Docker容器里，如果我们想测试两个容器是否建立关系，我们可以用ping来测试。在默认情况下，docker的centenos镜像由于做了精简，去掉了ping命令，我们需要自己安装：
这时会提示没有找到ping命令
[root@localhost ~]# docker exec -it test1 /bin/bash
root@e0e5054c2593:/# ping test2
bash: ping: command not found
则在容器内执行一下命令安装ping
检查并更新：
apt-get update
root@e0e5054c2593:/# apt-get update
Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
Get:2 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Get:3 http://security.ubuntu.com/ubuntu focal-security/multiverse amd64 Packages [1078 B]
Get:4 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [66.4 kB]
Get:5 http://archive.ubuntu.com/ubuntu focal-updates InRelease [111 kB]
Get:6 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages [207 kB]
Get:7 http://archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
Get:8 http://archive.ubuntu.com/ubuntu focal/multiverse amd64 Packages [177 kB]
Get:9 http://security.ubuntu.com/ubuntu focal-security/restricted amd64 Packages [39.1 kB]
Get:10 http://archive.ubuntu.com/ubuntu focal/restricted amd64 Packages [33.4 kB]
Get:11 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages [1275 kB]
Get:12 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages [11.3 MB]
Get:13 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [430 kB]
Get:14 http://archive.ubuntu.com/ubuntu focal-updates/multiverse amd64 Packages [17.3 kB]
Get:15 http://archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages [202 kB]
Get:16 http://archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages [39.3 kB]
apt install inputils-ping
root@e0e5054c2593:/# apt install iputils-ping
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
libcap2 libcap2-bin libpam-cap
The following NEW packages will be installed:
iputils-ping libcap2 libcap2-bin libpam-cap
0 upgraded, 4 newly installed, 0 to remove and 1 not upgraded.
Need to get 90.5 kB of archives.
After this operation, 333 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://archive.ubuntu.com/ubuntu focal/main amd64 libcap2 amd64 1:2.32-1 [15.9 kB]
Get:2 http://archive.ubuntu.com/ubuntu focal/main amd64 libcap2-bin amd64 1:2.32-1 [26.2 kB]
Get:3 http://archive.ubuntu.com/ubuntu focal/main amd64 iputils-ping amd64 3:20190709-3 [40.1 kB]
Get:4 http://archive.ubuntu.com/ubuntu focal/main amd64 libpam-cap amd64 1:2.32-1 [8352 B]
Fetched 90.5 kB in 5s (19.8 kB/s)
以上安装成功，我们测试一下：
root@e0e5054c2593:/# ping test2
PING test2 (172.18.0.3) 56(84) bytes of data.
64 bytes from test2.test-net (172.18.0.3): icmp_seq=1 ttl=64 time=0.124 ms
64 bytes from test2.test-net (172.18.0.3): icmp_seq=2 ttl=64 time=0.089 ms
64 bytes from test2.test-net (172.18.0.3): icmp_seq=3 ttl=64 time=0.083 ms
64 bytes from test2.test-net (172.18.0.3): icmp_seq=4 ttl=64 time=0.566 ms
64 bytes from test2.test-net (172.18.0.3): icmp_seq=5 ttl=64 time=0.118 ms