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Dockerfile 中的 ENTRYPOINT 指令.md

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Dockerfile 中的 ENTRYPOINT 指令

Ref:

示例脚本

Postgres Official Image 使用如下脚本内容作为其 ENTRYPOINT ：

Set options directly on the run line. The entrypoint script is made so that any options passed to the docker command will be passed along to the postgres server daemon.

好处在于，可以在执行 docker run 等命令时，指定需要传入到内部具体应用的 options ；

If you need to write a starter script for a single executable, you can ensure that the final executable receives the Unix signals by using exec and gosu commands:

在使用了 exec 和 gosu 后，能确保目标应用程序收到 Unix signals ；

#!/bin/bash
set -e

if [ "$1" = 'postgres' ]; then
    chown -R postgres "$PGDATA"

    if [ -z "$(ls -A "$PGDATA")" ]; then
        gosu postgres initdb
    fi

    exec gosu postgres "$@"
fi

exec "$@"

注意：gosu 后面的参数 postgres 为 user-spec ；

基础

ENTRYPOINT 指令具有两种使用形式：

  • ENTRYPOINT ["executable", "param1", "param2"] -- exec 形式，推荐
  • ENTRYPOINT command param1 param2 -- shell 形式

若存在多条 ENTRYPOINT 指令，则只有最后一条生效；

常规用法

The best use for ENTRYPOINT is to set the image’s main command, allowing that image to be run as though it was that command (and then use CMD as the default flags).

在 ENTRYPOINT 指令中设置镜像主命令；在 CMD 中设置默认 flags ；

This is useful because the image name can double as a reference to the binary as shown in the command above.

好处在于，可以直接镜像名字当做二进制程序命令来用；

高级用法

The ENTRYPOINT instruction can also be used in combination with a helper script, allowing it to function in a similar way to the command above, even when starting the tool may require more than one step.

将 ENTRYPOINT 指令和 script 脚本结合使用；

The helper script is copied into the container and run via ENTRYPOINT on container start:

使用套路如下

COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["postgres"]

使用了上述脚本后，用户可以通过如下几种方式和 Postgres 进行销户：

  • 简单启动 Postgres
$ docker run postgres
  • 启动 Postgres 的同时传递一些 parameters 到内部
$ docker run postgres postgres --help

注：postgres --help 将作为替代 Dockerfile 中 CMD 指令的内容使用；

  • 启动 Bash 等其他工具（方便调试使用）
$ docker run --rm -it postgres bash

Q&A

The exec builtin command

The exec builtin command is used to

  • replace the shell with a given program (executing it, not as new process)
  • set redirections for the program to execute or for the current shell

exec 的用途：

  • 使用指定程序替代当前 shell 进程（不创建新进程）；
  • 用于重定向指定程序的 stdin/stdout/stderr ；

If only redirections are given, the redirections affect the current shell without executing any program.

Why we want to configure app as PID 1

This script uses the exec Bash command so that the final running application becomes the container’s PID 1. This allows the application to receive any Unix signals sent to the container.

  • 使用 exec 执行目标程序后，目标程序将会成为容器的 PID 1 进程；
  • 容器的 PID 1 进程可以接收到任何发送给容器的 Unix signals ；

The shell form (of ENTRYPOINT) prevents any CMD or run command line arguments from being used, but has the disadvantage that your ENTRYPOINT will be started as a subcommand of /bin/sh -c, which does not pass signals. This means that the executable will not be the container’s PID 1 - and will not receive Unix signals - so your executable will not receive a SIGTERM from docker stop <container>.

  • shell 形式的 ENTRYPOINT 指令在运行指定程序时，会将其作为 /bin/sh -c 的子命令（子进程）；
  • 因为常规 shell 进程不会负责 signals 转发，通常情况下 signals 转发是由具有 daemon 特性的程序来负责的，例如 init 等；因此，无法进行 Unix signals 的传递就没啥奇怪的了；
  • 理论上讲，PID 1 的特殊性在于其作为容器内外沟通的桥梁，例如针对 signals 的处理，所以，要么将业务进程直接作为 PID 1 运行，要么就需要使用一个具有 signals 处理功能的 daemon 进程来管理业务进程；很明显，常规 shell 不具备该能力；

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.

  • Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
  • ENTRYPOINT should be defined when using the container as an executable.
  • 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.
  • 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_entry p1_entry exec_cmd p1_cmd
CMD [“p1_cmd”, “p2_cmd”] p1_cmd p2_cmd /bin/sh -c exec_entry p1_entry 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 exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd