{"id":1078,"date":"2023-03-16T17:13:52","date_gmt":"2023-03-16T17:13:52","guid":{"rendered":"https:\/\/blog.ngocha.biz\/?p=1078"},"modified":"2023-03-16T17:13:52","modified_gmt":"2023-03-16T17:13:52","slug":"entrypoint-vs-cmd-explained","status":"publish","type":"post","link":"https:\/\/blog.ngocha.biz\/?p=1078","title":{"rendered":"Docker ENTRYPOINT Vs CMD Explained With Examples"},"content":{"rendered":"

In this blog we will look at the key differences between Docker<\/a> ENTRYPOINT vs CMD instruction using a practical example.<\/p>\n

What is ENTRYPOINT and CMD<\/h2>\n

In a Dockerfile, ENTRYPOINT<\/code> and CMD<\/code> are two different instructions that are used to define how a container<\/a> should run.<\/p>\n

ENTRYPOINT<\/code> is used to specify the main command that should be executed when a container is started using the image. The default ENTRYPOINT command is \/bin\/sh -c<\/code><\/p>\n

CMD<\/code>, on the other hand, is used to specify the default command and arguments that should be executed when a container is started.<\/p>\n

If both ENTRYPOINT<\/code> and CMD<\/code> are specified in a Dockerfile, the command specified in CMD<\/code> will be appended to the ENTRYPOINT<\/code> command. It acts as an argument for ENTRYPOINT. The resulting command will be executed when the container is started.<\/p>\n

Lets understand these concepts practically.<\/p>\n

Executing Commands Using ENTRYPOINT and CMD<\/h2>\n

Let’s take an example of the following Dockerfile. It installs http-tools and starts the ab (apache benchmark<\/a>) utility using CMD and Entrypoint. Both of them perform the same job<\/p>\n

