Unable to find a suitable output format for 'ffmpeg' for joining mp4 audio with mp4 audio

Question

I want to add audio from a mp4 file to a video form a mp4 file with FFmpeg. I am using this command ffmpeg -i video.mp4 -i audio.mp4 -c:v copy -c:a aac output.mp4. However FFmpeg is throwing Unable to find a suitable output format for 'ffmpeg'. I've added the log below for more information. How can I fix this? Is this because both and audio files are in mp4 format?

Here's the error log:

2021-11-15 23:52:20.698658+0200 app[666:70371] [javascript]   libavutil      56. 55.100 / 56. 55.100
2021-11-15 23:52:20.698892+0200 app[666:70371] [javascript]   libavcodec     58. 96.100 / 58. 96.100
2021-11-15 23:52:20.699091+0200 app[666:70371] [javascript]   libavformat    58. 48.100 / 58. 48.100
2021-11-15 23:52:20.699299+0200 app[666:70371] [javascript]   libavdevice    58. 11.101 / 58. 11.101
2021-11-15 23:52:20.699501+0200 app[666:70371] [javascript]   libavfilter     7. 87.100 /  7. 87.100
2021-11-15 23:52:20.705405+0200 app[666:70371] [javascript]   libswscale      5.  8.100 /  5.  8.100
2021-11-15 23:52:20.705768+0200 app[666:70371] [javascript]   libswresample   3.  8.100 /  3.  8.100
2021-11-15 23:52:20.716141+0200 app[666:70371] [javascript] Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/var/mobile/Containers/Data/Application/Documents/video.mp4':
2021-11-15 23:52:20.716431+0200 app[666:70371] [javascript]   Metadata:
2021-11-15 23:52:20.716646+0200 app[666:70371] [javascript]     major_brand     :
2021-11-15 23:52:20.716849+0200 app[666:70371] [javascript] isom
2021-11-15 23:52:20.717051+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.717248+0200 app[666:70371] [javascript]     minor_version   :
2021-11-15 23:52:20.717443+0200 app[666:70371] [javascript] 512
2021-11-15 23:52:20.717637+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.717827+0200 app[666:70371] [javascript]     compatible_brands:
2021-11-15 23:52:20.718035+0200 app[666:70371] [javascript] isomiso2avc1mp41
2021-11-15 23:52:20.718232+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.718460+0200 app[666:70371] [javascript]     encoder         :
2021-11-15 23:52:20.718659+0200 app[666:70371] [javascript] Lavf58.48.100
2021-11-15 23:52:20.718866+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.719064+0200 app[666:70371] [javascript]   Duration:
2021-11-15 23:52:20.719276+0200 app[666:70371] [javascript] 00:00:28.67
2021-11-15 23:52:20.719480+0200 app[666:70371] [javascript] , start:
2021-11-15 23:52:20.719682+0200 app[666:70371] [javascript] 0.000000
2021-11-15 23:52:20.719872+0200 app[666:70371] [javascript] , bitrate:
2021-11-15 23:52:20.720425+0200 app[666:70371] [javascript] 6170 kb/s
2021-11-15 23:52:20.720860+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.721073+0200 app[666:70371] [javascript]     Stream #0:0
2021-11-15 23:52:20.721385+0200 app[666:70371] [javascript] (und)
2021-11-15 23:52:20.721623+0200 app[666:70371] [javascript] : Video: h264 (avc1 / 0x31637661), yuv420p(tv), 720x1280, 6166 kb/s
2021-11-15 23:52:20.721831+0200 app[666:70371] [javascript] ,
2021-11-15 23:52:20.722030+0200 app[666:70371] [javascript] 30 fps,
2021-11-15 23:52:20.722229+0200 app[666:70371] [javascript] 30 tbr,
2021-11-15 23:52:20.722426+0200 app[666:70371] [javascript] 19200 tbn,
2021-11-15 23:52:20.722622+0200 app[666:70371] [javascript] 38400 tbc
2021-11-15 23:52:20.722818+0200 app[666:70371] [javascript]  (default)
2021-11-15 23:52:20.723013+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.723223+0200 app[666:70371] [javascript]     Metadata:
2021-11-15 23:52:20.723424+0200 app[666:70371] [javascript]       handler_name    :
2021-11-15 23:52:20.723624+0200 app[666:70371] [javascript] Core Media Video
2021-11-15 23:52:20.723820+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.724032+0200 app[666:70371] [javascript] Input #1, mov,mp4,m4a,3gp,3g2,mj2, from '/var/mobile/Containers/Data/Application/Documents/audio.mp4':
2021-11-15 23:52:20.724232+0200 app[666:70371] [javascript]   Metadata:
2021-11-15 23:52:20.724430+0200 app[666:70371] [javascript]     major_brand     :
2021-11-15 23:52:20.724627+0200 app[666:70371] [javascript] isom
2021-11-15 23:52:20.724822+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.725025+0200 app[666:70371] [javascript]     minor_version   :
2021-11-15 23:52:20.725227+0200 app[666:70371] [javascript] 512
2021-11-15 23:52:20.725426+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.725624+0200 app[666:70371] [javascript]     compatible_brands:
2021-11-15 23:52:20.725820+0200 app[666:70371] [javascript] isomiso2mp41
2021-11-15 23:52:20.726026+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.726230+0200 app[666:70371] [javascript]     encoder         :
2021-11-15 23:52:20.726427+0200 app[666:70371] [javascript] Lavf58.48.100
2021-11-15 23:52:20.726624+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.726827+0200 app[666:70371] [javascript]   Duration:
2021-11-15 23:52:20.727025+0200 app[666:70371] [javascript] 00:00:28.65
2021-11-15 23:52:20.727230+0200 app[666:70371] [javascript] , start:
2021-11-15 23:52:20.727428+0200 app[666:70371] [javascript] 0.000000
2021-11-15 23:52:20.727627+0200 app[666:70371] [javascript] , bitrate:
2021-11-15 23:52:20.727829+0200 app[666:70371] [javascript] 93 kb/s
2021-11-15 23:52:20.728025+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.728220+0200 app[666:70371] [javascript]     Stream #1:0
2021-11-15 23:52:20.728414+0200 app[666:70371] [javascript] (eng)
2021-11-15 23:52:20.728733+0200 app[666:70371] [javascript] : Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 91 kb/s
2021-11-15 23:52:20.728941+0200 app[666:70371] [javascript]  (default)
2021-11-15 23:52:20.729134+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.729339+0200 app[666:70371] [javascript]     Metadata:
2021-11-15 23:52:20.729593+0200 app[666:70371] [javascript]       handler_name    :
2021-11-15 23:52:20.729799+0200 app[666:70371] [javascript] SoundHandler
2021-11-15 23:52:20.729992+0200 app[666:70371] [javascript] 
2021-11-15 23:52:20.730261+0200 app[666:70371] [javascript] FFmpeg process exited with rc=1.
2021-11-15 23:52:20.731495+0200 app[666:70371] [javascript] Unable to find a suitable output format for 'ffmpeg'
2021-11-15 23:52:20.731715+0200 app[666:70371] [javascript] ffmpeg: Invalid argument

Answer 1

silly me, after hours of research I found that I actually had to run the command without ffmeg, as the error states Unable to find a suitable output format for 'ffmpeg'. So all i had to do is change it to -i video.mp4 -i audio.mp4 -c:v copy -c:a aac output.mp4