---> 3c26b6a8ef1b
Step 4 : RUN chmod a+x /opt/startup
---> Using cache
---> 04d9b53d6148
Step 5 : ENTRYPOINT /opt/startup arg1
---> Running in 54947233dc3d
---> 15a485253b4e
Removing intermediate container 54947233dc3d
Step 6 : CMD arg2
---> Running in 18c43d2d90fd
---> 4684ba457cc2
Removing intermediate container 18c43d2d90fd
Successfully built 4684ba457cc2
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下面运行docker run启动两个test:latest镜像的容器，第一条docker run命令没有指定参数，第二条docker run命令指定了参数arg3，其运行结果如下，
lienhua34@test$ sudo docker run -ti --rm=true test
in startup, args: arg1 arg2
lienhua34@test$ sudo docker run -ti --rm=true test arg3
in startup, args: arg1 arg3
从上面第一个容器的运行结果可以看出CMD指令为ENTRYPOINT指令设置了默认参数;从第二个容器的运行结果看出，docker run命令指定的参数覆盖了CMD指令指定的参数。
4.3注意点
CMD指令为ENTRYPOINT指令提供默认参数是基于镜像层次结构生效的，而不是基于是否在同个Dockerfile文件中。意思就是说，如果Dockerfile指定基础镜像中是ENTRYPOINT指定的启动命令，则该Dockerfile中的CMD依然是为基础镜像中的ENTRYPOINT设置默认参数。
例如，我们有如下一个Dockerfile文件，
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FROM ubuntu:14.04
MAINTAINER lienhua34@xxx.com
ADD startup /opt
RUN chmod a+x /opt/startup
ENTRYPOINT ["/opt/startup", "arg1"]
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通过运行docker build命令生成test:0.0.1镜像，然后创建该镜像的一个容器，查看运行结果，
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lienhua34@test$ sudo docker build -t test:0.0.1 .
Sending build context to Docker daemon 6.144 kB
Step 1 : FROM ubuntu:14.04
---> a5a467fddcb8
Step 2 : MAINTAINER lienhua34@163.com
---> Running in 57a96522061a
---> c3bbf1bd8068
Removing intermediate container 57a96522061a
Step 3 : ADD startup /opt
---> f9884fbc7607
Removing intermediate container 591a82b2f382
Step 4 : RUN chmod a+x /opt/startup
---> Running in 7a19f10b5513
---> 16c03869a764
Removing intermediate container 7a19f10b5513
Step 5 : ENTRYPOINT /opt/startup arg1
---> Running in b581c32b25c3
---> c6b1365afe03
Removing intermediate container b581c32b25c3
Successfully built c6b1365afe03
lienhua34@test$ sudo docker run -ti --rm=true test:0.0.1
in startup, args: arg1
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下面新建一个Dockerfile文件，基础镜像是刚生成的test:0.0.1，通过CMD指定要通过echo打印字符串“in test:0.0.2”。文件内容如下所示，
FROM test:0.0.1
MAINTAINER lienhua34@xxx.com
CMD ["/bin/bash", "-c", "echo in test:0.0.2"]
运行docker build命令生成test:0.0.2镜像，然后通过运行docker run启动一个test:0.0.2镜像的容器来查看结果，
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lienhua34@test$ sudo docker build -t test:0.0.2 .
Sending build context to Docker daemon 6.144 kB
Step 1 : FROM test:0.0.1
---> c6b1365afe03
Step 2 : MAINTAINER lienhua34@163.com
---> Running in deca95cf4c15
---> 971b5a819b48
Removing intermediate container deca95cf4c15
Step 3 : CMD /bin/bash -c echo in test:0.0.2
---> Running in 4a31c4652e1e
---> 0ca06ba31405
Removing intermediate container 4a31c4652e1e
Successfully built 0ca06ba31405
lienhua34@test$ sudo docker run -ti --rm=true test:0.0.2
in startup, args: arg1 /bin/bash -c echo in test:0.0.2
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从上面结果可以看到，镜像test:0.0.2启动的容器运行时并不是打印字符串”in test:0.0.2”，而是将CMD指令指定的命令当做基础镜像test:0.0.1中ENTRYPOINT指定的运行脚本startup的参数。
