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docker - Why does the entrypoint in Docker starts with sh in running jar?

问题描述

Why does this entrypoint uses sh in running the jar? ENTRYPOINT ["sh", "-c", "java -jar app.jar"]. What would be its difference in just using ENTRYPOINT ["java", "-jar", "app.jar"]?

标签： dockerdockerfile

解决方案

该ENTRYPOINT指令（与 CMDand一起RUN）有两种形式。如果您使用JSON 数组语法，则没有解释、分词或任何其他处理；你传入的正是运行的内容。

# Three words
ENTRYPOINT ["java", "-jar", "app.jar"]
# You're already quoting things so spaces stay as part of each word
ENTRYPOINT ["java", "-jar", "my app.jar"]
# There is no interpolation so variable names do not get expanded --
# this looks for a file literally named `$JARFILE`
ENTRYPOINT ["java", "-jar", "$JARFILE"]
# There is only one command-line option with embedded spaces
ENTRYPOINT ["java", "-jar", "app.jar", "-a -b -c"]

在某些情况下（尤其是变量扩展），您确实需要一个 shell 来运行。这就是sh -c进来的地方。它需要一个单词并将其作为 shell 命令处理。然后它（大部分）忽略所有其他选项，这意味着CMD实际上被忽略了。

# This expands the environment variable `$JARFILE`, but if
# you pass additional options in `CMD`, they're lost
ENTRYPOINT ["sh", "-c", "java -jar $JARFILE"]

不过，您实际上不应该使用这种语法，因为 Docker 提供了一种可以自动为您插入的纯字符串语法sh -c。

# Exactly the same as above
ENTRYPOINT java -jar $JARFILE
# Three additional command-line options, since the shell does splitting
ENTRYPOINT java -jar app.jar -a -b -c

请记住，Docker 会将and组合成一个命令ENTRYPOINTCMD。这可能导致“容器即命令”模式，但您必须使用 JSON 数组形式ENTRYPOINT，并且它不能有sh -c包装器。

ENTRYPOINT ["java", "-jar", "app.jar"]
CMD ["-a", "-b", "-c"]

（还请记住，CMD在命令中覆盖要容易得多docker run，并且有一种有用的模式，即使用ENTRYPOINT包装脚本进行首次设置，然后CMD作为主容器进程执行。对于像您展示的简单命令，您可能更喜欢CMD。）

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