Dockerfile RUN, CMD & ENTRYPOINT

在使用Dockerfile建立image時, 有幾條指令比較容易混淆, RUN, CMD, ENTRYPOINT.

RUN是在building image時會運行的指令, 在Dockerfile中能夠寫多條RUN指令.

CMD和ENTRYPOINT則是在運行container 時會運行的指令, 都只能寫一條, 若是寫了多條, 則最後一條生效.

CMD和ENTRYPOINT的區別是: 

CMD在運行時會被command覆蓋, ENTRYPOINT不會被運行時的command覆蓋, 可是也能夠指定.

例如 : 

Usage: docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

  --entrypoint=""            Overwrite the default entrypoint of the image

 

docker run postgres:9.3.5 psql 

這裏的psql就是command, 將覆蓋Dockerfile的CMD, 可是不會覆蓋ENTRYPOINT.

若是要覆蓋ENTRYPOINT, 那麼能夠在docker run運行時輸入 --entrypoint="....".

 

CMD和ENTRYPOINT通常用於製做具有後臺服務的image, 例如apache, database等. 在使用這種image啓動container時, 自動啓動服務.

 

這幾條指令的詳細用法 : 

RUN

RUN has 2 forms:

• RUN <command> (the command is run in a shell - /bin/sh -c - shell form)
• RUN ["executable", "param1", "param2"] (exec form)

The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.

Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control.

The exec form makes it possible to avoid shell string munging, and to RUN commands using a base image that does not contain /bin/sh.

Note: To use a different shell, other than '/bin/sh', use the exec form passing in the desired shell. For example, RUN ["/bin/bash", "-c", "echo hello"]

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

The cache for RUN instructions isn't invalidated automatically during the next build. The cache for an instruction like RUN apt-get dist-upgrade -y will be reused during the next build. The cache for RUNinstructions can be invalidated by using the --no-cache flag, for example docker build --no-cache.

See the Dockerfile Best Practices guide for more information.

The cache for RUN instructions can be invalidated by ADD instructions. See below for details.

Known Issues (RUN)

• Issue 783 is about file permissions problems that can occur when using the AUFS file system. You might notice it during an attempt to rm a file, for example. The issue describes a workaround.

 

CMD

The CMD instruction has three forms:

• CMD ["executable","param1","param2"] (exec form, this is the preferred form)
• CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
• CMD command param1 param2 (shell form)

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the lastCMD will take effect.

The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINTinstruction as well.

Note: If CMD is used to provide default arguments for the ENTRYPOINT instruction, both the CMD andENTRYPOINT instructions should be specified with the JSON array format.

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

When used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image.

If you use the shell form of the CMD, then the <command> will execute in /bin/sh -c:

FROM ubuntu
CMD echo "This is a test." | wc -

If you want to run your <command> without a shell then you must express the command as a JSON array and give the full path to the executable. This array form is the preferred format of CMD. Any additional parameters must be individually expressed as strings in the array:

FROM ubuntu
CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every time, then you should consider usingENTRYPOINT in combination with CMD. See ENTRYPOINT.

If the user specifies arguments to docker run then they will override the default specified in CMD.

Note: don't confuse RUN with CMD. RUN actually runs a command and commits the result; CMDdoes not execute anything at build time, but specifies the intended command for the image.

 

 

ENTRYPOINT

ENTRYPOINT has two forms:

• ENTRYPOINT ["executable", "param1", "param2"] (exec form, the preferred form)
• ENTRYPOINT command param1 param2 (shell form)

There can only be one ENTRYPOINT in a Dockerfile. If you have more than one ENTRYPOINT, then only the last one in the Dockerfile will have an effect.

An ENTRYPOINT helps you to configure a container that you can run as an executable. That is, when you specify an ENTRYPOINT, then the whole container runs as if it was just that executable.

Unlike the behavior of the CMD instruction, The ENTRYPOINT instruction adds an entry command that willnot be overwritten when arguments are passed to docker run. This allows arguments to be passed to the entry point, i.e. docker run <image> -d will pass the -d argument to the entry point.

You can specify parameters either in the ENTRYPOINT JSON array (as in "like an exec" above), or by using a CMD instruction. Parameters in the ENTRYPOINT instruction will not be overridden by the docker runarguments, but parameters specified via a CMD instruction will be overridden by docker run arguments.

Like a CMD, you can specify a plain string for the ENTRYPOINT and it will execute in /bin/sh -c:

FROM ubuntu
ENTRYPOINT ls -l

For example, that Dockerfile's image will always take a directory as an input and return a directory listing. If you wanted to make this optional but default, you could use a CMD instruction:

FROM ubuntu
CMD ["-l"]
ENTRYPOINT ["ls"]

Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

Note: It is preferable to use the JSON array format for specifying ENTRYPOINT instructions.