【Docker】Dockerfile 之 ARG（一）

参考教程：https://docs.docker.com/engine/reference/builder/

环境

virtual box 6.1
centos 7.8
docker 19.03

ARG

ARG <name>[=<default value>]

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag. If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning.

ARG 指令定义了一个变量，用户可以在构建时使用 docker build 命令使用--build-arg <varname>=<value> 标志将其传递给构建器。如果用户指定了未在 Dockerfile 中定义的构建参数，则构建会输出警告。

[Warning] One or more build-args [foo] were not consumed.

A Dockerfile may include one or more ARG instructions. For example, the following is a valid Dockerfile:

Dockerfile 可能包含一个或多个 ARG 指令。例如，以下是有效的 Dockerfile：

FROM busybox
ARG user1
ARG buildno
# ...

Warning:

It is not recommended to use build-time variables for passing secrets like github keys, user credentials etc. Build-time variable values are visible to any user of the image with the docker history command.

警告：

不建议使用构建时变量来传递诸如 github 密钥，用户凭据等机密。构建时变量值对于使用 docker history 命令的镜像的任何用户都是可见的。

默认值

An ARG instruction can optionally include a default value:

ARG 指令可以选择包含默认值：

FROM busybox
ARG user1=someuser
ARG buildno=1
# ...

If an ARG instruction has a default value and if there is no value passed at build-time, the builder uses the default.

如果 ARG 指令具有默认值，并且在构建时未传递任何值，则构建器将使用默认值。

范围

An ARG variable definition comes into effect from the line on which it is defined in the Dockerfile not from the argument’s use on the command-line or elsewhere. For example, consider this Dockerfile:

ARG 变量从 Dockerfile 中定义的行开始生效，而不是从命令行或其他地方的自变量使用开始。例如，考虑以下 Dockerfile：

FROM busybox
USER ${user:-some_user}
ARG user
USER $user
# ...

A user builds this file by calling:

$ docker build --build-arg user=what_user .

The USER at line 2 evaluates to some_user as the user variable is defined on the subsequent line 3. The USER at line 4 evaluates to what_user as user is defined and the what_user value was passed on the command line. Prior to its definition by an ARG instruction, any use of a variable results in an empty string.

第 2 行的 USER 评估为 some_user，因为在随后的第 3 行中定义了 USER 变量。第 4 行的 USER 评估为 what_user，因为定义了 user，并且为 what_user 在命令行中传递。在通过 ARG 指令对其进行定义之前，对变量的任何使用都会导致一个空字符串。

An ARG instruction goes out of scope at the end of the build stage where it was defined. To use an arg in multiple stages, each stage must include the ARG instruction.

ARG 指令在定义它的构建阶段结束时超出范围。要在多个阶段使用变量，每个阶段都必须包含 ARG 指令。

FROM busybox
ARG SETTINGS
RUN ./run/setup $SETTINGS

FROM busybox
ARG SETTINGS
RUN ./run/other $SETTINGS

总结

介绍了 Dockerfile 中 ARG 指令的说明，默认值和范围。