build.cake

Task("Version")
  .Does(() =>
  {
    IEnumerable<string> redirectedStandardOutput;
    IEnumerable<string> redirectedErrorOutput;
    var exitCodeWithArgument =
        StartProcess(
            "git",
            new ProcessSettings {
                Arguments = "rev-parse HEAD",
                RedirectStandardOutput = true
            },
            out redirectedStandardOutput,
            out redirectedErrorOutput
        );

    string gitCommitHash = redirectedStandardOutput.Single();

    ReplaceTextInFiles("path_to_your_appsettings", "%BUILD_COMMIT_HASH%", gitCommitHash);
  });