Linux operation and maintenance management services through systemctl

Since centOS7, a new command for managing services - systemctl has been added. Through this command, it is very convenient to manage services on the system.

Open and close the service through systemctl

The following are the options related to opening and closing the service

• start Start the service

• stop Close the service

• restart Restart the service

• ##status View Service status

• reload Reload the configuration file (without closing the service)

• enable Start the service automatically after booting

• disable Turn off auto-start at boot

Let’s learn through cases

# 查看服务运行状况
# systemctl status atd
● atd.service - Job spooling tools
   Loaded: loaded (/usr/lib/systemd/system/atd.service; enabled; vendor preset: enabled)
   Active: active (running) since Sat 2020-11-07 16:20:59 CST; 3 days ago
 Main PID: 3002 (atd)
   CGroup: /system.slice/atd.service
           └─3002 /usr/sbin/atd -f
Nov 07 16:20:59 iz8vb626ci0aehwsivxaydz systemd[1]: Started Job spooling tools.
Nov 07 16:20:59 iz8vb626ci0aehwsivxaydz systemd[1]: Starting Job spooling tools...

Through the information displayed above, we can get a lot of information. From the second line Loaded, enabled appears, indicating that the service starts automatically at boot. From the running in the third line, we know that the service is running.

Let’s demonstrate shutting down and starting the service

# systemctl stop atd
# systemctl start atd

Regarding the running status of the service, in addition to the common running, dead, etc., there are some others, which are listed below

• active(running) Running

• active(waiting): waiting for execution (wait for other services to execute before executing)

• active(exited): A service that ends normally after being executed only once

• inactive(dead): The service is not started

In addition, about starting the service There are also several options for whether to start:

• enable: Start at boot

• disable: Do not start at boot

• static: It cannot start automatically when booting, but it can be started by other self-starting services.

• mask: It will not start anyway. The service has been forced to log off.

Observe services on the system through systemctl

Using systemctl, you can check which services are currently running, and you can also check all services on the system. (Including those that are not started), you can also view a certain type of service.

list-units Lists all currently started services. If the task option is not used, this option will be used by default. Add -a to display all services, including unstarted services

--type=TYPE List services of a certain type

See the demo below

# 列出系统所有正在运行的服务
# systemctl 
  UNIT                                                     LOAD      ACTIVE SUB       DESCRIPTION
  proc-sys-fs-binfmt_misc.automount                        loaded    active running   Arbitrary Executable File Formats File System Automoun
  sys-devices-pci0000:00-0000:00:03.0-virtio0-net-eth0.device loaded    active plugged   Virtio network device
  sys-devices-pci0000:00-0000:00:04.0-virtio1-virtio\x2dports-vport1p1.device loaded    active plugged   /sys/devices/pci0000:00/0000:00:04.
  ……
  
# 列出系统正在运行的service类型的服务
# systemctl --type=service
   UNIT                               LOAD      ACTIVE SUB     DESCRIPTION
  aegis.service                      loaded    active running LSB: aegis update.
  aliyun.service                     loaded    active running aliyun-assist
  atd.service                        loaded    active running Job spooling tools
  auditd.service                     loaded    active running Security Auditing Service
# 列出系统上所有服务，包括未运行的
# systemctl -a
……

Manage different operating environments (target units) through systemctl

In centos7, there are many target units, but here we only need to know a few commonly used target units. Commonly used target units are listed below:

• graphical.target graphical mode

• multi-user.target text mode

• rescue.target General rescue mode

• emergency.target Emergency rescue mode

Next, let’s see how to check the system default operating environment, and how to modify the operating environment.

systemctl [选项] [unit.target]
[选项]
  get-default：查看系统默认操作环境
  set-default:设置系统默认操作环境
  
# 查看默认的操作环境
# systemctl get-default
multi-user.target
# 重新设置新的默认操作环境
# systemctl set-default graphical.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target.
# systemctl get-default
graphical.target

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