Systemd.md

Systemd

Common commands

systemctl status

List all services systemctl list-units -t service

List dependencies systemctl list-dependencies A good overview of what is "on" and what is "off".

To enable the service to start at boot systemctl enable name.service

To manually start a service systemctl start name.service

To restart a service systemctl restart name.service

Reload config systemctl reload name.service reload will reload a specific service. That means that the systemd will send a SIGHUP signal to a service, and that signal will tell the service to reload its configuration files, which has nothing to do with systemd config files.

Reload custom service files systemctl daemon-reload Location of custom systemd units is "/etc/systemd/system". This command reloads the service files in that directory.

Check the configuration of a unit file systemctl cat sshd.service

Edit unit file systemctl edit sshd.service By default it uses Nano. To change that to Vim. export SYSTEMD_EDITOR=/usr/bin/vim This creates a drop-in-file in "/etc/systemd/system/" If it does not create the drop-in-file automatically, do systemctl daemon-reload

To see tunables for a service do systemctl show httpd.service

/user/lib/systemd/system/ is for configuration files provided by packages. Do not edit those directly since they can be overwritten by newer packages.

/etc/tuned/ and /usr/lib/tuned/ need an explanation for it here.

Mask To prevent certain units from starting up, use systemctl mask. It links a unit to the /dev/null device, which ensures it cannot be started. For instance systemctl mask nginx

systemctl unmask removes the unit mask.

Systemd Journal (journalctl)

Show all boots that have been logged. Needs to have persistent journalling. journalctl --list-boots

journalctl -xrb -x = Augment log lines with explanation texts from the message catalog. This will add explanatory help texts to log messages -r = Reverse output so that the newest entries are displayed first. -b = Show messages from a specific boot. This will add a match for "_BOOT_ID=". The argument may be empty, in which case logs for the current boot will be shown.

How to make the journal persistent

You could use rsyslog to make the journal persistent.

"/etc/systemd/journal.conf" The setting "Storage=auto" ensures that persistent storage is happening automatically after manually creating the directory "/var/log/journal"

Then we need to restart the journal service. systemctl restart systemd-journal-flush.service

Common commands

View only messages with a priority error and higher. journalctl -p err

View the last 10 lines, and adds new messages when they are added. journalctl -f

Show messages for the sshd service only. journalctl -u sshd.service

See space used by Journalctl

See current settings for growth and what it's currently using. journalctl | grep -E 'Runtime Journal|System Journal'

Systemd Timers (Scheduling)

When using systemd timers, the timer should be started, and not the service unit.

systemctl list-units -t timer systemctl list-unit-files dnf-makecache* systemctl status dnf-makecache.timer Checkout the "Trigger and Triggers".

To schedule and activate a timer you use the "OnCalendar" option.

OnCalendar=*:00/20 runs every 20 minutes.

You can use "OnUnitActiveSec" to start the unit at a specific time after the unit was last activated.

You can use "OnBootSec" or "OnStartupSec" to start the unit a specific time after booting.

Working with Tuned

Install tuned. dnf install tuned

To see available commands, type this in and press double tab. tuned-adm

To see available profiles. tune-adm list

Config files for Tuned are located in "/usr/lib/tuned/".