Concatenating media files

If you have media files with exactly the same codec and codec parameters you can concatenate them as described in "Concatenation of files with same codecs". If you have media with different codecs you can concatenate them as described in "Concatenation of files with different codecs" below.

Concatenation of files with same codecs

There are two methods within ffmpeg that can be used to concatenate files of the same type:

1. the concat ''demuxer''
2. 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 protocol only works with a select few containers.

Concat demuxer

You can read about the concat demuxer in the documentation. This demuxer reads a list of files and other directives from a text file and demuxes them one after the other, as if all their packets had been muxed together. All files must have the same streams (same codecs, same time base, etc.) but can be wrapped in different container formats.

Instructions

Create a file mylist.txt with all the files you want to have concatenated in the following form (lines starting with a # are ignored):

# this is a comment
file '/path/to/file1.wav'
file '/path/to/file2.wav'
file '/path/to/file3.wav'

Note that these can be either relative or absolute paths. Then you can stream copy or re-encode your files:

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

The -safe 0 above is not required if the paths are relative.

Automatically generating the input file

It is possible to generate this list file with a bash for loop, or using printf. Either of the following would generate a list file containing every *.wav in the working directory:

# with a bash for loop
for f in *.wav; do echo "file '$f'" >> mylist.txt; done
# or with printf
printf "file '%s'\n" *.wav > mylist.txt

On Windows Command-line:

(for %i in (*.wav) do @echo file '%i') > mylist.txt

Or for Windows Powershell:

foreach ($i in Get-ChildItem .\*.wav) {"file '$i'" | Out-File mylist.txt -Encoding utf8 -Append}

Or for Windows bat-file:

(for %%i in (*.wav) do @echo file '%%i') > mylist.txt

If your shell supports process substitution (like Bash and Zsh), you can avoid explicitly creating a list file and do the whole thing in a single line. This would be impossible with the concat protocol (see below). Make sure to generate absolute paths here, since ffmpeg will resolve paths relative to the list file your shell may create in a directory such as "/proc/self/fd/". Further, this process will fail if file names contain a single quote (e.g. 's) and an actual file must be generated. (From docs: https://ffmpeg.org/ffmpeg-formats.html#concat-1 3.5.1 Syntax: special characters and spaces must be escaped with backslash or single quotes)

From docs: https://ffmpeg.org/ffmpeg-formats.html#concat-1 3.5.1 Syntax The script is a text file in extended-ASCII, with one directive per line. Empty lines, leading spaces and lines starting with ’#’ are ignored. The following directive is recognized: file path Path to a file to read; special characters and spaces must be escaped with backslash or single quotes. All subsequent file-related directives apply to that file.

ffmpeg -f concat -safe 0 -i <(for f in ./*.wav; do echo "file '$PWD/$f'"; done) -c copy output.wav
ffmpeg -f concat -safe 0 -i <(printf "file '$PWD/%s'\n" ./*.wav) -c copy output.wav
ffmpeg -f concat -safe 0 -i <(find . -name '*.wav' -printf "file '$PWD/%p'\n") -c copy output.wav

You can also loop a video. This example will loop input.mkv 10 times:

for i in {1..10}; do printf "file '%s'\n" input.mkv >> mylist.txt; done
ffmpeg -f concat -i mylist.txt -c copy output.mkv

Changing playlist files on the fly

The concat demuxer opens the referenced files only when they are needed. This allows us to swap the referenced files atomically behind the demuxers back to be able to use the concat demuxer as a changeable live source. Check out the following example file list.txt:

ffconcat version 1.0
file dummy.mxf
file dummy.mxf

dummy.mxf is referenced twice to make sure the concat demuxer reopens the file when it reaches it. Combine this with infinite looping and you are done:

ffmpeg -re -stream_loop -1 -f concat -i list.txt -flush_packets 0 -f mpegts udp://127.0.0.1:5000?pkt_size=1316

Now you can change the looping clip by a simple move command:

mv next_clip.mxf dummy.mxf

Automatically appending to the list file

Concatenation does not work if the next clip for does not exist at the moment, because decoding won't start until the whole list is read. However, it is possible to refer to another list at the end of the current list. The following script provides an example for this mechanism:

#!/bin/bash

fn_concat_init() {
    echo "fn_concat_init"
    concat_pls=`mktemp -u -p . concat.XXXXXXXXXX.txt`
    concat_pls="${concat_pls#./}"
    echo "concat_pls=${concat_pls:?}"
    mkfifo "${concat_pls:?}"
    echo
}

fn_concat_feed() {
    echo "fn_concat_feed ${1:?}"
    {
        >&2 echo "removing ${concat_pls:?}"
        rm "${concat_pls:?}"
        concat_pls=
        >&2 fn_concat_init
        echo 'ffconcat version 1.0'
        echo "file '${1:?}'"
        echo "file '${concat_pls:?}'"
    } >"${concat_pls:?}"
    echo
}

fn_concat_end() {
    echo "fn_concat_end"
    {
        >&2 echo "removing ${concat_pls:?}"
        rm "${concat_pls:?}"
        # not writing header.
    } >"${concat_pls:?}"
    echo
}

fn_concat_init

echo "launching ffmpeg ... all.mkv"
timeout 60s ffmpeg -y -re -loglevel warning -i "${concat_pls:?}" -pix_fmt yuv422p all.mkv &

ffplaypid=$!

echo "generating some test data..."
i=0; for c in red yellow green blue; do
    ffmpeg -loglevel warning -y -f lavfi -i testsrc=s=720x576:r=12:d=4 -pix_fmt yuv422p -vf "drawbox=w=50:h=w:t=w:c=${c:?}" test$i.mkv
    fn_concat_feed test$i.mkv
    ((i++));
    echo
done
echo "done"

fn_concat_end

wait "${ffplaypid:?}"

echo "done encoding all.mkv"

Note that recursively referencing playlist files will cause ffmpeg to eventually run out of file descriptors (or other resources) because ffmpeg only closes the playlist file when the playlist has finished, but in the example above because of the recursive chaining none of the playlist files actually end.

