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Systemd:Systemd is the latest initialization system (init) in Linux system.

Summary explanation reference: https://www.ibm.com/developerworks/cn/linux/1407_liuming_init3/index.html

POST-- > Boot Sequence-- > Bootloader-- > kernel + initramfs (initrd)-- > rootfs-- > / sbin/init

Init:

CentOS 5: SysV init

CentOS 6: Upstart

CentOS 7: Systemd

To reduce system startup time, the goal of systemd is to:

Start as few processes as possible

Start as many processes in parallel as possible

What's new in Systemd:

Realize service parallel startup when the system is booted

Activate the process on demand

System status snapshot

Define service control logic based on dependency relationship

Systemd has its own log service journald, which is originally designed to overcome the shortcomings of existing syslog services.

Core concept: unit

The concept of unit

There are many things that need to be done to initialize the system. Background services need to be started, such as SSHD services, and configuration work needs to be done, such as mounting the file system. Each step in this process is abstracted by systemd as a configuration unit.

That is, unit. You can think of a service as a configuration unit; a mount point as a configuration unit; a swap partition configuration as a configuration unit; and so on. Systemd classifies the configuration units into the following different types.

Each configuration unit has a corresponding configuration file, and the system administrator's task is to write and maintain these different configuration files, such as a MySQL service corresponding to a mysql.service file. The syntax of this configuration file is so simple that users no longer need to write and maintain complex system scripts.

Configuration file for identification and configuration; the file mainly contains system services, monitoring socket, saved system snapshots and other init-related information

When systemd runs as a system instance (--system), the order in which the units are loaded (higher priority than the previous directory):

System unit catalog description

/ etc/systemd/system locally configured system unit

/ run/systemd/system system units configured at run time

System unit installed by / usr/lib/systemd/system software package

Type of Unit:

Service unit: file extension .service, used to define system services

Target unit: the file extension is .target, which is used to simulate the implementation of the run level

Device unit: .device, which defines the device recognized by the kernel

Mount unit: .mount, which defines the file system mount point

Socket unit: .socket, which is used to identify socket files for interprocess communication

Snapshot unit: .snapshot to manage system snapshots

Swap unit: .swap, used to identify swap devices

Automount unit: .automount, the automatic mount point of the file system

Path unit: .path, which defines a file or directory in the file system

Manage system services:

CentOS 7: service unit

Note: compatible with earlier service scripts

Command: systemctl COMMAND name.service

Start: service name start = = > systemctl start name.service

Stop: service name stop = > systemctl stop name.service

Restart: service name restart = > systemctl restart name.service

Status: service name status = = > systemctl status name.service

Conditional restart: service name condrestart = > systemctl try-restart name.service

Reload or restart the service: systemctl reload-or-restart name.service

Reload or conditional restart service: systemctl reload-or-try-restart name.service

Disable is set to boot: systemctl mask name.service

Disable is set to self-boot: systemctl unmask name.service

Check the status of whether a service is currently active or not: systemctl is-active name.service

View all services that have been activated:

Systemctl list-units-type service

View all services:

Systemctl list-units-type service-all

Correspondence of the chkconfig command:

Set a service to boot automatically: chkconfig name on = > systemctl enable name.service

Prohibit: chkconfig name off = = > systemctl disable name.service

Check the post status of all services:

Chkconfig-- list = = > systemctl list-unit-files-- type service

Check whether the service is self-booting: systemctl is-enabled name.service

Other commands:

View the dependencies of the service: systemctl list-dependencies name.service

Target and run level

Systemd replaces the concept of runlevel with target, providing more flexibility

Target units:

Unit configuration file: .target

Run level:

0 = = > runlevel0.target, poweroff.target

1 = = > runlevel1.target, rescue.target

2 = = > runlevel2.target, multi-user.target

3 = = > runlevel3.target, multi-user.target

4 = = > runlevel4.target, multi-user.target

5 = = > runlevel5.target, graphical.target

6 = > runlevel6.target, reboot.target

(it makes no difference for systemd to run level 2, 3, 4, 4)

The: runlevel command is not recommended for Centos7

Level switching:

Init N = > systemctl isolate name.target (.target cannot be omitted)

View level:

Runlevel = > systemctl list-units-- type target

Get the default run level:

/ etc/inittab = = > systemctl get-default

Modify the default level:

/ etc/inittab = = > systemctl set-default name.target

Switch to emergency rescue mode:

Systemctl rescue

Switch to emergency mode:

Systemctl emergency

(emergency: driver does not load, system does not initialize, service does not start)

Other common commands:

Shutdown: systemctl halt, systemctl poweroff

Restart: systemctl reboot

Pending: systemctl suspend

Snapshots: systemctl hibernate

Snapshot and suspend: systemctl hybrid-sleep

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