How do I enable FFMPEG logging and where can I find the FFMPEG log file?

Question

I want to be able to log FFMPEG processes because I am trying to work out how long a minute of video takes to convert to help with capacity planning of my video encoding server.

Answer 1

ffmpeg logs to stderr, and can log to a file with a different log-level from stderr. The -report command-line option doesn't give you control of the log file name or the log level, so setting the environment variable is preferable.

(-v is a synonym for -loglevel. Run ffmpeg -v help to see the levels. Run ffmpeg -h full | less to see EVERYTHING. Or consult the online docs, or their wiki pages like the h.264 encode guide).

#!/bin/bash

of=out.mkv
FFREPORT="level=32:file=$of.log" ffmpeg -v verbose   -i src.mp4 -c:a copy -preset slower -c:v libx264 -crf 21 "$of"

That will trancode src.mp4 with x264, and set the log level for stderr to "verbose", and the log level for out.mkv.log to "status".

(AV_LOG_WARNING=24, AV_LOG_INFO=32, AV_LOG_VERBOSE=40, etc.). Support for this was added 2 years ago, so you need a non-ancient version of ffmpeg. (Always a good idea anyway, for security / bugfixes and speedups)

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A few codecs, like -c:v libx265, write directly to stderr instead of using ffmpeg's logging infrastructure. So their log messages don't end up in the report file. I assume this is a bug / TODO-list item.

To log stderr, while still seeing it in a terminal, you can use tee(1).

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If you use a log level that includes status line updates (the default -v info, or higher), they will be included in the log file, separated with ^M (carriage return aka \r). There's no log level that includes encoder stats (like SSIM) but not status-line updates, so the best option is probably to filter that stream.

If don't want to filter (e.g. so the fps / bitrate at each status-update interval is there in the file), you can use less -r to pass them through directly to your terminal so you can view the files cleanly. If you have .enc logs from several encodes that you want to flip through, less -r ++G *.enc works great. (++G means start at the end of the file, for all files). With single-key key bindings like . and , for next file and previous file, you can flip through some log files very nicely. (the default bindings are :n and :p).

If you do want to filter, sed 's/.*\r//' works perfectly for ffmpeg output. (In the general case, you need something like vt100.py, but not for just carriage returns). There are (at least) two ways to do this with tee + sed: tee to /dev/tty and pipe tee's output into sed, or use a process substitution to tee into a pipe to sed.

# pass stdout and stderr through to the terminal, 
## and log a filtered version to a file (with only the last status-line update).

of="$1-x265.mkv"
ffmpeg -v info -i "$1" -c:a copy -c:v libx265 ... "$of" |&    # pipe stdout and stderr
   tee /dev/tty | sed 's/.*\r//' >> "$of.enc"

## or with process substitution where tee's arg will be something like /dev/fd/123

ffmpeg -v info -i "$1" -c:a copy -c:v libx265 ... "$of" |&
  tee >(sed 's/.*\r//' >> "$of.enc")

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For testing a few different encode parameters, you can make a function like this one that I used recently to test some stuff. I had it all on one line so I could easily up-arrow and edit it, but I'll un-obfuscate it here. (That's why there are ;s at the end of each line)

ffenc-testclip(){
  # v should be set by the caller, to a vertical resolution.  We scale to WxH, where W is a multiple of 8 (-vf scale=-8:$v)
  db=0;   # convenient to use shell vars to encode settings that you want to include in the filename and the ffmpeg cmdline
  of=25s@21.15.${v}p.x265$pre.mkv; 
  [[ -e "$of.enc" ]]&&echo "$of.enc exists"&&return;   # early-out if the file exists

  # encode 25 seconds starting at 21m15s (or the keyframe before that)
  nice -14 ffmpeg -ss $((21*60+15))  -i src.mp4 -t 25  -map 0 -metadata title= -color_primaries bt709 -color_trc bt709 -colorspace bt709 -sws_flags lanczos+print_info -c:a copy -c:v libx265 -b:v 1500k -vf scale=-8:$v  -preset $pre -ssim 1 -x265-params ssim=1:cu-stats=1:deblock=$db:aq-mode=1:lookahead-slices=0 "$of" |&
   tee /dev/tty | sed 's/.*\r//' >> "$of.enc";
}

# and use it with nested loops like this.
for pre in fast slow;  do for v in  360 480 648 792;do  ffenc-testclip ;done;done

less -r ++G *.enc       # -r is useful if you didn't use sed

Note that it tests for existence of the output video file to avoid spewing extra garbage into the log file if it already exists. Even so, I used and append (>>) redirect.

It would be "cleaner" to write a shell function that took args instead of looking at shell variables, but this was convenient and easy to write for my own use. That's also why I saved space by not properly quoting all my variable expansions. ($v instead of "$v")