Systemctl User Guide: Comparison between CentOS 7.x systemd and CentOS 6.x daemon

Systemctl User Guide: Comparison between CentOS 7.x systemd and CentOS 6.x daemon

Starting from CentOS 7.x, CentOS began to use the systemd service to replace daemon. The commands related to the management system startup and management system services were all replaced by the systemctl command.

1. Comparison between original service commands and systemctl commands

Daemon command	Systemctl command	Description
Service [service] start	Systemctl start [unit type]	Start the service
Service [service] stop	Systemctl stop [unit type]	Stop Service
Service [service] restart	Systemctl restart [unit type]	Stop Service

In addition, the two systemctl parameters do not match the service command parameters.

Status: parameter to view the service running status reload: reload the service, load the updated configuration file (not all services support this parameter, such as network. service)

Example:

# Start network service systemctl start network. service # stop network service systemctl stop network. service # restart network service systemctl restart network. service # view network service status systemctl status network. serivce

2. Comparison between the original chkconfig command and the systemctl command

2.1 set startup/no startup

Daemon command	Systemctl command	Description
Chkconfig [SERVICE] on	Systemctl enable [unit type]	Set Service Startup
Chkconfig [SERVICE] off	Systemctl disable [unit type]	Disable device Service Startup

Example:

# Stop the CPU power management service systemctl stop cups. service # disable the cups service from starting systemctl disable cups. service # view the status of the cups service systemctl status cups. service # reset the cups service to start systemctl enable cups. service

2.2 view all services on the System
Command Format:

systemctl [command] [–type=TYPE] [–all]

Parameters:

Command
List-units: lists all started units based on the unit. -All is added to list the units not started;
List-unit-files: list the startup files based on the startup files in/usr/lib/systemd/system /.
-Type = TYPE
Unit type, mainly including service, socket, target

Example:

Systemctl command	Description
Systemctl	List all system services
Systemctl list-units	List all startup units
Systemctl list-unit-files	List all startup files
Systemctl list-units-type = service-all	List units of all service types
Systemctl list-units-type = service-all | grep cpu	List services with cpu power management mechanism
Systemctl list-units-type = target-all	List all targets

3. Special usage of systemctl

Systemctl command	Description
Systemctl is-active [unit type]	Check whether the service is running
Systemctl is-enable [unit type]	Check whether the service is set to boot
Systemctl mask [unit type]	Deregistering a specified service]
Systemctl unmask [unit type]	Cancels the cancellation of a specified service

Example:

# Check whether the network service starts the systemctl is-active network. service # Check whether the network service is set to start systemctl is-enable network. service # stop cups service systemctl stop cups. service # log out of the cups service systemctl mask cups. service # view the status of the cups service systemctl status cups. service # cancel the cancellation of the cups service systemctl unmask cups. service

4. Comparison between the init command and the systemctl command

Init command	Systemctl command	Description
Init 0	Systemctl poweroff	System Shutdown
Init 6	Systemctl reboot	Restart

Other commands related to the on/off server:

Systemctl command	Description
Systemctl suspend	Enter sleep mode
Systemctl hibernate	Enter sleep mode
Systemctl rescue	Force enter rescue mode
Systemctl emergency	Forced access to emergency rescue mode

5. Set the system running level

5.1. Table of Operation Level

Init level	Systemctl target
0	Shutdown.tar get
1	Emergency.tar get
2	Rescure.tar get
3	Multi-user.target
4	None
5	Graphical.tar get
6	None

Getty.tar get is used to set the number of tty.

5.2 set the running level
Command Format:

systemctl [command] [unit.target]

Parameters:

Command:
Get-default: get the current target
Set-default: set the specified target to the default running level.
Isolate: switch to the specified running level
Unit.tar get: The running level listed in table 5.1

Systemctl command	Description
Systemctl get-default	Obtain the current running level
Systemctl set-default multi-user.target	Set the default running level to muyun-user.
Systemctl isolate multi-user.target	Switch to the muyun-user running level without restarting.
Systemctl isolate graphical.tar get	Switch to the graphic interface without restarting

6. Use systemctl to analyze dependencies between services

Command Format:

systemctl list-dependencies [unit] [–reverse]

-Reverse is used to check which unit is used.

Example:

# Obtain the target [root @ www ~] at the current running level # Systemctl get-defaultmulti-user.target # view the services started by the current run-level target (mult-user) [root @ www ~] # Systemctl restart get │ ─ ── alsa-restore.service │ ─ ── alsa-state.service ..... (omitted in the middle )..... │ ├─ets.tar get │ ─ ── avahi-daemon.socket │ ── ↓. socket ..... (omitted in the middle )..... │ unzip sysinit.tar get │ ── dev-hugepages.mount │ ── dev-mqueue.mount ..... (omitted in the middle )..... │ unzip timers.tar get │ unzip systemd-tmpfiles-clean.timer?#getty.tar get │ unzip-gett Y@tty1.service‑policremote-fs.tar get # Check which targets reference the current running-level target [root @ www ~] # Systemctl list-dependencies --reversedefault.target?#graphical.tar get

7. Disable Network Services

When you use systemctl to disable network services, there are some special issues.
Both unit. servce and unit. socket must be disabled.

Use systemctl to view the sshd service enabled

[root@www system]#  systemctl list-units --all | grep sshdsshd-keygen.service loaded inactive dead        OpenSSH Server Key Generationsshd.service        loaded active   running     OpenSSH server daemonsshd.socket         loaded inactive dead        OpenSSH Server Socket

We can see that sshd is enabled at the same time. service and sshd. socket, if only sshd is closed. service so sshd. the socket is still listening to the network. sshd will be started when there is a requirement to connect to sshd on the network. service. Therefore, if you want to completely disable the sshd service, you need to disable both sshd. service and sshd. socket.

systemctl stop sshd.servicesystemctl stop sshd.socketsystemctl disable sshd.service sshd.socket

Net-tools is not installed in centos 7.x by default, so netstat cannot be used to view the product developed by the host. You need to install yum to obtain the Toolkit:

 yum -y install net-tools

Check whether port 22 is disabled

netstat -lnp |grep sshd

8. Disable firewall

Iptables is removed from Centos 7.x and replaced with firewall. To disable the firewall and disable Service Startup, run the following command:

systemctl stop firewalld.servicesystemctl disable firewalld.service

Full guide to systemctl commands

Usage of systemctl on CentOS 7