CMD 与 ENTRYPOINT 区别

CMD 命令演示

编写 Dockerfile

[root@sail dockerfile]# vim Dockerfile-cmd-test
[root@sail dockerfile]# cat Dockerfile-cmd-test
FROM centos
CMD ["ls","-a"]

构建镜像

[root@localhost home]# docker build -f dockerfile-cmd-test -t cmd .

[+] Building 0.0s (5/5) FINISHED                          doc
 => [internal] load build definition from dockerfile-cmd-test
 => => transferring dockerfile: 133B                         
 => [internal] load metadata for docker.io/library/centos:lat
 => [internal] load .dockerignore                            
 => => transferring context: 2B                              
 => CACHED [1/1] FROM docker.io/library/centos:latest        
 => exporting to image                                       
 => => exporting layers                                      
 => => writing image sha256:2b06f96416291f005301cc48ccf63a8fa
 => => naming to docker.io/library/cmd 

运行一下

先看id

[root@localhost home]# docker images
REPOSITORY   TAG       IMAGE ID       CREATED          SIZE
mycentos     latest    c7defb0511c1   19 minutes ago   312MB
centos       latest    5d0da3dc9764   2 years ago      231MB
cmd          latest    2b06f9641629   2 years ago      231MB
cmdtest      latest    2b06f9641629   2 years ago      231MB

[root@localhost home]# docker run 2b06f9641629
.
..
.dockerenv
bin
dev
etc
home
lib
lib64
lost+found
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var

此时 Dockerfile 中编写的命令生效了。

追加 -l 命令

[root@localhost home]# docker run cmd -l
docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "-l": executable file not found in $PATH: unknown.

没有达到预期的 ls -al 命令。

CMD 是替换的方式， -l 不是命令，所以报错。

ENTRYPOINT 命令演示

编写 Dockerfile

[root@sail dockerfile]# vim dockerfile-entrypoint-test
[root@sail dockerfile]# cat dockerfile-entrypoint-test
FROM centos
ENTRYPOINT ["ls","-a"]

构建镜像

[root@localhost home]# docker build -f docker-entrypoint-test -t entrypoint .
[+] Building 0.0s (5/5) FINISHED             docker:default
 => [internal] load build definition from docker-entr  0.0s
 => => transferring dockerfile: 144B                   0.0s
 => [internal] load metadata for docker.io/library/ce  0.0s
 => [internal] load .dockerignore                      0.0s
 => => transferring context: 2B                        0.0s
 => CACHED [1/1] FROM docker.io/library/centos:latest  0.0s
 => exporting to image                                 0.0s
 => => exporting layers                                0.0s
 => => writing image sha256:a58b53c50da3074855495de14  0.0s
 => => naming to docker.io/library/entrypoint          0.0s

运行镜像

[root@localhost home]# docker images
REPOSITORY   TAG       IMAGE ID       CREATED          SIZE
cmd          latest    2b06f9641629   2 years ago      231MB
entrypoint   latest    a58b53c50da3   2 years ago      231MB
[root@localhost home]# docker run entrypoint
.
..
.dockerenv
bin
dev
etc
home
lib
lib64
lost+found
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var

此时 Dockerfile 中编写的命令也生效了。

追加 -l 命令

[root@localhost home]# docker run entrypoint -l
total 4
drwxr-xr-x.   1 root root    6 May 30 12:44 .
drwxr-xr-x.   1 root root    6 May 30 12:44 ..
-rwxr-xr-x.   1 root root    0 May 30 12:44 .dockerenv
lrwxrwxrwx.   1 root root    7 Nov  3  2020 bin -> usr/bin
drwxr-xr-x.   5 root root  340 May 30 12:44 dev
drwxr-xr-x.   1 root root   66 May 30 12:44 etc
drwxr-xr-x.   2 root root    6 Nov  3  2020 home
lrwxrwxrwx.   1 root root    7 Nov  3  2020 lib -> usr/lib
lrwxrwxrwx.   1 root root    9 Nov  3  2020 lib64 -> usr/lib64
drwx------.   2 root root    6 Sep 15  2021 lost+found
drwxr-xr-x.   2 root root    6 Nov  3  2020 media
drwxr-xr-x.   2 root root    6 Nov  3  2020 mnt
drwxr-xr-x.   2 root root    6 Nov  3  2020 opt
dr-xr-xr-x. 342 root root    0 May 30 12:44 proc
dr-xr-x---.   2 root root  162 Sep 15  2021 root
drwxr-xr-x.  11 root root  163 Sep 15  2021 run
lrwxrwxrwx.   1 root root    8 Nov  3  2020 sbin -> usr/sbin
drwxr-xr-x.   2 root root    6 Nov  3  2020 srv
dr-xr-xr-x.  13 root root    0 May 30 00:36 sys
drwxrwxrwt.   7 root root  171 Sep 15  2021 tmp
drwxr-xr-x.  12 root root  144 Sep 15  2021 usr
drwxr-xr-x.  20 root root 4096 Sep 15  2021 var

运行了预期的 ls -al 命令。

ENTRYPOINT 是追加的方式。

Docker 中许多命令都十分相似，我们需要了解他们的区别，最好的方式就是这样对比测试。