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Docs: add note about CMD and ENTRYPOINT commands

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Signed-off-by: Tomasz Kopczynski <tomek@kopczynski.net.pl>
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Tomasz Kopczynski 2016-02-23 22:03:45 +01:00
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docs/reference/builder.md
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@ -950,6 +950,29 @@ If you then run `docker stop test`, the container will not exit cleanly - the
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### Understand how CMD and ENTRYPOINT interact
Both `CMD` and `ENTRYPOINT` instructions define what command gets executed when running a container.
There are few rules that describe their co-operation.
1. Dockerfile should specify at least one of `CMD` or `ENTRYPOINT` commands.
2. `ENTRYPOINT` should be defined when using the container as an executable.
3. `CMD` should be used as a way of defining default arguments for an `ENTRYPOINT` command
or for executing an ad-hoc command in a container.
4. `CMD` will be overridden when running the container with alternative arguments.
The table below shows what command is executed for different `ENTRYPOINT` / `CMD` combinations:
| | No ENTRYPOINT | ENTRYPOINT exec_entry p1_entry | ENTRYPOINT ["exec_entry", "p1_entry"] |
|--------------------------------|----------------------------|-----------------------------------------------------------|------------------------------------------------|
| **No CMD** | *error, not allowed* | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry |
| **CMD ["exec_cmd", "p1_cmd"]** | exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry exec_cmd p1_cmd | exec_entry p1_entry exec_cmd p1_cmd |
| **CMD ["p1_cmd", "p2_cmd"]** | p1_cmd p2_cmd | /bin/sh -c exec_entry p1_entry p1_cmd p2_cmd | exec_entry p1_entry p1_cmd p2_cmd |
| **CMD exec_cmd p1_cmd** | /bin/sh -c exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd | exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd |
## VOLUME
VOLUME ["/data"]