systemd is the new way of running services on Linux. systemd has a superceded sysvinit. systemd brings faster boot-times to Linux and is now, a standard way to manage Linux services. While stable, systemd is still evolving.
systemd as an init system, is used to manage both services and daemons that need status changes after the Linux kernel has been booted. By status change starting, stopping, reloading, and adjusting service state is applied.
First, let's check the version of systemd currently running on our server.
[centos@localhost ~]$ systemctl --version systemd 219 +PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 -SECCOMP +BLKID +ELFUTILS +KMOD +IDN [centos@localhost ~]$
As of CentOS version 7, fully updated at the time of this writing systemd version 219 is the current stable version.
We can also analyze the last server boot time with systemd-analyze
[centos@localhost ~]$ systemd-analyze Startup finished in 1.580s (kernel) + 908ms (initrd) + 53.225s (userspace) = 55.713s [centos@localhost ~]$
When the system boot times are slower, we can use the systemd-analyze blame command.
[centos@localhost ~]$ systemd-analyze blame 40.882s kdump.service 5.775s NetworkManager-wait-online.service 4.701s plymouth-quit-wait.service 3.586s postfix.service 3.121s systemd-udev-settle.service 2.649s tuned.service 1.848s libvirtd.service 1.437s network.service 875ms packagekit.service 855ms gdm.service 514ms firewalld.service 438ms rsyslog.service 436ms udisks2.service 398ms sshd.service 360ms boot.mount 336ms polkit.service 321ms accounts-daemon.service
When working with systemd, it is important to understand the concept of units. Units are the resources systemd knows how to interpret. Units are categorized into 12 types as follows −
- .service
- .socket
- .device
- .mount
- .automount
- .swap
- .target
- .path
- .timer
- .snapshot
- .slice
- .scope
For the most part, we will be working with .service as unit targets. It is recommended to do further research on the other types. As only .service units will apply to starting and stopping systemd services.
Each unit is defined in a file located in either −
/lib/systemd/system − base unit files
/etc/systemd/system − modified unit files started at run-time
Manage Services with systemctl
To work with systemd, we will need to get very familiar with the systemctl command. Following are the most common command line switches for systemctl.
| Switch | Action |
|---|---|
| -t | Comma separated value of unit types such as service or socket |
| -a | Shows all loaded units |
| --state | Shows all units in a defined state, either: load, sub, active, inactive, etc.. |
| -H | Executes operation remotely. Specify Host name or host and user separated by @. |
Basic systemctl Usage
systemctl [operation] example: systemctl --state [servicename.service]
For a quick look at all the services running on our box.
[root@localhost rdc]# systemctl -t service UNIT LOAD ACTIVE SUB DESCRIPTION abrt-ccpp.service loaded active exited Install ABRT coredump hook abrt-oops.service loaded active running ABRT kernel log watcher abrt-xorg.service loaded active running ABRT Xorg log watcher abrtd.service loaded active running ABRT Automated Bug Reporting Tool accounts-daemon.service loaded active running Accounts Service alsa-state.service loaded active running Manage Sound Card State (restore and store) atd.service loaded active running Job spooling tools auditd.service loaded active running Security Auditing Service avahi-daemon.service loaded active running Avahi mDNS/DNS-SD Stack blk-availability.service loaded active exited Availability of block devices bluetooth.service loaded active running Bluetooth service chronyd.service loaded active running NTP client/server
Stopping a Service
Let's first, stop the bluetooth service.
[root@localhost]# systemctl stop bluetooth [root@localhost]# systemctl --all -t service | grep bluetooth bluetooth.service loaded inactive dead Bluetooth service [root@localhost]#
As we can see, the bluetooth service is now inactive.
To start the bluetooth service again.
[root@localhost]# systemctl start bluetooth [root@localhost]# systemctl --all -t service | grep bluetooth bluetooth.service loaded active running Bluetooth service [root@localhost]#
Note − We didn't specify bluetooth.service, since the .service is implied. It is a good practice to think of the unit type appending the service we are dealing with. So, from here on, we will use the .service extension to clarify we are working on service unit operations.
The primary actions that can be performed on a service are −
| Start | Starts the service |
| Stop | Stops a service |
| Reload | Reloads the active configuration of a service w/o stopping it (like kill -HUP in system v init) |
| Restart | Stops, then starts a service |
| Enable | Starts a service at boot time |
| Disable | Stops a service from automatically starting at run time |
The above actions are primarily used in the following scenarios −
| Start | To bring a service up that has been put in the stopped state. |
| Stop | To temporarily shut down a service (for example when a service must be stopped to access files locked by the service, as when upgrading the service) |
| Reload | When a configuration file has been edited and we want to apply the new changes while not stopping the service. |
| Restart | In the same scenario as reload, but the service does not support reload. |
| Enable | When we want a disabled service to run at boot time. |
| Disable | Used primarily when there is a need to stop a service, but it starts on boot. |
To check the status of a service −
[root@localhost]# systemctl status network.service network.service - LSB: Bring up/down networking Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled) Active: active (exited) since Sat 2017-01-14 04:43:48 EST; 1min 31s ago Docs: man:systemd-sysv-generator(8) Process: 923 ExecStart = /etc/rc.d/init.d/network start (code=exited, status = 0/SUCCESS) localhost.localdomain systemd[1]: Starting LSB: Bring up/down networking... localhost.localdomain network[923]: Bringing up loopback interface: [ OK ] localhost.localdomain systemd[1]: Started LSB: Bring up/down networking. [root@localhost]#
Show us the current status of the networking service. If we want to see all the services related to networking, we can use −
[root@localhost]# systemctl --all -t service | grep -i network network.service loaded active exited LSB: Bring up/ NetworkManager-wait-online.service loaded active exited Network Manager NetworkManager.service loaded active running Network Manager ntpd.service loaded inactive dead Network Time rhel-import-state.service loaded active exited Import network [root@localhost]#
For those familiar with the sysinit method of managing services, it is important to make the transition to systemd. systemd is the new way starting and stopping daemon services in Linux.
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