using System;
using System.Diagnostics;

public static void Run(string input, TraceWriter log)
{
    log.Info("Executing");
    using (var process = new Process())
    {
        process.StartInfo.FileName = @"D:homeffmpeg.exe";
        process.StartInfo.Arguments = @"-i D:homeAmnestyInternational.mp4 -ss 00:00:03 -t 00:00:08 -async 1 D:homecut.mp4";
        process.StartInfo.UseShellExecute = false;
        process.StartInfo.RedirectStandardOutput = true;
        process.StartInfo.RedirectStandardError = true;
        log.Info(Directory.GetCurrentDirectory());
        log.Info("Cutting starts:"+DateTime.Now.ToString("h:mm:ss tt"));
        process.Start();
        string output = process.StandardOutput.ReadToEnd();
        process.WaitForExit();
        log.Info("Cutting ends :"+DateTime.Now.ToString("h:mm:ss tt"));
        log.Info(output);
    }
}

2017-03-24T11:06:00.705 Function started (Id=df082f54-719a-415f-b7f1-b10548a213be)
2017-03-24T11:06:00.721 Executing
2017-03-24T11:06:00.721 D:Windowssystem32
2017-03-24T11:06:00.721 Cutting start :11:06:00 AM
2017-03-24T11:07:14  No new trace in the past 1 min(s).
2017-03-24T11:08:14  No new trace in the past 2 min(s).
2017-03-24T11:09:14  No new trace in the past 3 min(s).
2017-03-24T11:10:14  No new trace in the past 4 min(s).
2017-03-24T11:11:00.758 Microsoft.Azure.WebJobs.Host: Timeout value of 00:05:00 was exceeded by function: Functions.ManualTriggerCSharp1.

Directory.SetCurrentDirectory(@"D:home");

process.StartInfo.WorkingDirectory = @"D:home";
process.StartInfo.FileName = "ffmpeg.exe";