Dockerfile difference in ENTRYPOINT with the CMD
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Dockerfile There are two boot configuration, CMD and ENTRYPOINT, configuration commands can be executed automatically when the vessel started Dockerfile in, but what difference does both, then summarized as follows:
First look at the CMD, there are three formats of configuration, are:
- CMD [ "executable", "param1", "param2"] (exec format, the format recommended)
- CMD [ "param1", "param2"] (as ENTRYPOINT default parameters)
- CMD command param1 param2 (shell format)
In use, should pay attention to the problem:
- When contains an executable file, execute the command specified as the default container, this is the main purpose;
- When that does not contain an executable file, it must provide ENTRYPOINT configuration, CMD as the default parameters;
- In only one CMD Dockerfile onset, if a plurality appears, the last onset;
And ENTRYPOINT, there are two formats configured, namely:
- ENTRYPOINT [ "executable", "param1", "param2"] (exec format is recommended)
- ENTRYPOINT command param1 param2 (shell format)
CMD and ENTRYPOINT format is the difference between the two configurations:
- exec format this format is recommended when using this mode, you do not need to shell process, so that the executable program Docker applications become PID 1 process container, Unix can receive signals, such as the implementation of docker when the stop can receive SIGTERM.
- shell format will always invoke a shell process, become the / bin / sh -c sub-command, Unix executable program can not respond to the signal.
Note: When using the shell ENTRYPOINT format, ignore CMD and run the incoming parameters, if you want to replace the default ENTRYPOINT command, you need to specify --entrypoint parameters in the implementation of docker run.
When using the CMD and ENTRYPOINT summarized as follows:
- In Dockerfile, the CMD and should specify at least one EntryPoint;
- When Docker as an executable program, you should use ENTRYPOINT be configured;
- ENTRYPOINT CMD can be used as default parameters or as the default command Docker;
- CMD can be covered docker run parameters passed;
- After docker run the incoming parameters will be appended to the ENTRYPOINT, provided that the use of exec format.
Finally, we look at an example:
ENTRYPOINT use exec format, ENTRYPOINT the argument always used, and CMD additional parameters can be dynamically replaced when the container starts.
Dockerfile example, the following fragment:
ENTRYPOINT ["/bin/echo", "Hello"] CMD ["world"]
When activated by the container docker run -it [image], output:
$ docker run -it [image] Hello world
If -it [image] CloudMan initiated by docker run, the output is:
$ docker run -it [image] CloudMan
Hello CloudMan