user26767: Basically I was following https://video.stackexchange.com/a/15701/26767, which states:

This should work faster...
ffmpeg -ss 1:00 -t 5 -i in.mkv -ss 2:00 -t 5 -i in.mkv \
 -filter_complex "[0][1]concat=n=2:v=1:a=1" \ 
 out.mkv


Is it really working? Or how can I make it works for nowadays ffmpeg?
First, I found the command there is incorrect, as I'm getting this:
$ ffmpeg -codec copy -i inp.mp4 clip1.mp4
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
  configuration: --prefix=/usr --extra-version=1ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Unknown decoder 'copy'

I found this works: ffmpeg -i inp.mp4 -codec copy clip1.mp4
Then when I want to put things together, I got Unknown decoder 'copy' again:
$ ffmpeg -i inp.mp4 -ss 1:09 -t 76 -codec copy -i inp.mp4 -ss 2:55 -codec copy -filter_complex "[0][1]concat=n=2:v=1:a=1" out.mp4
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
. . .
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'inp.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2018-03-08T18:56:55.000000Z
  Duration: 00:03:46.81, start: 0.000000, bitrate: 3137 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1280x720 [SAR 1:1 DAR 16:9], 2943 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 191 kb/s (default)
    Metadata:
      creation_time   : 2018-03-08T18:57:03.000000Z
      handler_name    : IsoMedia File Produced by Google, 5-11-2011
Unknown decoder 'copy'

How can I make the 5-year-old answer works for nowadays ffmpeg without re-encoding?

user36485: You should put -i in.mkv  for the first place.
Please try
ffmpeg -i in.mkv -ss 1:00 -t 5 -ss 2:00 -t 5 

I think your code have multiple "-i in.mkv" parts.
Remove all but one "-i
Edit:
Then I think the easiest way is :
ffmpeg -i in.mkv -ss 1:00 -t 5 -c copy clip1.mkv
ffmpeg -i in.mkv -ss 2:00 -t 5 -c copy clip2.mkv
echo file clip1.mkv >> mylist.txt
echo file clip2.mkv >> mylist.txt
ffmpeg -f concat -i mylist.txt -c copy out.mkv
rm mylist.txt

Just make two clips from the original video "in.mkv" and just concat them in the usual ffmpeg concat way.

26767: <p>Basically I was following <a href="https://video.stackexchange.com/a/15701/26767">https://video.stackexchange.com/a/15701/26767</a>, which states:</p>
<blockquote>
<p>This should work faster...</p>
<pre class="lang-bash prettyprint-override"><code>ffmpeg -ss 1:00 -t 5 -i in.mkv -ss 2:00 -t 5 -i in.mkv \
 -filter_complex &quot;[0][1]concat=n=2:v=1:a=1&quot; \ 
 out.mkv
</code></pre>
</blockquote>
<p>Is it really working? Or how can I make it works for nowadays <code>ffmpeg</code>?</p>
<p>First, I found the command there is incorrect, as I'm getting this:</p>
<pre><code>$ ffmpeg -codec copy -i inp.mp4 clip1.mp4
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
  configuration: --prefix=/usr --extra-version=1ubuntu1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Unknown decoder 'copy'
</code></pre>
<p>I found this works: <code>ffmpeg -i inp.mp4 -codec copy clip1.mp4</code></p>
<p>Then when I want to put things together, I got <code>Unknown decoder 'copy'</code> again:</p>
<pre><code>$ ffmpeg -i inp.mp4 -ss 1:09 -t 76 -codec copy -i inp.mp4 -ss 2:55 -codec copy -filter_complex &quot;[0][1]concat=n=2:v=1:a=1&quot; out.mp4
ffmpeg version 4.2.2-1ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-3ubuntu1)
. . .
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'inp.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2018-03-08T18:56:55.000000Z
  Duration: 00:03:46.81, start: 0.000000, bitrate: 3137 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1280x720 [SAR 1:1 DAR 16:9], 2943 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 191 kb/s (default)
    Metadata:
      creation_time   : 2018-03-08T18:57:03.000000Z
      handler_name    : IsoMedia File Produced by Google, 5-11-2011
Unknown decoder 'copy'
</code></pre>
<p>How can I make the 5-year-old answer works for nowadays <code>ffmpeg</code> without re-encoding?</p>
 36485: <p>You should put -i in.mkv  for the first place.
Please try</p>
<pre><code>ffmpeg -i in.mkv -ss 1:00 -t 5 -ss 2:00 -t 5 
</code></pre>
<p>I think your code have multiple &quot;-i in.mkv&quot; parts.
Remove all but one &quot;-i</p>
<p>Edit:
Then I think the easiest way is :</p>
<pre><code>ffmpeg -i in.mkv -ss 1:00 -t 5 -c copy clip1.mkv
ffmpeg -i in.mkv -ss 2:00 -t 5 -c copy clip2.mkv
echo file clip1.mkv &gt;&gt; mylist.txt
echo file clip2.mkv &gt;&gt; mylist.txt
ffmpeg -f concat -i mylist.txt -c copy out.mkv
rm mylist.txt
</code></pre>
<p>Just make two clips from the original video &quot;in.mkv&quot; and just concat them in the usual ffmpeg concat way.</p>
