linux解决bash: nc: command not found 安装nc、telnet、yum

一、最近在用zookeeper搭建集群改造配置中心的单点故障问题：使用"四字命令"echo ruok|nc localhost 2181时报错：发现命令无法被识别：

[root@centos6 bin]# echo ruok|nc localhost 2181
-bash: nc: command not found
从提示信息中我们可以知道，是由于nc命令无法被找到。出现该情况有两种可能：

（1）没有配置nc命令的环境变量；
（2）该系统没有安装nc命令；

我查看了一下在/usr/bin目录中并没有nc命令，所以我可以认为出现该情况的原因是第二种情况

二、安装
2.1、配置yum

yum是一种在线安装和更新软件的工具，使用该工具要保证网络连接，还要有一个可用的软件源，可以使用163或sohu的软件源，软件源地址如下：

a：163的网址：http://mirrors.163.com/ ，进入后点击右列的“fedora使用帮助”，有使用说明
b：sohu的网址：http://mirrors.sohu.com/
c: Centos自带的源

在此我使用的是163的源，使用163下载源，则步骤如下：

a：下载fedora-163.repo和fedora-updates-163.repo, 通过winCP放入Linux中/etc/yum.repos.d/目录
b：运行以下命令生成缓存：yum clean all、yum makecache

2.2、执行yum install -y nc在线安装nc

[root@centos6 bin]# yum install -y nc

再次执行nc命令可得状态信息

# 再次执行 echo stat | nc localhost 2181
[root@centos6 bin]# echo stat | nc localhost 2181
Zookeeper version: 3.4.10-39d3a4f269333c922ed3db283be479f9deacaa0f, built on 03/23/2017 10:13 GMT
Clients:
 /0:0:0:0:0:0:0:1:55071[0](queued=0,recved=1,sent=0)
 
Latency min/avg/max: 0/0/0
Received: 1
Sent: 0
Connections: 1
Outstanding: 0
Zxid: 0xa00000001
Mode: follower
Node count: 5

2.3、安装远程命令telnet

执行命令：yum list telnet*；查看telnet相关的安装包，结果如下：

telnet.x86_64表示客户端，telnet-server.x86_64表示服务端。我们需要的是客户端，安装telnet.x86_64即可。

执行命令 yum install telnet.x86_64安装即可

[root@centos6 bin]# yum install telnet.x86_64
Loaded plugins: fastestmirror, security
Loading mirror speeds from cached hostfile
 * base: mirrors.163.com
 * extras: mirrors.163.com
 * updates: mirrors.163.com
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package telnet.x86_64 1:0.17-48.el6 will be installed
--> Finished Dependency Resolution
 
Dependencies Resolved
 
======================================================================================================================================
 Package                       Arch                          Version                                Repository                   Size
======================================================================================================================================
Installing:
 telnet                        x86_64                        1:0.17-48.el6                          base                         58 k
 
Transaction Summary
======================================================================================================================================
Install       1 Package(s)
 
Total download size: 58 k
Installed size: 109 k
Is this ok [y/N]: y
Downloading Packages:
telnet-0.17-48.el6.x86_64.rpm                                                                                  |  58 kB     00:00     
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : 1:telnet-0.17-48.el6.x86_64                                                                                        1/1
  Verifying  : 1:telnet-0.17-48.el6.x86_64                                                                                        1/1
 
Installed:
  telnet.x86_64 1:0.17-48.el6                                                                                                         
 
Complete!
验证执行命令 telnet --help ，如下所示安装成功
[root@centos6 bin]# telnet --help
telnet: invalid option -- '-'
Usage: telnet [-8] [-E] [-L] [-S tos] [-a] [-c] [-d] [-e char] [-l user]
    [-n tracefile] [-b hostalias ] [-r]
 [host-name [port]]