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make the cache miss clear 

Signed-off-by: mikelinjie <294893458@qq.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
master
mikelinjie 2016-03-16 15:26:57 +08:00 committed by Tibor Vass
parent b394f05fee
commit bc060f1f19
1 changed files with 29 additions and 8 deletions
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37
docs/reference/builder.md
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@ -1174,8 +1174,10 @@ To use these, simply pass them on the command line using the `--build-arg
`ARG` variables are not persisted into the built image as `ENV` variables are.
However, `ARG` variables do impact the build cache in similar ways. If a
Dockerfile defines an `ARG` variable whose value is different from a previous
build, then a "cache miss" occurs upon its first usage, not its declaration.
For example, consider this Dockerfile:
build, then a "cache miss" occurs upon first use of the `ARG` variable. The
declaration of the `ARG` variable does not count as a use.
For example, consider these two Dockerfile:
```
1 FROM ubuntu
@ -1183,12 +1185,17 @@ For example, consider this Dockerfile:
3 RUN echo $CONT_IMG_VER
```
If you specify `--build-arg CONT_IMG_VER=<value>` on the command line the
specification on line 2 does not cause a cache miss; line 3 does cause a cache
miss. The definition on line 2 has no impact on the resulting image. The `RUN`
on line 3 executes a command and in doing so defines a set of environment
variables, including `CONT_IMG_VER`. At that point, the `ARG` variable may
impact the resulting image, so a cache miss occurs.
```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo hello
```
If you specify `--build-arg CONT_IMG_VER=<value>` on the command line, in both
cases, the specification on line 2 does not cause a cache miss; line 3 does
cause a cache miss.`ARG CONT_IMG_VER` causes the RUN line to be identified
as the same as running `CONT_IMG_VER=<value>` echo hello, so if the `<value>`
changes, we get a cache miss.
Consider another example under the same command line:
@ -1203,6 +1210,20 @@ the variable's value in the `ENV` references the `ARG` variable and that
variable is changed through the command line. In this example, the `ENV`
command causes the image to include the value.
If an `ENV` instruction overrides an `ARG` instruction of the same name, like
this Dockerfile:
```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER hello
4 RUN echo $CONT_IMG_VER
```
Line 3 does not cause a cache miss because the value of `CONT_IMG_VER` is a
constant (`hello`). As a result, the environment variables and values used on
the `RUN` (line 4) doesn't change between builds.
## ONBUILD
ONBUILD [INSTRUCTION]