Systemd on Linux – Manage Services, Run Levels and Logs

Systemd is not only a service manager but also a system manager for Linux. It’s designed to be backward compatible with SysV init scripts and used by many popular Linux distributions like Debian 8+, Ubuntu 15.04+, Fedora, Redhat 7+ /CentOS 7+, Arch Linux, OpenSUSE. So it will be very helpful to know how to use systemd.

This tutorial shows you some basic commands that will be useful to manage services, run levels, and logs. First, you might want to know the systemd version on your Linux distribution, run

systemd --version

or

systemctl --version

systemd check version

Managing Services

Systemd can help you start, stop, restart services and also allows you to auto-start services at boot time.

To list active systemd services, run

systemctl list-units --type service

Check the status of a specific service, for example, SSH daemon.

systemctl status ssh

To start a service, use the start subcommand:

sudo systemctl start ssh

Check if it’s active (running)

systemctl is-active ssh

Enable a service to autostart at boot time

sudo systemctl enable ssh

Check if it’s enabled

systemctl is-enabled ssh

Prevent a service from starting up at boot time

sudo systemctl disable ssh

Stop a service

sudo systemctl stop ssh

To prevent a service from starting manually, use the mask subcommand

sudo systemctl mask ssh

Masked services can’t be started with systemctl start command until it’s unmasked.

sudo systemctl unmask ssh

Sometimes, you may find an unknown process is running with the htop command. You want to stop it, but don’t know which systemd service it belongs to. In this case, you can run the following command

systemctl status

This command will give you a list of active systemd service units and the commands used to start them.

Managing Run Levels

The concept of run levels is replaced by targets in systemd. The multi-user.target is equivalent to run level 3 and the graphical.target is equivalent to run level 5. You can still use the runlevel command to show the current run level.

runlevel

Use the following command to check the status of the default target, which by default is usually the graphical target (run level 5).

systemctl status default.target

systemd default boot target

You use systemctl isolate command to change target. For example, change to the multi-user.target (run level 3):

sudo systemctl isolate multi-user.target

And to change back to graphical target (run level 5):

sudo systemctl isolate graphical.target

To set multi-user.target as the default target, use the following command:

sudo systemctl set-default multi-user.target

This command creates a symbolic link.

systemd default target

If you reboot now, you will be taken to multi-user target.

multi-user-target

To list active targets, run

systemctl list-units --type target

Managing Logs

The systemd software suite includes a journalctl utility which can be used to manage logs on Linux. Linux traditionally saves logs under /var/log/ directory. On a Linux distribution with systemd, you may not be able to find logs such as postfix mail logs (/var/log/mail.log) in that directory.

You can view recent logs with the following command. You can press F key to jump to the next page and press B key to going back to the previous page. Press Q to quit.

sudo journalctl

Use grep to search for logs related to your search term

sudo journalctl | grep <search term>

View logs since the current boot

sudo journalctl -b

View logs since the previous boot

sudo journalctl -b -1

Here’s a little trick to show logs being generated in real time.

sudo journalctl -f

journalctl examples

Show logs of a unit, for instance, the ssh service unit.

sudo journalctl -u ssh

To view Postfix logs, run

sudo journalctl -u postfix

Some packages may still be configured to save logs to plain files such as the Postfix log files on Ubuntu (/var/log/mail.log), so you can use a command-line text editor to peruse the logs. Sometimes a systemd service might stop producing log messages, you can try to restart rsyslog to solve this problem.

sudo systemctl restart rsyslog

Conflicting Services

When two services shouldn’t run at the same time, you can use the Conflicts parameter. For example, I always use a VPN on my Ubuntu desktop computer. It’s a self-hosted VPN running on a Vultr VPS. Then I sometimes use Transmission to download torrents. When I start Transmission, I must close the VPN connection, because Vultr will shut down your account if you use their servers to download torrents (I mean downloading copyright-infringing materials via BitTorrent).

I can edit the transmission-daemon.service.

sudo nano /lib/systemd/system/transmission-daemon.service

Add the following line in the [Unit] section.

Conflicts=openconnect.service

systemd conflicting services

Save and close the file. Then reload systemd for the changes to take effect.

sudo systemctl daemon-reload

openconnect.service is the service that starts VPN connection on my computer. Now if I start transmission-daemon.service, openconnect.service will be automatically shut down, and vice versa.

Wants

If you want service B to start when service A starts, you can use the Wants parameter to make it happen. In the [Unit] section of a systemd .service file, add a line like this:

[Unit]
Wants = b.service

You can also use Before = or After = to specify the order.

[Unit]
Wants = b.service
Before = b.service

Sometimes, you may want service B to wait for service A to complete before starting itself. Then you can add the following lines in the [Service] section of the b.service file.

[Service]
ExecStartPre=/bin/sleep 20

This tells b.service to wait 20 seconds.

Remember to reload systemd for the changes to take effect.

sudo systemctl daemon-reload

Run /etc/rc.local Scripts

If you are running a Linux distro that uses Systemd, you may find that your commands in /etc/rc.local file would not run at system boot time. The following tutorial explains how to enable /etc/rc.local script to run on system startup.

Accounting

Systemd allows you to check how much CPU and memory is used by a service and how many tasks are running by the service. If you are using a recent Linux distro like Ubuntu 20.04, Debian 10, RHEL 8, accounting is enabled by default. On old Linux distros like Ubuntu 18.04, you need to manually enable accounting. Let’s say you want to enable accounting for Nginx on Ubuntu 18.04, then follow the instruction below.

First, we make a directory to store custom systemd configurations for Nginx.

sudo mkdir -p /etc/systemd/system/nginx.service.d/

Create the custom config file under this directory. The filename extension must be .conf.

sudo nano /etc/systemd/system/nginx.service.d/custom.conf

Then add the following lines in this file.

[Service]
CPUAccounting=true
MemoryAccounting=true
TasksAccounting=true

Save and close the file. Then reload systemd.

sudo systemctl daemon-reload

There’s no need to restart Nginx. Now you can check Nginx status with:

systemctl status nginx

systemd accounting

Custom Configuration for Template Systemd Service

You may have seen a template systemd service that contains the @ symbol like the Syncthing service.

/lib/systemd/system/[email protected]

This kind of service allows you to run multiple instances. For example, you can run Syncthing for two users with:

sudo systemctl start [email protected]user1.service
sudo systemctl start [email protected]user2.service

If you want to add custom configuration to this kind of service, you need to create a folder for each of these services.

sudo mkdir -p /etc/systemd/system/[email protected]user1.service.d
sudo mkdir -p /etc/systemd/system/[email protected]user2.service.d

Then you can create the custom configuration file.

sudo nano /etc/systemd/system/[email protected]user1.service.d/custom.conf
sudo nano /etc/systemd/system/[email protected]user2.service.d/custom.conf

After saving your changes. Reload systemd.

sudo systemctl daemon-reload

Wrapping Up

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3 Responses to “Systemd on Linux – Manage Services, Run Levels and Logs

  • Sacha Wharton
    2 months ago

    Absolutely brilliant! Thank you so much for all the amazing articles you put out on Linux!!!

  • I hate typing out “systemctl” every time I want to call it up. So I just put an “alias sc=systemctl” (and “alias jc=journalctl”) in my .bashrc, and save myself some annoying repetitive typing. Just a suggestion.

  • Awesome source of information about systemd and the commands to manage it. Thanks very much!

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