Building msys2 runtime

Installing a build environment

1. Download and run the Git for Windows SDK installer.

Building msys2-runtime

2. Open the Git for Windows SDK MSys shell by double-clicking the msys2_shell.cmd script in the install folder (by default C:\git-sdk-<arch>).

3. From the MSys shell do: sdk build msys2-runtime

This last step may take a while, so please be patient!

Rebuilding the msys2-runtime

When there are bugs in the msys2-runtime (e.g. problems with the POSIX-to-Windows path mangling), you need to rebuild the msys-2.0.dll, possibly frequently.

To rebuild the msys-2.0.dll -- assuming that you have built it already as described above -- you first need to start an MSys shell by double-clicking the msys2_shell.cmd script in your Git SDK's top-level directory. Then call cd /usr/src/MSYS2-packages/msys2-runtime/src/msys2-runtime/winsup/cygwin. Now you can modify the source code to your heart's content. It is actually a local clone of https://github.com/git-for-windows/msys2-runtime; you probably want to add your own fork as a remote.

To actually build the msys-2.0.dll, switch to src/build-<arch>-pc-msys/<arch>-pc-msys/winsup/cygwin and type make. This will generate an msys0.dll file in that directory. This is the new msys-2.0.dll, but you cannot simply copy it into /usr/bin/ because it is in use by the current mintty as well as by the current Bash. To update, you need to close all programs using the msys2-runtime (including all of the terminal windows), then copy the msys0.dll file, replacing the existing /usr/bin/msys-2.0.dll, either using Explorer, cmd.exe or a separate Git SDK (for example, when debugging the 64-bit msys2-runtime, a 32-bit Git SDK comes in real handy). You will want to make a backup copy of the old msys-2.0.dll...

Related: building/rebuilding Bash

The process to rebuild the Bash is very similar to the msys2-runtime one; You will just need to work in the src/bash-<version>/ subdirectory of /usr/src/MSYS2-packages/bash/ (both the sources and the generated bash.exe live there). Bash also requires all Bash instances to be exited before the executable is replaced by the new one. And like with the msys2-runtime, you will want to make a backup copy of the old bash.exe.