About the Systemd

January 18, 2025 · 2 min read · Page View:

A real powerful tool

If you have any questions, feel free to comment below.

It has been a long time that the linux use init to manage the startup process, such as sudo /etc/init.d/apache2 start or service apache2 start, but the init is serial. To address this issue, the systemd was born. The d is the abbreviation of daemon, which means the systemd is a daemon manager. The systemd substitutes the initd and becomes the default main process PID 1.

You can check the version systemctl --version.

systemctl #

sudo systemctl reboot
sudo systemctl poweroff
sudo systemctl suspend

hostnamectl #

Look up the host info, Architecture, Hardware, Kernel, Operating System, etc.

You can also query via uname -a.

timedatectl #

Query the timezone.

loginctl #

Manage the login session.

loginctl list-sessions
loginctl list-users
loginctl show-user

Unit #

There are 12 types of units:

Service unit
Target unit which is a group of units
Device Unit
Mount Unit
Automount Unit
Path Unit
Scope Unit: not started by systemd
Slice Unit: process group
Snapshot Unit: Systemd snapshot, can switch back to a snapshot
Socket Unit: process communication socket
Swap Unit: swap file
Timer Unit

You can also query them:

systemctl list-units –all
systemctl list-units –all –state=inactive
systemctl list-units –type=service

systemctl status #

systemctl status bluetooth.service

systemctl about service #

systemctl start
systemctl stop
systemctl restart
systemctl reload
systemctl enable
systemctl disable
systemctl show service

Unit config #

The unit config file is located at /etc/systemd/system/. But most of them are the symbolic links to the real config file in /usr/lib/systemd/system/.

The systemctl enable command will create a symbolic link to the real config file in /etc/systemd/system/(if you config start on boot in the unit file, it will start on boot) And the systemctl disable command will remove the symbolic link. Such as

sudo systemctl enable [email protected]
# which is equivalent to
$ sudo ln -s '/usr/lib/systemd/system/[email protected]' '/etc/systemd/system/multi-user.target.wants/[email protected]'

You can list all the config files:

systemctl list-unit-files
# the tail is the kind of unit, such as service(default), socket, etc.

There are four status of the unit:

enabled: the unit is enabled
disabled: the unit is disabled
static: the unit is static, which only served as other unit’s dependency
masked: the unit is banned to be enabled

Adjust file #

systemctl cat atd.service can show the specific unit file.

The detail you can refer to the official document.

Once you adjust the unit file, you need to reload the systemd and restart the service:

sudo systemctl daemon-reload
sudo systemctl restart httpd.service

Target #

The target is a group of units, once the target is enabled, the units in the target will be enabled.

journalctl #

You can check the kernel log and the service log by only journalctl. The config file is /etc/systemd/journald.conf.

sudo journalctl
sudo journalctl -k # kernel log
sudo journalctl –since yesterday
sudo journalctl -f # follow the log
sudo journalctl _PID=1 # the log of the specific process
sudo journalctl -u # the log of the specific service

Refer to https://www.ruanyifeng.com/blog/2016/03/systemd-tutorial-commands.html