Red hat linux service

Chapter 12. Introduction to systemd

As a system administrator, you can manage critical aspects of your system with systemd . Serving as a system and service manager for Linux operating systems, systemd software suite provides tools and services for controlling, reporting, and system initialization. Key features of systemd include:

• Parallel start of system services during boot
• On-demand activation of daemons
• Dependency-based service control logic

The basic object that systemd manages is a systemd unit , a representation of system resources and services. A systemd unit consists of a name, type and a configuration file that defines and manages a particular task. You can use unit files to configure system behavior. See the following examples of various systemd unit types:

Service Controls and manages individual system services. Target Represents a group of units that define system states. Device Manages hardware devices and their availability. Mount Handles file system mounting. Timer Schedules tasks to run at specific intervals.

To display all available unit types:

# systemctl -t help

12.1. Systemd unit files locations

You can find the unit configuration files in one of the following directories:

Table 12.1. systemd unit files locations

systemd unit files distributed with installed RPM packages.

systemd unit files created at run time. This directory takes precedence over the directory with installed service unit files.

systemd unit files created by using the systemctl enable command as well as unit files added for extending a service. This directory takes precedence over the directory with runtime unit files.

The default configuration of systemd is defined during the compilation and you can find the configuration in the /etc/systemd/system.conf file. By editing this file, you can modify the default configuration by overriding values for systemd units globally.

For example, to override the default value of the timeout limit, which is set to 90 seconds, use the DefaultTimeoutStartSec parameter to input the required value in seconds.

DefaultTimeoutStartSec=required value

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How to use systemctl to manage Linux services

Suppose you’re making configuration changes to a Linux server. Perhaps you just fired up Vim and made edits to the /etc/ssh/sshd_config file, and it’s time to test your new settings. Now what?

Services such as SSH pull their settings from configuration files during the startup process. To let the service know about changes to the file, you need to restart the service so that it rereads the file. You can use the systemctl command to manage services and control when they start.

Restart a service

After editing the /etc/ssh/sshd_config file, use the systemctl restart command to make the service pick up the new settings:

$ sudo systemctl restart sshd

You can verify the service is running by using the status subcommand:

$ sudo systemctl status sshd

Stop and start a service

Perhaps while troubleshooting you need to stop a service to determine whether it is the culprit or interfering with some other process. Use the stop subcommand for this:

Once you determine if this service is associated with the issue, you can restart it:

$ sudo systemctl start sshd

Training & certification

While the restart subcommand is useful for refreshing a service’s configuration, the stop and start features give you more granular control.

Control whether the service starts with the system

One consideration with using stop and start is that the two commands apply only to the current runtime. The next time you boot the system, the service will either start or not start, depending on its default settings. You can use the enable and disable subcommands to manage those defaults.

When you disable the service, it doesn’t start the next time the system boots. You might use this setting as part of your security hardening process or for troubleshooting:

$ sudo systemctl disable sshd

Reboot the system with reboot sudo systemctl reboot , and the service won’t automatically start.

You may determine that you need the service to start automatically. In that case, use the enable subcommand:

$ sudo systemctl enable sshd

The enable subcommand doesn’t start a service, it only marks it to start automatically at boot. To enable and start a service at the same time, use the —now option:

$ sudo systemctl enable --now sshd

Mask a service

You can manually start a disabled service with the systemctl start command after the system boots. To prevent this, use the mask subcommand. Masking the service links its configuration to /dev/null . A user or process will not be able to start this service at all (whereas with a disabled service, a user or process can still start it). Use the unmask subcommand to reverse the setting:

Display all subcommands

Bash’s built-in tab-completion feature is one of my favorite tricks for systemctl (and other commands). When working with commands that support subcommands, this feature saves you a lot of time. Simply type systemctl and add a space, then tap the Tab key twice. Bash displays all available subcommands.

The challenge

Do you think you’re ready to use systemctl to manage your services? Fire up a lab virtual machine and choose a service to work with. Don’t do this on a production system! Make sure you can accomplish the following tasks:

Automation advice

Wrap up

Many management tasks involve the systemctl command, but the ones covered above represent the majority of them. Service management is critical, especially when editing configuration files and hardening a system. Plan to be confident, competent, and quick at using systemctl and its common subcommands.

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