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Dockerfile ARG指令

Docker的文档关于ARG和FROM指令配合使用做了详细说明:

ARG用于传入外部参数,定义在FROM指令前,FROM后的其他指令无法使用ARG定义的环境变量,如果FROM指令后的指令要使用ARG定义的值,需要在FROM后再次定义。如果FROM不使用定义的ARG,可以直接定义到FROM后。

传递参数

定义在FROM前

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ARG UBUNTU_VERSION=16.04
FROM ubuntu:${UBUNTU_VERSION}
RUN env | grep UBUNTU_VERSION

从结果可以看到FROM后的指令无法访问ARG定义的UBUNTU_VERSION

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$ docker build -t test:afterfrom .
Sending build context to Docker daemon 37.38kB
Step 1/3 : ARG UBUNTU_VERSION=16.04
Step 2/3 : FROM ubuntu:${UBUNTU_VERSION}
---> 4b22027ede29
Step 3/3 : RUN env | grep UBUNTU_VERSION
---> Running in 8e93ca5e376b
The command '/bin/sh -c env | grep UBUNTU_VERSION' returned a non-zero code: 1

定义在FROM后

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ARG UBUNTU_VERSION=16.04
FROM ubuntu:${UBUNTU_VERSION}
ARG UBUNTU_VERSION
RUN env | grep UBUNTU_VERSION

从结果可以看到FROM后的指令可以访问ARG定义的UBUNTU_VERSION

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$ docker build -t test:afterfrom .
Sending build context to Docker daemon 37.38kB
Step 1/4 : ARG UBUNTU_VERSION=16.04
Step 2/4 : FROM ubuntu:${UBUNTU_VERSION}
---> 4b22027ede29
Step 3/4 : ARG UBUNTU_VERSION
---> Using cache
---> e3bae0875e66
Step 4/4 : RUN env | grep UBUNTU_VERSION
---> Running in 02438aff1f75
UBUNTU_VERSION=16.04
Removing intermediate container 02438aff1f75
---> 646eec496165
Successfully built 646eec496165
Successfully tagged test:afterfrom

传递多个ARG参数

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ARG UBUNTU_VERSION=16.04
ARG FILE_NAME=test
FROM ubuntu:${UBUNTU_VERSION}

ARG FILE_NAME
WORKDIR /test/
RUN touch ${FILE_NAME}
RUN ls ${FILE_NAME}

传递多个参数需要多次使用--build-arg,可以看到传递的FILE_NAME=app生效了,而不是默认值。

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$ docker build -t test:twoargs --build-arg UBUNTU_VERSION=18.04 --build-arg FILE_NAME=app .
Sending build context to Docker daemon 37.38kB
Step 1/7 : ARG UBUNTU_VERSION=16.04
Step 2/7 : ARG FILE_NAME=test
Step 3/7 : FROM ubuntu:${UBUNTU_VERSION}
18.04: Pulling from library/ubuntu
171857c49d0f: Pull complete
419640447d26: Pull complete
61e52f862619: Pull complete
Digest: sha256:646942475da61b4ce9cc5b3fadb42642ea90e5d0de46111458e100ff2c7031e6
Status: Downloaded newer image for ubuntu:18.04
---> 56def654ec22
Step 4/7 : ARG FILE_NAME
---> Running in bee623328f35
Removing intermediate container bee623328f35
---> 52f803da2959
Step 5/7 : WORKDIR /test/
---> Running in fa4542584af1
Removing intermediate container fa4542584af1
---> 5ebd782db9b8
Step 6/7 : RUN touch ${FILE_NAME}
---> Running in 0cd5723b744d
Removing intermediate container 0cd5723b744d
---> 920c6bd75bab
Step 7/7 : RUN ls ${FILE_NAME}
---> Running in b94a33093ddf
app
Removing intermediate container b94a33093ddf
---> 804bf831059a
Successfully built 804bf831059a
Successfully tagged test:twoargs

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