Concat protocol

While the demuxer works at the stream level, the concat protocol works at the file level. Certain files (MPEG-2 transport streams, possibly others) can be concatenated. This is analogous to using cat on UNIX-like systems or copy on Windows.

Instructions

The following command concatenates three MPEG-2 TS files and concatenates them without re-encoding:

ffmpeg -i "concat:input1.ts|input2.ts|input3.ts" -c copy output.ts

Using intermediate files

If you have MP4 files, these could be losslessly concatenated by first transcoding them to MPEG-2 transport streams. With H.264 video and AAC audio, the following can be used:

ffmpeg -i input1.mp4 -c copy intermediate1.ts
ffmpeg -i input2.mp4 -c copy intermediate2.ts
ffmpeg -i "concat:intermediate1.ts|intermediate2.ts" -c copy output.mp4

Using named pipes to avoid intermediate files

If you're using a system that supports named pipes, you can use those to avoid creating intermediate files. This sends stderr (to which ffmpeg sends all the written data) to /dev/null, to avoid cluttering up the command-line:

mkfifo temp1 temp2
ffmpeg -y -i input1.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts temp1 2> /dev/null & \
ffmpeg -y -i input2.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts temp2 2> /dev/null & \
ffmpeg -f mpegts -i "concat:temp1|temp2" -c copy -bsf:a aac_adtstoasc output.mp4

The additional -y switch is needed to force ffmpeg to write to existing files temp1 and temp2, which are the named pipes. Without the switch, the first two ffmpeg programs running in the background will not produce any output because they wait for interactive yes/no answers to the questions whether to overwrite existing files.

All MPEG codecs (MPEG-4 Part 10 / AVC, MPEG-4 Part 2, MPEG-2 Video, MPEG-1 Audio Layer II, MPEG-2 Audio Layer III (MP3), MPEG-4 Part III (AAC)) are supported in the MPEG-TS container format, although the commands above would require some alteration (e.g., the -bsf bitstream filters will have to be changed).

Concatenation of files with different codecs

In many cases, input files will have different codecs or different codec properties, which makes it impossible to use any of the above methods.

Concat filter

See the concat filter documentation for more info. The filter works on segments of synchronized video and audio streams. All segments must have the same number of streams of each type, and that will also be the number of streams at output.

Note: Filters are incompatible with stream copying; you can't use -c copy with this method. Since you have to re-encode the video and audio stream(s), and since re-encoding may introduce compression artifacts, make sure to add proper target bitrate or quality settings. See the encoding guides for more info.

For the concat filter to work, the inputs have to be of the same frame dimensions (e.g., 1920⨉1080 pixels) and should have the same framerate. Therefore, you may at least have to add a ​scale or ​scale2ref filter before concatenating videos. A handful of other attributes have to match as well, like the stream aspect ratio. Refer to the documentation of the filter for more info.

Instructions

Let's say we have three files that we want to concatenate – each of them with one video and audio stream. The concat filter command would look like this:

ffmpeg -i input1.mp4 -i input2.webm -i input3.mov \
-filter_complex "[0:v:0][0:a:0][1:v:0][1:a:0][2:v:0][2:a:0]concat=n=3:v=1:a=1[outv][outa]" \
-map "[outv]" -map "[outa]" output.mkv

Now, let's dissect that command. We first specify all the input files, then instantiate a -filter_complex filtergraph – this is needed instead of -filter:v because it has multiple inputs and outputs.

The following line:

[0:v:0][0:a:0][1:v:0][1:a:0][2:v:0][2:a:0]

tells ffmpeg which streams to take from the input files and send as input to the concat filter. In this case, video stream 0 [0:v:0] and audio stream 0 [0:a:0] from input 0 (input1.mp4 in this example), and video stream 0 [1:v:0] and audio stream 0 [1:v:0] from input 1 (input2.webm), etc.

concat=n=3:v=1:a=1[outv][outa]'

This is the concat filter itself. n=3 is telling the filter that there are three input segments; v=1 is telling it that there will be one video stream per segment; a=1 is telling it that there will be one audio stream per segment. The filter then concatenates these segments and produces two output streams. [outv] and [outa] are names for these output streams. Note that the quotes around the filter section are required.

The following image shows the stream mapping to and from the filter in the above example:

You can then either re-use these streams in other filters, or map them to the output file:

-map "[outv]" -map "[outa]" output.mkv

This tells ffmpeg to use the results of the concat filter rather than the streams directly from the input files.

Using an external script

There is a Bash script called mmcat which was useful for older versions of ffmpeg that did not include the concat filter.