Merge Two Videos without Re-encoding

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Concatenation of Files with Same Codecs

There are two methods within FFmpeg that can be used to concatenate files of the same type: the concat demuxer & the concat protocol

The demuxer is more flexible – it requires the same codecs, but different container formats can be used; and it can be used with any container formats, while the concat protocol only works with a select few containers.

The Concat Demuxer

create a text file named vidlist.txt in the following format:

file '/path/to/clip1'
file '/path/to/clip2'
file '/path/to/clip3'

Note that these can be either relative or absolute paths.

Then issue the command:

ffmpeg -f concat -safe 0 -i vidlist.txt -c copy output

The files will be stream copied in the order they appear in the vidlist.txt into the output container. the "copy codec" is blazing fast.

Edit: Note that although the docs say you don't need -safe 0 if the paths are relative, my testing indicates it's a requirement. It's possible that this may vary with your version of FFmpeg.

There are tips for auto generating the file available in the docs.

Note: All the clips must already exist or the command will fail because decoding won't start until the whole list is read.

The Concat Protocol

ffmpeg -i "concat:video1.ts|video2.ts|video3.ts" -c copy output.ts

Note: as mentioned above the concat protocol is severely limited in what streams and containers it supports so I never use it. The above is only included in an attempt to create a thorough answer. The concat demuxer is a far better choice for most projects.

An alternative suggestion: Personally I prefer using the Matroska container due to it's flexibility and low overhead and join videos with the same encoding using

mkvmerge -o output.mkv input1.mkv + input2.mkv

Concatenation of Files with Different Codecs

If your clips don't use the same codecs for audio and video and/or have different rates, your stuck re-encoding to intermediate files prior to joining which as we all know is both time and resource consuming.

Note that special characters can break things so if you have these in your filenames you'll need to deal with them.