自动同步系统时间
安装ntp服务
sudo apt-get install ntp
CentOs 没有 apt-get命令
sudo yum install ntp
在安装完后，在/etc/ntp.conf文件以配置ntp服务，添加：
server ntp.aliyun.com
启动ntp服务
sudo /etc/init.d/ntp start
CentOs用
systemctl start ntpd.service
ntp服务将会启动并自动同步本地计算机的时间。
可以用命令查看 ntpd.service 服务器是否起来
systemctl list-units --type=service --state=running
运行：
PID TTY TIME CMD
19239 pts/0 00:00:00 bash
35768 pts/0 00:00:00 ps
[root@localhost etc]# systemctl list-units --type=service --state=running
UNIT LOAD ACTIVE SUB DESCRIPTION
auditd.service loaded active running Security Auditing Service
crond.service loaded active running Command Scheduler
dbus.service loaded active running D-Bus System Message Bus
emqx.service loaded active running emqx daemon
getty@tty1.service loaded active running Getty on tty1
irqbalance.service loaded active running irqbalance daemon
mysqld.service loaded active running MySQL Server
NetworkManager.service loaded active running Network Manager
ntpd.service loaded active running Network Time Service ----------------------在这里
polkit.service loaded active running Authorization Manager
rc-local.service loaded active running /etc/rc.d/rc.local Compatibility
redis.service loaded active running Redis persistent key-value databasersyslog.service loaded active running System Logging Service
smb.service loaded active running Samba SMB Daemon
sshd.service loaded active running OpenSSH server daemon
systemd-journald.service loaded active running Journal Service
systemd-logind.service loaded active running Login Service
systemd-udevd.service loaded active running udev Kernel Device Manager
tuned.service loaded active running Dynamic System Tuning Daemon
vgauthd.service loaded active running VGAuth Service for open-vm-tools
vmtoolsd.service loaded active running Service for virtual machines hosted
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.
21 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
