Linux developers were handed a surprise yesterday when Microsoft announced it will ship a full Linux kernel with Windows 10, starting this summer. Although Microsoft shipped a Linux kernel last year with the announcement of Azure Sphere, this is the first time Windows has ever included one as a component.

"Beginning with Windows Insiders builds this Summer, we will include an in-house custom-built Linux kernel to underpin the newest version of the Windows Subsystem for Linux (WSL)," program manager of the Linux Systems Group at Microsoft Jack Hammons wrote in a blog post yesterday. "The kernel itself will initially be based on version 4.19, the latest long-term stable release of Linux. The kernel will be rebased at the designation of new long-term stable releases to ensure that the WSL kernel always has the latest Linux goodness."

"Microsoft ♥ Linux," Microsoft's message for the last few years

Microsoft is revealing a host of new products and features this week in conjunction with Build 2019, the company's developer conference. Yesterday's announcements also included the release of WSL 2 and a new command line app called Windows Terminal. As Hammons wrote, the Windows Linux kernel will be provided in new builds to support WSL 2.

"For developers [the kernel] should dramatically improve the performance of Microsoft's Linux subsystem in Windows," The Verge predicted. "Microsoft is also promising to update this kernel through Windows Update, and it will be fully open source with the ability for developers to create their own WSL kernel and contribute changes."

The decision to include a full Linux kernel marks a logical, if surprising, conclusion to a big shift in company philosophy for Microsoft — one that many Linux developers seem to appreciate. Former Microsoft CEO Steve Ballmer famously (or maybe infamously) said in 2001 that "Linux is a cancer." In recent years, though, Microsoft's public attitude toward open source has evolved significantly, joining the Linux Foundation in 2016 and acquiring GitHub last year, for example. A significant part of Azure's revenue growth was also due to its customers' virtual machines running on Linux.

Open source developer Steve Klabnik tweeted about yesterday's kernel announcement and the sense of bemused shock in the Linux community:

Steve Klabnik's tweet

It seems as though Linus Torvalds, the creator of Linux, has won — and then some. Thomas Zander replied to Klabnik's tweet:

Thomas Zander's tweet

Here's hoping Tux the Linux Penguin enjoys the new view (pun intended) from Windows builds.