Question

Got the following from FFmpeg FAQ:

mkfifo intermediate1.mpg
mkfifo intermediate2.mpg
ffmpeg -i input1.avi -sameq -y intermediate1.mpg < /dev/null &
ffmpeg -i input2.avi -sameq -y intermediate2.mpg < /dev/null &
cat intermediate1.mpg intermediate2.mpg |\
ffmpeg -f mpeg -i - -sameq -vcodec mpeg4 -acodec libmp3lame output.avi

Before i use or modify it I would like to understand it completely.

What does the < /dev/null & do?

I understand | is pipe but why |\ ?

What is the -f mpeg after ffmpeg (Seems, it tells ffmpeg to accept the piped in output from the cat(?) )

Answer 1

< /dev/null &

This is actually two parts:

1. < /dev/null
2. &

1 (< /dev/null) is just a simple way to pass no input/EOF to a program. I’m not sure it’s needed but it may be because you are using named pipes.

2 (&) simply pushes the command to the background and allows you to do other things. This is necessary because otherwise, ffmpeg would just sit there waiting for the other end of the named pipe to “open”.

Backslash after pipe

The backslash after the pipe is simply there to allow you to enter the long command on multiple lines. If you want to write it on a single line, you should omit the backslash. You’ll notice that the prompt changes from your usual [user@machine directory]$ (or whatever) to something like > after you enter the first line (ending with a backslash). This signifies that your command is being continued from an earlier line.

ffmpeg -f switch

The man page for ffmpeg indicates that the -f switch allows you to force a file format. In the example in the FAQ, you want to force an input format (read: tell ffmpeg what input format to expect) since your using piped bits as input. Usually, it would try to guess the input format based on the file extension and/or “file magic”.