![docker run image specify entrypoint docker run image specify entrypoint](https://phoenixnap.com/kb/wp-content/uploads/2021/04/dockerfile-entrypoint-with-cmd.png)
Some images contain a so-called “wrapper” that decorates a legacy program or otherwise prepares it for use in a containerized environment. Consider the following use cases: Wrappers My experience, though, is that ENTRYPOINT suits almost every case I’ve encountered. The choice you make is largely an artistic one, and it will depend significantly on your use case. But now we need to know which to choose: When is it better to use ENTRYPOINT, and when is it better to use CMD? At this point, we understand the mechanics of how ENTRYPOINT and CMD work together to construct a default argument list for a container. Supposed we’re building our own Dockerfile for a project. Here’s the corresponding argument list - by this point there should be no surprises: When Should I Use ENTRYPOINT? How About CMD? Per our general rule, the following argument list will be constructed: Overriding Both ENTRYPOINT and CMDĬan we override both ENTRYPOINT and CMD? Certainly: docker run -entrypoint /bin/logwrap myservice /bin/service -e Now, let’s change the ENTRYPOINT by running the following: docker run -entrypoint /bin/logwrap myservice Suppose, as before, we have the following Dockerfile and create an image from it called myservice: ENTRYPOINT To do so, we specify the -entrypoint option argument to docker run. We can easily override the ENTRYPOINT declared in a Dockerfile as well. Note that CMD is entirely replaced - it’s not prepended or appended. If we instead invoke docker run myservice /bin/debug, the container will be created with these arguments instead: If we invoke docker run myservice, the container will be created with the following arguments: To illustrate, suppose we have the following Dockerfile and create an image from it called myservice: ENTRYPOINT Specifying CMD in a Dockerfile merely creates a default value: if we pass non-option arguments to docker run, they will override the value of CMD. This is among the reasons I always recommend specifying arrays whenever possible. This is an unfortunate but unavoidable consequence of the way argument strings are converted to arrays. They can both be array values, and ENTRYPOINT can be an array value and CMD can be a string value but if ENTRYPOINT is a string value, CMD will be ignored. Note: ENTRYPOINT and CMD cannot both be string values. Here’s an example that illustrates the point.
![docker run image specify entrypoint docker run image specify entrypoint](https://devopspoints.com/uploads/library/docker/img_5dafcb835d8c1.png)
![docker run image specify entrypoint docker run image specify entrypoint](https://phoenixnap.com/kb/wp-content/uploads/2021/04/dockerfile-entrypoint.png)
So when we declare both an ENTRYPOINT and a CMD, and ENTRYPOINT is a list, the two are concatenated together to form a default argument list - even if we declare CMD as a string. Suppose we declare a CMD that starts a web server as follows: CMD /usr/bin/httpd -DFOREGROUNDĭocker will automatically convert CMD to an array that looks like this: (I always recommend declaring them as arrays, though, to avoid ambiguity.) It’s important to understand that, in a Dockerfile, ENTRYPOINT, and CMD are always converted to arrays - even if you declare them as strings. (In fact, this relates to what shells primarily do: they take space-separated “commands” at the prompt, and ultimately turn them into arrays of arguments for passing to the exec system call.) Arguments are Always Arrays This list roughly approximates the shell command /bin/chamber exec production - /bin/service -d. Putting these together, the default arguments to the container will be. Here’s a Dockerfile snippet that has both an ENTRYPOINT and a CMD, both specified as arrays: ENTRYPOINT For example, if a key /production/mysql/password is found, then Chamber would set the MYSQL_PASSWORD environment variable to the secure value within. A typical invocation is chamber exec production - program, which fetches all Parameter Store values whose keys are prefixed with /production, converts slashes in the keys to underscores, and populates the environment with the returned keys and values. To demonstrate the benefit of ENTRYPOINTs, we introduce Chamber, an open-source utility that populates the container’s environment with values found in AWS Systems Manager Parameter Store.
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If there’s one lesson you should take away today, it’s the following general rule:ĮNTRYPOINT + CMD = default container command arguments 中文版 – As you begin your Docker container creation journey, you might find yourself faced with a puzzling question: Should your Dockerfile contain an ENTRYPOINT instruction, a CMD instruction, or both? In this post, I discuss the differences between the two in detail, and explain how best to use them in various use cases you might encounter.