Fetch frame count with ffmpeg

ffprobe

ffprobe -v error -select_streams v:0 -count_packets -show_entries stream=nb_read_packets -of csv=p=0 input.mp4

This actually counts packets instead of frames but it is much faster. Result should be the same. If you want to verify by counting frames change -count_packets to -count_frames and nb_read_packets to nb_read_frames.

What the ffprobe options mean

-v error This hides "info" output (version info, etc) which makes parsing easier (but makes it harder if you ask for help since it hides important info).
-count_frames Count the number of packets per stream and report it in the corresponding stream section.
-select_streams v:0 Select only the first video stream.
-show_entries stream=nb_read_packets Show only the entry for nb_read_frames.
-of csv=p=0 sets the output formatting. In this case it hides the descriptions and only shows the value. See FFprobe Writers for info on other formats including JSON.

Only counting keyframes

See Checking keyframe interval?

MP4 Edit List

The presence of an edit list in MP4/M4V/M4A/MOV can affect your frame count.

Also see

FFprobe Documentation
FFmpeg Wiki: FFprobe Tips

mediainfo

The well known mediainfo tool can output the number of frames:

mediainfo --Output="Video;%FrameCount%" input.avi

MP4Box

For MP4/M4V/M4A files.

MP4Box from gpac can show the number of frames:

MP4Box -info input.mp4

Refer to the Media Info line in the output for the video stream in question:

Media Info: Language "Undetermined (und)" - Type "vide:avc1" - 2525 samples

In this example the video stream has 2525 frames.

boxdumper

For MP4/M4V/M4A/MOV files.

boxdumper is a simple tool from l-smash. It will output a large amount of information. Under the stsz sample size box section refer to sample_count for the number of frames. In this example the input has 1900 video frames:

boxdumper input.mp4
  ...
  [stsz: Sample Size Box]
    position = 342641
    size = 7620
    version = 0
    flags = 0x000000
    sample_size = 0 (variable)
    sample_count = 1900

Be aware that a file may have more than one stsz atom.

In Unix, this works like a charm:

ffmpeg -i 00000.avi -vcodec copy -acodec copy -f null /dev/null 2>&1 \
                                          | grep 'frame=' | cut -f 2 -d ' '