cleaning up temporary files.
systemd-tmpfiles-clean.timer unit can be configured to automatically clearn up temporary files.
- Triggers the systemd-tmpfiles-clean.service.
- Runs systemd-tmpfiles --clean.

To view the manual for tmpfiles.d:

man tmpfile
man tmpfiles.d

Example of the tmpfiles.d configuration files:

$ systemctl cat systemd-tmpfiles-
systemd-tmpfiles-clean.service      systemd-tmpfiles-setup-dev.service
systemd-tmpfiles-clean.timer        systemd-tmpfiles-setup.service

To check the clean.timer:

$ systemctl cat systemd-tmpfiles-clean.timer

# /usr/lib/systemd/system/systemd-tmpfiles-clean.timer
#  SPDX-License-Identifier: LGPL-2.1+
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Daily Cleanup of Temporary Directories
Documentation=man:tmpfiles.d(5) man:systemd-tmpfiles(8)

[Timer]
OnBootSec=15min
OnUnitActiveSec=1d

Automatic File Cleanup

The usr/lib/tmpfiles.d/tmp.conf contains the settings for the automatic tmp file cleanup.

When making any changes, you should create a copy of /usr/lib/tmpfiles.d/tmp.conf to /etc/tmpfiles.d. After making modifications, run the command to ensure the files doesn't contain any errors:

systemd-tmpfiles --clean /etc/tmpfiles.d/tmp.conf

Modifying tmp.conf

To modify tmp.conf and manage temporary files using systemd's tmpfiles.d mechanism, follow these steps:

Copy the tmp.conf File: You should copy the original tmp.conf from /usr/lib/tmpfiles.d/ to /etc/tmpfiles.d/ where local modifications are intended.
```
$ sudo cp /usr/lib/tmpfiles.d/tmp.conf /etc/tmpfiles.d/
```
Edit tmp.conf: Use your preferred text editor (e.g., vim) to modify /etc/tmpfiles.d/tmp.conf. For example:
```
$ sudo vim /etc/tmpfiles.d/tmp.conf
```
Modify the tmp.conf file according to your requirements. For instance, to clean /tmp every 5 days instead of 10 days, adjust the q line:
```
# Clear tmp directories separately, to make them easier to override
q /tmp 1777 root root 5d
q /var/tmp 1777 root root 30d
```
Save and exit the editor after making your changes.
Cleaning Temporary Files:
- To create and manage a custom temporary directory using a separate configuration file (mytemp.conf), follow these steps:
```
$ sudo vim /etc/tmpfiles.d/mytemp.conf
```
  Add a line to create a temporary directory /run/mytemp with appropriate permissions and cleanup time:
```
d /run/mytemp 0700 root root 30s
```
  Save and exit the editor.
- Apply the configuration changes using systemd-tmpfiles:
```
$ sudo systemd-tmpfiles --create /etc/tmpfiles.d/mytemp.conf
```
- Verify the creation of the temporary directory and file:
```
$ sudo ls -l /run/mytemp/
-rw-r--r--. 1 root root 0 Dec 26 16:31 myfile
```
- Clean up temporary files using systemd-tmpfiles:
```
$ sudo systemd-tmpfiles --clean /etc/tmpfiles.d/tmp.conf
```
  This command cleans up according to the modified rules in /etc/tmpfiles.d/tmp.conf.

Cron​

Crontab Format​

Cron - Simplified​

User Cron - Format​

Anacron​

Systemd Timers​

At​

Unit atd.service could not be found.​

Using atd​

Managing Temporary Files​

Automatic File Cleanup​

Modifying tmp.conf​