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Linux版本號意義

在2011年7月以前，linux版本號有三個數字，例如2.6.39 其中

  • 2 -> 主版本號
  • 6 -> 副版本號，奇數為開發版本，偶數為穩定版本
  • 39 -> 修訂版本號

2011/7/21，核心版本3.0釋出，Linus為了整齊把版本號換成兩個。 現在的開發流程，新核心版本會從一個為期兩週的"merge window"開始，Linus在這期間會接受各方面新功能的patch，把他們merge到核心中。Merge window過了之後，穩定化階段開始，持續的測試並debug新的核心，每週會發布一個"release candidate"，表示方法是在版本號之後加上-rc#，第幾個rc, '#'就代入那個數字，舉例現在最新的核心5.9-rc2就是第二個5.9版的release candidate。目前每個新核心大概會有8個rc，之後就會正式釋出，下一個版本的merge window重新開始。

上述的核心版本是最新的，版本號走在最前面的，叫做mainline kernel，由Linux原作者Linus Torvalds維護。

Mainline核心正式釋出之後，會轉移到stable分支上，這個分支由Greg Kroah-Hartman維護，這裡的核心會持續受到維護、debug等等，並在版本號上加上第三個數字表示修訂的版本。 通常，stable分支只會維護到最新開發循環的前一到兩個版本，例如現在是mainline 5.9的rc2階段，5.8和5.7就在stable中維護，5.6就不再受到更新。

除此之外，有些核心版本被挑選為longterm，會受到持續的測試與維護，約一兩年。每年至少有一個版本會被選為longterm，滿足對於穩定核心的需求。

這樣看下來點進https://www.kernel.org就應該看得懂那些版本號的意思了！

參考： Chris Simmonds. (2017). Mastering Embedded Linux Programming Second Edition. Packt Publishing

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