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When writing Dockerfile, include an entrypoint configuration, which is used to do some initialization configuration, or some custom configuration, before the container starts. It is usually a script in which the relevant predefined items are configured. This document details the writing skills of entrypoint entry files.

Take the entrypoint file docker-entrypoint.sh in the official mysql image as an example. The file address is:

Docker-entrypoint.sh

Set-e

Every script you write should add set-e at the beginning of the file, which tells bash to quit if the result of any statement is not true. The advantage is to prevent errors from snowballing into a fatal error that should have been dealt with before. If you want to increase readability, you can use set-o errexit, which works the same as set-e

Set-o pipefail

The purpose of the design is the same as above, that is, I hope to exit immediately after the execution error and not to execute it down again. The scope of-o pipefail is pipes, that is, pipes in Linux scripts. If there is a problem with the execution of the previous command, you should exit immediately.

Shopt-s nullglob

When using wildcards in Linux *? If there is no match to any file, the No such file or directory will not be reported but the parameters after the command will be removed for execution.

If ["${1:0:1}" ='-']; then...

This is a judgment statement, and in the official document, the previous line has given a comment: if command starts with an option, prepend mysqld

This judgment statement is ${1:0:1} which means to judge $1 (the first parameter to call the script), offset 0 (no offset), and take a character (take the length of the string).

If it is determined that the first character of the parameter followed by the call to the script is the-middle dash, all the subsequent strings are considered to be the startup parameters of mysqld.

The above operation is similar to the string slicing of Python

Set-mysqld "$@"

After determining that the first argument begins with -, the command set-- mysqld "$@" is executed. The use of set-- is used. Set-- stores all the strings separated by spaces in the variables $1, $2, and $3 in order, where the new $@ is set-- everything that follows.

For example: bash docker-entrypoint.sh-f xxx.conf

In this case, the value of $@ in set-- mysqld "$@" is-f xxx.conf

After executing the command set-- mysqld "$@":

$1=mysqld$2=-f$3=xxx.conf$@=mysqld-f xxx.conf

You can see that when the docker-entrypoint.sh script is executed with a parameter in the form of-x, the value of $@ is changed, based on the original value of $@, and the mysqld command is pre-added before

Exec "$@"

On the last line of almost every docker-entrypoint.sh script, the exec "$@" command is executed

The meaning of this command is that you have anticipated the invocation situation for your image. When the person actually using the image executes an executable command that you did not expect, it will go to the last line of the script to execute the user's new executable command.

Situation judgment

The last line of the script is directly mentioned above. In the previous script, you need to fully consider the situation in which your own script might be called. Or take MySQL's official dockerfile, who judges the following:

It starts with -, if you think it is a parameter, it starts with mysqld, and if the user id is 0 (root user), it starts with mysqld. After judging all the invocation patterns of your application, you should add the exec "$@" command to cover the background.

${mysql [@]}

An array in Shell, executing ${mysql [@]} directly will execute the array as an executable program.

Mysql= (mysql-- protocol=socket-uroot-hlocalhost-- socket= "${SOCKET}") echo ${mysql [1]}-- output: mysqlecho ${mysql [2]}-- output:-- protocol=socketecho ${mysql [3]}-- output:-urootecho ${mysql [4]}-output:-hlocalhostecho ${mysql [@]}-output: mysql-- protocol=socket-uroot-hlocalhost-- socket=

Exec gosu mysql "$BASH_SOURCE"$@"

The gosu command here is a lightweight "substitute" for the sudo command in Linux.

Gosu is a tool developed by the golang language to replace the sudo command in shell. Su and sudo commands have some defects, mainly causing uncertain TTY, and there are also problems in forwarding semaphores. Using su or sudo is too heavy just to run programs with specific users, so gosu arises at the historic moment.

Gosu directly borrows the principle that libcontainer starts the application in the container, using / etc/passwd to process the application. Gosu first finds the specified user or user group, and then switches to that user or user group. Next, start the application using exec. So far, gosu has done its job and will not participate in the declaration cycle later in the application. This approach avoids the problem of gosu handling TTY and forwarding semaphores, leaving these two tasks directly to the application.

The above is the whole content of this article, I hope it will be helpful to your study, and I also hope that you will support it.

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