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devenv.exe

devenv.exe starts the IDE of Visual Studio and allows to control it from the command line (cmd.exe).
The path of this executable can be queried with vswhere.exe in PowerShell: 
PS C:\> & 'C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe' -property productPath

devenv.exe directory

After running VsDevCmd.bat, the name of the directory that contains devenv.exe is stored in the Visual Studio environment variable %DevEnvDir%.

Building solutions or projects

Some options allow to build projects from the command line: 
C:\…> devenv tq84.sln /Build
C:\…> devenv tq84.sln /Build        proj
C:\…> devenv tq84.sln /Build /Debug proj
Microsoft recommends to use MSBuild

Diffing files

With the /diff flag, devenv.exe compares two files: 
C:\…> devenv /diff file1.cs file2.cs

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