Scheduling Tasks

Nov 10, 2025
rhcsa

Scheduling options

  • Systemd timers are the primary solution for scheduling recurring jobs
  • crond is an older scheduling solution which is still supported and a bit easier to schedule custom tasks
  • at is available to schedule non-recurring user tasks

Systemd timers

  • Systemd provides unit.timer files that go together with unit.service files
  • When using systemd timers, the timer should be enabled/started, NOT the service unit
  • Systemd timers are often installed from RPM packages
  • In the timer unit file, the OnCalendar option specifies when the service should be started
  • Systemd timers have been the default way for scheduling recurring services since RHEL 9

Timer activation

  • Use OnCalendar to schedule a recurring jobs (it’s not the same syntax as cron, very unfortunately)
  • Use OnUnitActivateSec to start the unit a specific time after the unit was last activated
  • Use OnBootSec or OnStartupSec to start the unit a specific time after booting
  • Read man 7 systemd.time for specification of the time format to be used

cron

  • Cron is an old UNIX scheduling option
  • It uses crond, a daemon that checks its configuration to run cron jobs periodically
  • Still on RHEL 10, crond is enabled as a systemd service by default
  • The crond process checks its configuration every minute
  • /etc/crontab is the main (managed) configuration file
  • /etc/cron.d can be used for drop-in files
  • User-specific cron jobs can be created using crontab -e
  • The /etc/crontab file has a nice syntax example
  • /etc/cron.{hourly,daily,weekly,monthly} can be used as a drop-in for scripts that need to be scheduled on a regular basis
    • anacron takes care of these jobs
    • They are executed on a regular basis, but not at a specific time
    • Configuration is in /etc/anacrontab

It’s important to note that user cron jobs run even when the user is logged out, but user level systemd timers do not by default. When a user needs to schedule jobs when they aren’t logged in, cron or at may be the best option

at

  • The atd service must be running to run once-only jobs using at
  • Use at {time} to schedule a job
    • Type one or more job specs in the interactive shell
    • Use Ctrl-D to close the shell
  • Use atq for a list of jobs currently scheduled
  • Use atrm to remove jobs from the list

Temporary files

  • In the past, temporary files were created in the /tmp directory

  • Without management, these files could stay around for a long time

  • As a solution, the /tmp directory could be created on a RAM drive

  • Nowadays, systemd-tmpfiles is started while booting and manages temp files and directories

  • Functionality has been expanded to manage other files also

  • It will create and delete tmp file automatically, according to the configuration files in the following locations:

    • /usr/lib/tmpfiles.d
    • /etc/tmpfiles.d
    • /run/tmpfiles.d

  • systemd-tmpfiles works with related services to manage temporary files

  • systemd-tmpfiles-setup.service creates and removes temp files according to the configuration

  • systemd-tmpfiles-clean.timer calls the systemd-tmpfiles-clean.service to remove temp files

    • By default 15 minutes after booting and also daily

  • d /run/myfiles 0750 root root will create the directory /run/myfiles if necessary. No action if it already exists

  • D /run/myfiles 0750 root root 1d will create the directory /run/myfiles and wipe its contents if it already exists. Files older than 1 day are eligible for automatic removal the next time systemd-tmpfiles runs

  • man tmpfiles.d provides detailed information and examples

Lab Exercise

  1. Ensure the systemd timer that cleans up tmp files is enabled
  2. Create a /tmp-fast directory for all users and ensure that files older than 5 days are removed
  3. Create a /tmp-very-fast directory for all users and ensure that all files will be removed every day and on every boot
  4. Run a cron job that will issue the command touch /tmp/cronfile 2 minutes from now as user mac
  5. Use at to schedule a job to reboot your system 5 minutes from now after you ensure mac’s cron job ran
Solution
  1. systemctl status systemd-tmpfiles-clean
  2. /etc/tmpfiles.d/tmp-fast.conf 
    d /tmp-fast 1777 root root 5d
  3. /etc/tmpfiles.d/tmp-very-fast.conf 
    D /tmp-very-fast 1777 root root 1d
  4. crontab -e, local time is Fri 2025-11-21 15:00 MST 
    02 15 21 11 fri /bin/touch /tmp/cronfile
    1. The /etc/crontab file specifies a user. That is because this is the system crontab file. When a user edits their crontab file, no user is specified. Crond will interpret a user-name as a command in a user’s crontab!
    2. The system’s local time is used, not UTC
  5. — 
    at 15:05
> systemctl reboot

- Clint Jordan