【Docker】Dockerfile 之 ENTRYPOINT（二）

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

环境

1. virtual box 6.1
2. centos 7.8
3. docker 19.03

ENTRYPOINT 的 exec 格式示例

You can use the exec form of ENTRYPOINT to set fairly stable default commands and arguments and then use either form of CMD to set additional defaults that are more likely to be changed.
您可以使用 ENTRYPOINT 的 exec 形式来设置相当稳定的默认命令和参数，然后使用 CMD 的任一种形式来设置更可能被更改的其他默认值。

FROM ubuntu
ENTRYPOINT ["top", "-b"]
CMD ["-c"]

When you run the container, you can see that top is the only process:
运行容器时，可以看到 top 是唯一的进程：

$ docker run -it --rm --name test  top -H
top - 03:06:16 up 2 days, 12:28,  0 users,  load average: 3.28, 4.85, 4.80
Threads:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us, 33.3 sy,  0.0 ni, 66.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :  32174.9 total,  10607.0 free,   9970.9 used,  11597.0 buff/cache
MiB Swap:      0.0 total,      0.0 free,      0.0 used.  21364.0 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
    1 root      20   0    5968   3336   2908 R   0.0   0.0   0:00.03 top

To examine the result further, you can use docker exec:
要进一步检查结果，可以使用 docker exec：

$ docker exec -it test ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.0   5968  3336 ?        Ss   03:08   0:00 top -b -H
root         8  0.0  0.0   5896  2928 pts/0    Rs+  03:08   0:00 ps aux

And you can gracefully request top to shut down using docker stop test.
您可以使用 docker stop test 优雅地请求 top 关闭。

The following Dockerfile shows using the ENTRYPOINT to run Apache in the foreground (i.e., as PID 1):
以下 Dockerfile 显示了使用 ENTRYPOINT 在前台运行 Apache（即，作为 PID 1）：

FROM debian:stable
RUN apt-get update && apt-get install -y --force-yes apache2
EXPOSE 80 443
VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"]
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]

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:
如果需要为单个可执行文件编写启动脚本，则可以使用 exec 和 gosu 命令确保最终的可执行文件接收 Unix 信号：

#!/usr/bin/env 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 "$@"

Lastly, if you need to do some extra cleanup (or communicate with other containers) on shutdown, or are co-ordinating more than one executable, you may need to ensure that the ENTRYPOINT script receives the Unix signals, passes them on, and then does some more work:
最后，如果您需要在关机时进行一些额外的清理工作（或与其他容器通信），或者要协调多个可执行文件，则可能需要确保 ENTRYPOINT 脚本接收 Unix 信号，然后将其传递，然后做更多的工作：

#!/bin/sh
# Note: I've written this using sh so it works in the busybox container too

# USE the trap if you need to also do manual cleanup after the service is stopped,
#     or need to start multiple services in the one container
trap "echo TRAPed signal" HUP INT QUIT TERM

# start service in background here
/usr/sbin/apachectl start

echo "[hit enter key to exit] or run 'docker stop <container>'"
read

# stop service and clean up here
echo "stopping apache"
/usr/sbin/apachectl stop

echo "exited $0"

If you run this image with docker run -it --rm -p 80:80 --name test apache, you can then examine the container’s processes with docker exec, or docker top, and then ask the script to stop Apache:
如果使用 docker run -it --rm -p 80:80 --name test apache 运行该映像，则可以使用 docker exec 或 docker top 检查容器的进程，然后询问脚本停止 Apache：

$ docker exec -it test ps aux

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.0   4448   692 ?        Ss+  00:42   0:00 /bin/sh /run.sh 123 cmd cmd2
root        19  0.0  0.2  71304  4440 ?        Ss   00:42   0:00 /usr/sbin/apache2 -k start
www-data    20  0.2  0.2 360468  6004 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
www-data    21  0.2  0.2 360468  6000 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
root        81  0.0  0.1  15572  2140 ?        R+   00:44   0:00 ps aux

$ docker top test

PID                 USER                COMMAND
10035               root                {run.sh} /bin/sh /run.sh 123 cmd cmd2
10054               root                /usr/sbin/apache2 -k start
10055               33                  /usr/sbin/apache2 -k start
10056               33                  /usr/sbin/apache2 -k start

$ /usr/bin/time docker stop test

test
real	0m 0.27s
user	0m 0.03s
sys	0m 0.03s

Note

You can override the ENTRYPOINT setting using --entrypoint, but this can only set the binary to exec (no sh -c will be used).

注意

您可以使用 --entrypoint 覆盖 ENTRYPOINT 设置，但这只能将二进制文件设置为 exec（将不使用 sh -c）。

Note

The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

注意

exec 格式被解析为 JSON 数组，这意味着您必须在单词引用时使用双引号而不是单引号。

Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, ENTRYPOINT [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: ENTRYPOINT [ "sh", "-c", "echo $HOME" ]. When using the exec form and executing a shell directly, as in the case for the shell form, it is the shell that is doing the environment variable expansion, not docker.

与 shell 格式不同，exec 格式不调用命令外壳程序。这意味着不会进行常规的外壳处理。例如，ENTRYPOINT [ "echo", "$HOME" ] 不会在 $HOME 上进行变量替换。如果要进行 shell 处理，则可以使用 shell 形式或直接执行 shell，例如： ENTRYPOINT [ "sh", "-c", "echo $HOME" ]。当使用 exec 格式并直接执行 shell 时，是由 shell 进行环境变量扩展，而不是 docker。

总结

介绍了 Dockerfile 中 ENTRYPOINT 指令的 exec 格式的使用。