$ ffmpeg -encoders 2>/dev/null | grep rav1e
 V..... librav1e             librav1e AV1 (codec av1)
$ ffmpeg -i test.y4m -c:v librav1e -b:v 500K av1.mp4
ffmpeg version 4.3.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9.3.0 (GCC)
  configuration: --disable-static --prefix=/nix/store/fk43qqifmhbjhq02v91mpv51d04xrpwk-ffmpeg-full-4.3.1 --target_os=linux --arch=x86_64 --enable-gpl --enable-version3 --disable-nonfree --enable-shared --enable-pic --disable-small --enable-runtime-cpudetect --disable-lto --enable-gray --enable-swscale-alpha --enable-hardcoded-tables --enable-safe-bitstream-reader --enable-pthreads --disable-w32threads --disable-os2threads --enable-network --enable-pixelutils --enable-ffmpeg --enable-ffplay --enable-ffprobe --enable-avcodec --enable-avdevice --enable-avfilter --enable-avformat --enable-avresample --enable-avutil --enable-postproc --enable-swresample --enable-swscale --enable-doc --disable-htmlpages --enable-manpages --disable-podpages --disable-txtpages --enable-bzlib --enable-libcelt --enable-libdav1d --disable-libfdk-aac --disable-libflite --enable-fontconfig --enable-libfreetype --enable-frei0r --enable-libfribidi --enable-libgme --enable-gnutls --enable-libgsm --enable-ladspa --enable-libmp3lame --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libdc1394 --enable-iconv --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmysofa --enable-libopus --enable-librsvg --enable-libsrt --enable-libssh --enable-libtheora --enable-libv4l2 --enable-vaapi --enable-vdpau --enable-libvorbis --enable-libvmaf --enable-libvpx --enable-libwebp --enable-xlib --enable-libxcb --enable-libxcb-shm --enable-libxcb-xfixes --enable-libxcb-shape --enable-lzma --enable-nvenc --enable-openal --enable-libopencore-amrnb --disable-opengl --enable-libopenjpeg --disable-openssl --enable-libpulse --enable-librav1e --enable-librtmp --enable-sdl2 --enable-libsoxr --enable-libspeex --enable-libvidstab --enable-libvo-amrwbenc --enable-libwavpack --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzmq --enable-zlib --disable-debug --enable-optimizations --disable-extra-warnings --disable-stripping
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, yuv4mpegpipe, from 'test.y4m':
  Duration: 00:00:02.04, start: 0.000000, bitrate: 33178 kb/s
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p(progressive), 384x288, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> av1 (librav1e))
Press [q] to stop, [?] for help
Output #0, mp4, to 'av1.mp4':
  Metadata:
    encoder         : Lavf58.45.100
    Stream #0:0: Video: av1 (librav1e) (av01 / 0x31307661), yuv420p, 384x288, q=-1--1, 500 kb/s, 25 fps, 12800 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.91.100 librav1e
frame=   51 fps=2.1 q=-0.0 Lsize=     167kB time=00:00:02.00 bitrate= 682.7kbits/s speed=0.0825x
video:166kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.533024%