9.Dockerfile详解【3】

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1. CMD说明

Dockerfile 中可以有多个CMD的命令，但只有最后一个生效，CMD会被docker run之后的参数替换掉

1.1查看tomcat的Dockerfile

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EXPOSE 8080
CMD ["catalina.sh", "run"]

从上面的可以看出最后执行了一个catalina.sh的命令  相当于./bin/catalina.sh  

运行如下命令:

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[root@localhost ~]# docker run -it -p 9999:8080 0eed806285b5 ls -lh
total 124K
-rw-r--r--. 1 root root  19K Oct  6 17:01 BUILDING.txt
-rw-r--r--. 1 root root 5.3K Oct  6 17:01 CONTRIBUTING.md
-rw-r--r--. 1 root root  56K Oct  6 17:01 LICENSE
-rw-r--r--. 1 root root 1.7K Oct  6 17:01 NOTICE
-rw-r--r--. 1 root root 3.2K Oct  6 17:01 README.md
-rw-r--r--. 1 root root 7.0K Oct  6 17:01 RELEASE-NOTES
-rw-r--r--. 1 root root  16K Oct  6 17:01 RUNNING.txt
drwxr-xr-x. 2 root root 4.0K Nov  3 02:52 bin
drwxr-xr-x. 2 root root  238 Oct  6 17:01 conf
drwxr-xr-x. 2 root root 4.0K Nov  3 02:52 lib
drwxrwxrwx. 2 root root    6 Oct  6 16:57 logs
drwxr-xr-x. 2 root root  134 Nov  3 02:52 native-jni-lib
drwxrwxrwx. 2 root root   30 Nov  3 02:52 temp
drwxr-xr-x. 2 root root    6 Nov  3 02:52 webapps
drwxr-xr-x. 7 root root   81 Oct  6 16:58 webapps.dist
drwxrwxrwx. 2 root root    6 Oct  6 16:57 work
[root@localhost ~]# docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
[root@localhost ~]#

发现这个tomcat的容器启动之后就退出了，这是因为在启动容器的时候后面加了ls -lh那么会覆盖Dockerfile里面的CMD ["catalina.sh", "run"]

2. ENTRYPOINT说明

使用ENTRYPOINT来运行命令，在run运行的参数会追加到新的命令后面

3. CMD和ENTRYPOINT的区别

编写Dockerfile: 

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FROM centos
CMD ["ls", "-a"]

构建镜像: 

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[root@localhost myls]# docker build -t myls:1.0 .
Sending build context to Docker daemon 2.048 kB
Step 1/2 : FROM centos
 ---> 0d120b6ccaa8
Step 2/2 : CMD ls -a
 ---> Running in d4bb812b3f22
 ---> 7ef84a2f1392
Removing intermediate container d4bb812b3f22
Successfully built 7ef84a2f1392

启动镜像: 

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[root@localhost myls]# docker run -it myls:1.0 -lh
/usr/bin/docker-current: Error response from daemon: oci runtime error: container_linux.go:235: starting container process caused "exec: \"-lh\": executable file not found in $PATH".

出现错误，这是因为Dockerfile里面使用的是CMD ["ls", "-a"]，会被-lh覆盖，-lh不是完整的命令

修改Dockerfile: 

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FROM centos
ENTRYPOINT ["ls", "-a"]

构建镜像: 

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[root@localhost myls]# docker build -t myls:1.1 .
Sending build context to Docker daemon 2.048 kB
Step 1/2 : FROM centos
 ---> 0d120b6ccaa8
Step 2/2 : ENTRYPOINT ls -a
 ---> Running in b01a46cd4255
 ---> 4d66a0a91d3f
Removing intermediate container b01a46cd4255
Successfully built 4d66a0a91d3f
[root@localhost myls]# docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             SIZE
myls                    1.1                 4d66a0a91d3f        15 seconds ago      215 MB
myls                    1.0                 7ef84a2f1392        2 minutes ago       215 MB
mycentos01              2.0                 f503701c7737        24 hours ago        272 MB
mycentos01              1.0                 9aea850e12d7        2 days ago          215 MB
mytomcat01              1.0                 b14e2098d46d        3 days ago          533 MB
docker.io/tomcat        8.5                 0eed806285b5        2 weeks ago         533 MB
docker.io/nginx         latest              f35646e83998        5 weeks ago         133 MB
docker.io/centos        latest              0d120b6ccaa8        3 months ago        215 MB
docker.io/hello-world   latest              bf756fb1ae65        10 months ago       13.3 kB

运行镜像: 

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[root@localhost myls]# docker run -it myls:1.1 -lh
total 0
drwxr-xr-x.   1 root root  17 Nov 17 14:30 .
drwxr-xr-x.   1 root root  17 Nov 17 14:30 ..
-rwxr-xr-x.   1 root root   0 Nov 17 14:30 .dockerenv
lrwxrwxrwx.   1 root root   7 May 11  2019 bin -> usr/bin
drwxr-xr-x.   5 root root 360 Nov 17 14:30 dev
drwxr-xr-x.   1 root root  66 Nov 17 14:30 etc
drwxr-xr-x.   2 root root   6 May 11  2019 home
lrwxrwxrwx.   1 root root   7 May 11  2019 lib -> usr/lib
lrwxrwxrwx.   1 root root   9 May 11  2019 lib64 -> usr/lib64
drwx------.   2 root root   6 Aug  9 21:40 lost+found
drwxr-xr-x.   2 root root   6 May 11  2019 media
drwxr-xr-x.   2 root root   6 May 11  2019 mnt
drwxr-xr-x.   2 root root   6 May 11  2019 opt
dr-xr-xr-x. 134 root root   0 Nov 17 14:30 proc
dr-xr-x---.   2 root root 162 Aug  9 21:40 root
drwxr-xr-x.   1 root root  21 Nov 17 14:30 run
lrwxrwxrwx.   1 root root   8 May 11  2019 sbin -> usr/sbin
drwxr-xr-x.   2 root root   6 May 11  2019 srv
dr-xr-xr-x.  13 root root   0 Nov 17 13:46 sys
drwxrwxrwt.   7 root root 145 Aug  9 21:40 tmp
drwxr-xr-x.  12 root root 144 Aug  9 21:40 usr
drwxr-xr-x.  20 root root 262 Aug  9 21:40 var

-lh被追加到ls -a的后面了，最终为ls -a -lh

4. 总结

  1. CMD ["ls", "-a"]

docker run -it myls:1.0 -lh: 报错，因为-lh会覆盖ls -a，最终为-lh，不是完整的命令，会报错

docker run -it myls:1.0 ls -lh: 不报错，因为ls -lh会覆盖ls -a，最终为ls -lh，是完整的命令

  1. ENTRYPOINT ["ls", "-a"]

docker run -it myls:1.1 ls -lh: 报错，因为ls -lh会追加到ls -a之后，最终为ls -a ls -lh，不是正确的命令，会报错

docker run -it myls:1.1 -lh: 不报错，因为-lh会追加到ls -a之后，最终为ls -a -lh，是正确的命令