ffmpeg command for concatenate two mp3 files

To skip re-encoding, use the concat demuxer:

Create a text file

file '/path/to/first.mp3'
file '/path/to/second.mp3'

and then

ffmpeg -f concat -i list.txt -c copy out.mp3

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If re-encoding is fine,

ffmpeg -i first.mp3 -i second.mp3 -filter_complex [0:a][1:a]concat=n=2:v=0:a=1 out.mp3

Usually,

cat first.mp3 second.mp3 > out.mp3

should just work. You didn't say what goes wrong when you try it.

Alternatively, you can use mp3wrap:

mp3wrap out.mp3 first.mp3 second.mp3 third.mp3 ...

This doesn't re-encode the MP3s like ffmpeg would, it keeps the ID3 tags, and you can split the files again later with mp3split.

I advise against using ffmpeg or similar programs, because reencoding causes loss of quality.

you could also use cat [your files] > concat.mp3 which will produce a mp3 file that has multiple headers and ID3 tags. Afterwards you can correct this by invoking mp3val -f -nb concat.mp3.