Docker_Dockerfile中保留字指令CMD和ENTRYPOINT区别

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#docker #CMD #ENTRYPOINT #保留字指令

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本文详细解析了Docker中CMD与ENTRYPOINT指令的区别，通过实例演示了两者的使用场景与行为差异，CMD易被docker run参数覆盖，而ENTRYPOINT则会追加参数。

CMD

指定一个容器启动时要运行的命令
Dockerfile中可以有多个CMD指令,但只有最后一个生效,CMD会被 docker run之后的参数替换

ENTRYPOINT

指定一个容器启动时要运行的命令
ENTRYPOINT的目的和CMD一样,都是在指定容器启动程序及参数

验证

验证中使用的命令为curl

首先编写Dockerfile文件，内容如下：

FROM centos:7
RUN yum -y install curl
CMD ["curl","-s","https://ip.cn"]

build构建新的镜像

[root@localhost myDocker]# docker build -f /myDocker/DockerfileTree -t myip .
Sending build context to Docker daemon  4.096kB
Step 1/3 : FROM centos:7
 ---> 5e35e350aded
Step 2/3 : RUN yum -y install curl
 ---> Running in cb4a10126aa2
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
 * base: mirror.bit.edu.cn
 * extras: mirrors.aliyun.com
 * updates: mirrors.aliyun.com
Package curl-7.29.0-54.el7.x86_64 already installed and latest version
Nothing to do
Removing intermediate container cb4a10126aa2
 ---> 88d37e3c7f89
Step 3/3 : CMD ["curl","-s","https://ip.cn"]
 ---> Running in 80b594b5d5ae
Removing intermediate container 80b594b5d5ae
 ---> 38e3d380c971
Successfully built 38e3d380c971
Successfully tagged myip:latest

运行容器

[root@localhost myDocker]# docker run myip
{"ip": "118.113.40.151", "country": "四川省成都市", "city": "电信"}

现在需要在原有的 `curl -s https://ip.cn` 后面追加一个`-i`得到报文头

[root@localhost myDocker]# docker run myip -i
docker: Error response from daemon: OCI runtime create failed: container_linux.go:346: starting container process caused "exec: \"-i\": executable file not found in $PATH": unknown.

可以看到，CMD后面的命令，会被docker run后面的命令覆盖掉，也就是我刚才写的 -i

测试`ENTRYPOINT`指令，方法一样，只不过构建文件的最后`CMD`替换为`ENTRYPOINT`

FROM centos:7
RUN yum -y install curl
ENTRYPOINT ["curl","-s","https://ip.cn"]

build新镜像

[root@localhost myDocker]# docker build -f /myDocker/DockerfileFour -t myip2 .
Sending build context to Docker daemon   5.12kB
Step 1/3 : FROM centos:7
 ---> 5e35e350aded
Step 2/3 : RUN yum -y install curl
 ---> Using cache
 ---> 88d37e3c7f89
Step 3/3 : ENTRYPOINT ["curl","-s","https://ip.cn"]
 ---> Running in 41efe0250802
Removing intermediate container 41efe0250802
 ---> 0c174d9b9d07
Successfully built 0c174d9b9d07
Successfully tagged myip2:latest

运行myip2

[root@localhost myDocker]# docker run myip2
{"ip": "118.113.40.151", "country": "四川省成都市", "city": "电信"}

在末尾添加`-i`

[root@localhost myDocker]# docker run myip2 -i
HTTP/1.1 200 OK
Date: Mon, 25 Nov 2019 13:56:30 GMT
Content-Type: application/json; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
Set-Cookie: __cfduid=d66201e01a5389c244826811f2a98b9d31574690190; expires=Wed, 25-Dec-19 13:56:30 GMT; path=/; domain=.ip.cn; HttpOnly
CF-Cache-Status: DYNAMIC
Expect-CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"
Server: cloudflare
CF-RAY: 53b4205b9fc8eb91-LAX

{"ip": "118.113.40.151", "country": "四川省成都市", "city": "电信"}