chown 指令中點(".", dot)的用法

最近看到同事在寫 dockerfile 時，寫到:

chown apuser.apuser -R /data

一直覺得那個"點"很奇怪XD

後來查了一下才發現那個寫法已經不正確了，只為了向下相容，所以目前還支援，正確的寫法要改成:

chown apuser:apuser -R /data

原因:

Some older scripts may still use `.' in place of the `:' separator.
POSIX 1003.1-2001 (*note Standards conformance::) does not require
support for that, but for backward compatibility GNU `chown' supports
`.' so long as no ambiguity results. New scripts should avoid the use
of `.' because it is not portable, and because it has undesirable
results if the entire OWNER`.'GROUP happens to identify a user whose
name contains `.'.

ref:

https://unix.stackexchange.com/questions/245210/why-does-a-work-in-chown