Systemd is a system and service manager and like most major Linux distributions the init deamon was replaced by systemd in CentOS 7. One of the main functions of systemd is to manage the services, devices, mount points, sockets and other entities in a Linux system. Each of these entity that are managed by systemd is called a unit. Each unit is defined by a unit file (configuration file) which is located in one of the following directories.
| Directory | Description |
|---|---|
| /usr/lib/systemd/system/ | Unit files distributed with installed packages. Do not modify unit files in this location. |
| /run/systemd/system/ | Unit files that are dynamically created at runtime. Changes in this directory are lost when rebooted. |
| /etc/systemd/system/ | Unit files created by systemctl enable and custome unit files created by system administrators. |
Any custom unit files that you create should be placed in the /etc/system/system/ directory. This directory takes precedence over other directories.
Unit files names are of the form
unit_name.unit_type
Unit_type can be one of the following:
| Unit Type | Description |
|---|---|
| device | A device unit. |
| service | A system service. |
| socket | A socket for inter-process communication. |
| swap | A swap file or device. |
| target | A group of units. |
| timer | A systemd timer. |
| snapshot | A snapshot of systemd manager. |
| mount | A mount point. |
| slice | A group of unit that manage the system processes. |
| path | A file or directory. |
| automount | A automount point. |
| scope | An externally created process. |
Creating a new service (systemd unit)
To create a custom service to be managed by systemd, you create a unit file that defines the configuration of that service. To create a service named MyService for example, you create a file named MyService.service in /etc/systemd/system/
# vi /etc/systemd/system/MyService.service
The unit file of service consists of a set of directives that are organized in to three sections - Unit, Service and Install. Below is an example of a very simple unit file.
[Unit] Description=Service description [Service] ExecStart=path_to_executable [Install] WantedBy=default.target
Once you have created the unit file with all the necessary configuration options, save the file and set the correct file permissions.
# chmod 664 /etc/systemd/system/MyService.service
The next step is to reload all unit files to make systemd know about the new service.
# systemctl daemon-reload
Finally to start the service, run
# systemctl start MyService.service
[Unit] Section
The following are the main directives that you specify in the [Unit] section.
| Description | A short description of the unit. |
| Documentation | A list of URIs pointing to the documentation for the unit. |
| Requires | A list of units that must be started alongside the current unit. If the any these units fail to start then current unit will not be activated. |
| Wants | Similar to the Requires directive but the difference is the current unit will be activated even if the depended units fail to start. |
| Before | List of units that cannot be started before the current unit. |
| After | The current unit can started only after the units listed here. |
| Conflicts | List units that cannot be run concurrently with the current unit. |
[Service] Section
Some of the common directives that you'll see in service section are.
| Type | Defines the startup type of the unit which can be one of the values:
|
| ExecStart | Specifies the command to the executed to start service. |
| ExecStartPre | Specifies the command to be executed before the main process specified in the ExecStart is started. |
| ExecStartPost | Specifies the command to be executed after the main process specified in the ExecStart has finished. |
| ExecStop | Specifies the command to be executed when the service is stopped. |
| ExecReload | Specifies the command to be executed when the service is restarted. |
| Restart | Specifies when to restart the service automatically. Possible values are "always", "on-success", "on-failure", "on-abnormal", "on-abort", or "on-watchdog". |
[Install] Section
The [install] section provides information required to enable or disable the units using the systemctl command. The common options are:
| RequiredBy | A list of units that requires unit. A symbolic link of this unit is created in the .requires directory of the listed unit. |
| WantedBy | Specifies a list of targets under which the service should be started. A symbolic link of this unit is created in the .wants directory of the listed target. |
Using systemctl to manage services
systemctl is the command line tool you can use to control and manage services in systemd. Let's now take a look at the some of the important systemctl commands for service management.
Listing Service Units and Unit files
To list all the units that are loaded
# systemctl list-units
To list only units of type service
# systemctl list-units -t service
To list all installed unit files of type service
# systemctl list-unit-files -t service
To list all installed unit files of type service
# systemctl list-unit-files -t service
You can use the --state option to filter the output by the state of the unit. The following command lists all services that are enabled.
# systemctl list-unit-files --state enabled
Note the difference between list-units and list-unit-files is that list-unit will only show units that are loaded while list-unit-files shows all unit files that are installed on the system.
Start and Stop service
This is quite straightforward, start option to start a service and stop option to stop a service
# systemctl start service_name.service
# systemctl stop service_name.service
Restart and Reload services
The restart option will restart a service that is running. If the service is not running, it will be started.
# systemctl restart service_name.service
If you want to restart the service only if its running then use the try-restart option.
# systemctl try-restart service_name.service
The reload option will try to reload the service specific configuration of a unit if it is supported.
# systemctl reload service_name.service
Enable and Disable services
Units can be enabled or disabled using the enable or disable options of systemctl command. When a unit a enabled symbolic links are created in various locations as specified in the [install] section of the unit file. Disabling a unit will remove the symbolic links that wer created when the unit was enabled.
# systemctl enable service_name.service
# systemctl disable service_name.service
Reload Unit Files
Whenever you make any changes to the unit files you need to let systemd know by executing daemon-reload which reloads all unit files.
# systemctl daemon-reload
Modifying system services
The unit files that come with installed packages are stored in /usr/lib/systemd/system/. The unit files in this directory should not be modified directly as the changes will be lost when if you update the package. The recommended method is to first copy the unit file to /etc/systemd/system/ and make the changes in that location. The unit files in /etc/systemd/system/ takes precedence over unit files in /usr/lib/systemd/system/ so the original unit file will be overridden.
