subprocess call ffmpeg (command line)

Question

I have been incorporating subprocess calls in my program. I have had no issues with subprocess calls for other commands, but I am having trouble getting the command line inp

Answer 1

When you use subprocess, your command must either be a string that looks exactly like what you would type on the command line (and you set shell=True), or a list where each command is an item in the list (and you take the default shell=False). In either case, you have to deal with the variable part of the string. For instance, the operating system has no idea what "%03d" is, you have to fill it in.

I can't tell from your question exactly what the parameters are, but lets assume you want to convert frame 3, it would look something like this in a string:

my_frame = 3
subprocess.call(
    'ffmpeg -r 10 -i frame%03d.png -r ntsc movie%03d.mpg' % (my_frame, my_frame),
    shell=True)

Its kinda subtle in this example, but that's risky. Suppose these things were in a directory whose name name had spaces (e.g., ./My Movies/Scary Movie). The shell would be confused by those spaces.

So, you can put it into a list and avoid the problem

my_frame = 3
subprocess.call(['ffmpeg', '-r', '10', '-i', 'frame%03d.png' % my_frame,
    ['-r',  'ntsc', 'movie%03d.mpg' % my_frame])

More typing, but safer.