title: FFmpeg returns' Output must be specified' when it has been acknowledged
body: <p>Here's my command in python:</p>
  
  <pre><code>ffmpeg -i concat:803.ts|804.ts|805.ts -c copy output.ts
  </code></pre>
  
  <p>When I run the command it opens the output but it doesn't save the file to the directory.
  I run the command in FFmpeg and it gave the same result - it opened the output but it gave me this error at the end:</p>
  
  <pre><code>At least one output file must be specified
  </code></pre>
  
  <p>I don't understand what I did wrong as I specified the output clearly, I already checked for solutions but most of them are suited to OP's needs</p>