Using CMD<\/strong><\/p>\n

FROM centos:7\nMAINTAINER Devopscube\nRUN yum -y update && \\\n    yum -y install httpd-tools && \\\n    yum clean all\nCMD [\"ab\"]<\/code><\/pre>\n
Using ENTRYPOINT:<\/strong><\/p>\n
FROM centos:7\nMAINTAINER Devopscube\nRUN yum -y update && \\\n    yum -y install httpd-tools && \\\n    yum clean all\nENTRYPOINT [\"ab\"]<\/code><\/pre>\n
Now if you run the container from the above Dockerfile images, it will throw the following error.<\/p>\n
\u279c  docker run demo\nab: wrong number of arguments\nUsage: ab [options] [http[s]:\/\/]hostname[:port]\/path\nOptions are:\n    -n requests     Number of requests to perform<\/code><\/pre>\n
The reason is, ab command requires an http endpoint<\/strong> as an argument to start the service.<\/p>\n
We have two ways to get around this problem. Hardcode the HTTP endpoint argument as shown in the below examples.<\/p>\n
Using CMD<\/strong>: The ab<\/code> executable and HTTP URL arguments are added in separate square brackets.<\/p>\n
FROM centos:7\nMAINTAINER Devopscube\nRUN yum -y update && \\\n    yum -y install httpd-tools && \\\n    yum clean all\nCMD [\"ab\"] [\"http:\/\/google.com\/\"]<\/code><\/pre>\n
Using ENTRYPOINT:<\/strong> The executable and argument are separated by commas in the same square bracket.<\/p>\n
FROM centos:7 MAINTAINER Devopscube RUN yum -y update && \\ yum -y install httpd-tools && \\ yum clean all ENTRYPOINT [“ab” , “http:\/\/google.com\/” ]<\/p>\n
Difference Between ENTRYPOINT and CMD<\/h2>\n
Lets look at the difference between CMD<\/code> and ENTRYPOINT<\/code> by passing ab commands during docker run.<\/p>\n
Using CMD:<\/strong><\/p>\n
FROM centos:7\nMAINTAINER Devopscube\nRUN yum -y update && \\\n    yum -y install httpd-tools && \\\n    yum clean all\nCMD [\"ab\"]<\/code><\/pre>\n
While running the container, just add the full ab command at the end of the docker run command. It will override the whole CMD<\/strong> specified in the Dockerfile.<\/p>\n
docker run ab-demo ab http:\/\/google.com\/<\/code><\/pre>\n
Using ENTRYPOINT:<\/strong><\/p>\n
If you want to pass the URL argument to ENTRYPOINT, you need to pass the URL alone. The reason is we have the ab<\/code> command as part of the ENTRYPOINT definition.<\/p>\n
And the URL you pass in the run command will be appended to the ENTRYPOINT script. In this case, CMD instruction is not required in the Dockerfile.<\/p>\n
FROM centos:7\nMAINTAINER Devopscube\nRUN yum -y update && \\\n    yum -y install httpd-tools && \\\n    yum clean all\nENTRYPOINT [\"ab\"]<\/code><\/pre>\n
Docker Command:<\/strong><\/p>\n
docker run ab-demo http:\/\/google.com\/<\/code><\/pre>\n
You can override the whole ENTRYPOINT<\/strong> like you do with CMD using the --entrypoint<\/code> flag as shown below<\/p>\n
docker run --entrypoint \"ab\" ab-demo http:\/\/google.com\/<\/code><\/pre>\n
Using ENTRYPOINT and CMD in Same Dockerfile<\/h2>\n
You can also use both CMD<\/code> and ENTRYPOINT<\/code> instructions to achieve this. Here is how the Dockerfile looks.<\/p>\n
FROM centos:7\nMAINTAINER Devopscube\nRUN yum -y update && \\\n    yum -y install httpd-tools && \\\n    yum clean all\nENTRYPOINT [\"ab\"]\nCMD [\"http:\/\/dummy-url.com\/\"]<\/code><\/pre>\n
When ENTRYPOINT<\/code> and CMD<\/code> used in the same Dockerfile, everything in the CMD instruction will be appended to the ENTRYPOINT as an argument.<\/p>\n
If you run a container using the above Dockerfile, at container start, ab script will get executed with the dummy-url.com <\/code>as an argument.<\/p>\n
ENTRYPOINT and CMD in Kubernetes Pod<\/h2>\n
Lets understand how Docker Entrypoint and CMD translates to Kubernetes<\/a> Pod.<\/p>\n
When it comes to Kubernetes Pod, we have the option named “command<\/code>” and “args<\/code>” in the Pod Specification to override Entrypoint and CMD set in the Docker image.<\/p>\n
In Kubernetes Pod if you want to override Docker CMD or Entrypoint, you need to specify the command and args show below.<\/p>\n
apiVersion: v1\nkind: Pod\nmetadata:\n  name: my-pod\nspec:\n  containers:\n  - name: my-container\n    image: my-image\n    command: [\"my-script.sh\"]\n    args: [\"arg1\", \"arg2\"]<\/code><\/pre>\n
In this example, the entrypoint<\/code> is set to [\"\/bin\/bash\", \"-c\"]<\/code>, which means that when the container starts, it will run the bash<\/code> shell with the -c<\/code> flag to execute the command specified in the command<\/code> field (my-script.sh<\/code>). The args<\/code> field specifies any additional arguments to be passed to my-script.sh<\/code>.<\/p>\n
Here is what you should know.<\/p>\n
    \n
  1. In case a Command or Args is not provided for a container, the defaults specified by the image will be utilized.<\/li>\n
  2. However, if you provide a Command without any Args for the container, only the specified Command will be executed while disregarding the default arguments of the image.<\/li>\n
  3. On the other hand, if only Args are supplied, the image’s default command will be used in conjunction with the provided arguments.<\/li>\n
  4. Lastly, if both Command and Args are supplied, the defaults of the image will be overridden and the supplied values will be utilized.<\/li>\n<\/ol>\n
    \n
    Ngu\u1ed3n:<\/strong> Docker ENTRYPOINT Vs CMD Explained With Examples \u2014 DevOpsCube<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
    Source: https:\/\/devopscube.com\/entrypoint-vs-cmd-explained\/<\/p>\n","protected":false},"author":1,"featured_media":1079,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1078","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-devops"],"_links":{"self":[{"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=\/wp\/v2\/posts\/1078","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1078"}],"version-history":[{"count":0,"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=\/wp\/v2\/posts\/1078\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=\/wp\/v2\/media\/1079"}],"wp:attachment":[{"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1078"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1078"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.ngocha.biz\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1078"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}