Script to join many mp3 files into fewer ones (without re-encoding)

glue.sh

#!/bin/zsh

# first argument: number of files to join into new one, default 5
num=${1:-5}

tmpdir=$(mktemp --directory)
outdir=$(date +"%Y-%m-%d_%H-%M")
mkdir $outdir

ls > $tmpdir/files
(
    cd $tmpdir
    split --lines=$num files
    
    {
        print "#!/bin/zsh"
        print "out=$outdir"
    } > ff

    cat >> ff <<-'FOOBAR'
	(( $ARGC == 1 )) || { exit 1; }
	
        ffmpeg -loglevel quiet -i "concat:${(j:|:)${(f)$(cat $1)}}" -acodec copy "$out/$(basename $1).mp3"
	FOOBAR
    
    chmod u+x ff
)

parallel -j 4 $tmpdir/ff '{}' ::: $tmpdir/x*

Notable prerequisites:

• zsh
• ffmpeg
• GNU parallel (which could be easily avoided, though)

Usage:

cd /path/to/many/mp3files
/path/to/glue.sh 10

will take groups of 10 files from the current directory and join them into new files to be put in a subdirectory of the original one (i.e. if there were 300 files before, it will be 30 afterwards in this example).

Notes:

• Original filenames are lost in the process.
• In case of non-mp3 files in the directory, one needs to tweak line 10 to exclude, e.g. "folder.jpg" etc.
• In the same line, one might want to use ls -v instead to sort numbers naturally, i.e. 1,2,10 instead of 1,10,2.