Systemd Services Start and Stop

 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 systemd has a superceded sysvinit.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	Starts, then stops 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.