man：
centos 6.x
[root@Lnmp ~]# whatis man
man: nothing appropriate
[root@Lnmp ~]# makewhatis
[root@Lnmp ~]# man -f man
man (1) - format and display the on-line manual pages
man.config [man] (5) - configuration data for man
man [manpath] (1) - format and display the on-line manual pages
centos 7.x
[root@localhost ~]# man -f man
man: nothing appropriate.
[root@localhost ~]# makewhatis
-bash: makewhatis: command not found
[root@localhost ~]# mandb
[root@localhost ~]# man -f man
man (1) - an interface to the on-line reference manuals
服务启动：
centos6.x：
[root@Shell byrd]# /etc/init.d/sshd status #查询sshd服务状态
openssh-daemon (pid 1183) is running...
[root@Shell byrd]# /etc/init.d/sshd restart #重新启动sshd服务
Stopping sshd: [ OK ]
Starting sshd: [ OK ]
[root@Shell byrd]# service sshd restart #重新启动sshd服务
[root@Shell byrd]# chkconfig --list|grep 3:on #启动服务
centos7.x：
[root@hz ~]# systemctl status sshd.service #查询sshd服务状态
sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
Active: active (running) since Tue 2016-01-12 16:04:22 CST; 5min ago
Process: 2018 ExecStartPre=/usr/sbin/sshd-keygen (code=exited, status=0/SUCCESS)
Main PID: 2019 (sshd)
CGroup: /system.slice/sshd.service
└─2019 /usr/sbin/sshd -D
[root@hz ~]# systemctl restart sshd.service #重新启动sshd服务
[root@hz ~]# systemctl list-unit-files|grep enabled #查询所有启动服务
防火墙
centos6.x（默认是iptables）：
[root@Shell byrd]# iptables -t filter -A INPUT -p tcp --dport 22 -j ACCEPT #允许22端口，默认sshd服务
centos7.x（默认是firewalld）：
[root@hz ~]# firewall-cmd --permanent --zone=public --add-port=22/tcp #允许22端口，默认sshd服务
未完待续！！！