dotnet-installer/src/Microsoft.DotNet.Cli.Utils/Env.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;

namespace Microsoft.DotNet.Cli.Utils
{
    internal static class Env
    {
        private static IEnumerable<string> _searchPaths;
        private static IEnumerable<string> _executableExtensions;

        public static IEnumerable<string> ExecutableExtensions
        {
            get
            {
                if (_executableExtensions == null)
                {

                    _executableExtensions = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
                        ? Environment.GetEnvironmentVariable("PATHEXT").Split(';').Select(e => e.ToLower())
                        : new [] { string.Empty };
                }

                return _executableExtensions;
            }
        } 

        private static IEnumerable<string> SearchPaths 
        {
            get
            {
                if (_searchPaths == null)
                {
                    var searchPaths = new List<string> {AppContext.BaseDirectory};

                    searchPaths.AddRange(Environment.GetEnvironmentVariable("PATH").Split(Path.PathSeparator));

                    _searchPaths = searchPaths;
                }

                return _searchPaths;
            }
        }

        public static string GetCommandPath(string commandName, params string[] extensions)
        {
            if (!extensions.Any())
                extensions = Env.ExecutableExtensions.ToArray();

            var commandPath = Env.SearchPaths.Join(
                extensions,
                    p => true, s => true,
                    (p, s) => Path.Combine(p, commandName + s))
                .FirstOrDefault(File.Exists);

            return commandPath;
        }
    }
}