systemctl command

format:systemctl <COMMAND> <name.service>
- .service Can be omitted, the system will auto-complete
Profiles
- Each Unit has a configuration file that tells Systemd how to start this Unit
- Profiles on the main /usr/lib/systemd/systemdirectory, it may in /etc/systemd/systemdirectory

System Management

# 检查系统上是否安装了systemd
systemd --version

# 重启系统
systemctl reboot

# 显示主机信息
hostnamectl

#显示主机时区
timedatectl

Management Services

systemctlCommand enable, disableand maskthe sub-command has been added --nowat the same time option to be enabled at the same time to start the service, stop the service deactivated
start up
- systemctl start name.service
stop
- systemctl stop name.service
Restart
- systemctl restart name.service
status
- systemctl status name.service
Reload the configuration file
- Here's the configuration file refers to the /usr/lib/systemd/systemfiles in the directory
  - Not the program itself profile
- systemctl reload name.service
Setting a service boot from Kai
- Equivalent to /etc/systemd/systemadding a symbolic link directory, point /usr/lib/systemd/systeminside the name.servicefile
  - When turned on, Systemdonly perform /etc/systemd/systemdirectory profile
- After setting the boot, the software will not start immediately
- systemctl enable name.service
Cancellation boot from Kai
- systemctl disable name.service
Disable Service
- Other services to prevent the service is indirect start, the service can not be started by start or restart command
- systemctl mask name.service
Enable Service
- Only for service to disabled
- systemctl unmask name.service
Kill service and its child processes
- systemctl kill name.service
List all dependent services
- systemctl list-dependencies name.service

View Service

#查看服务激活与否
systemctl is-active name.service

#查看服务的开机自启状态
systemctl is-enabled name.service

#查看系统所有运行的服务项
systemctl list-units --type=service

journalctl command

systemd log configuration file is/etc/systemd/journald.conf

# 需要root权限

# 查看所有日志（默认情况下 ，只保存本次启动的日志）
journalctl
# 查看系统本次启动的日志
journalctl -b
# 显示尾部的最新n行日志;默认为10
journalctl -n 20
# 滚动显示
journalctl -f

# 查看指定服务(unit)的日志
journalctl -u <service name> 
# 根据pid查看服务日志
journalctl -b _PID=xxx

# 查看指定时间的日志
journalctl --since="2012-10-30 18:17:16"
journalctl --since "20 min ago"
journalctl --since yesterday
journalctl --since 09:00 --until "1 hour ago"

Daemon concept

Daemon is Daemon program is a program executed in the background, it is independent of the control terminal and perform some tasks or triggering event cycle
- End is typically designated as "d" letters, such as the httpd , the syslogd , systemd and dockerd etc.
By ampersand, you can put the command into the background, but it is different with the daemon
- Daemon has nothing to do with the terminal, the init process is adopted orphan process; and the parent process background process is a terminal, the terminal can still print
- Daemon is still strong in the closed terminal; and background process with the user exits and stop, unless coupled nohup
- Daemon changed the conversation, process group, the working directory and file descriptors, background processes directly inherit the parent process (shell) of

service and systenctl

service is the essence of the command to execute /etc/init.d/the script directory
systemctl command compatible with the service
Command substitution
- 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

syslog与systemd-journal

syslog It is a protocol that defines the format of the message
- Actually run in the system is syslogda daemon
- It listening socket /dev/loginformation from the socket and is written to a log file
- As /var/log/messagesor/var/log/syslog
systemd-journald is syslog supplement
- systemd logs stored in /run/log/journalthe system reboot will clear
- By the new /var/log/journaldirectory, the log will be automatically recorded into this directory, and perpetual