Crossfade two audio tracks using sox

crossfade.sh

#!/bin/bash

fade_length="$1"
first_file="$2"
second_file="$3"

fade_first="auto"
if [ "$4" != "" ]; then
    fade_first="$4"
fi

fade_second="auto"
if [ "$5" != "" ]; then
    fade_second="$5"
fi

fade_first_opts=
if [ "$fade_first" != "no" ]; then
    fade_first_opts="fade t 0 0:0:$fade_length 0:0:$fade_length"
fi

fade_second_opts=
if [ "$fade_second" != "no" ]; then
    fade_second_opts="fade t 0:0:$fade_length"
fi

first_length=`sox --info -D "$first_file"`

trim_length=`echo "$first_length - $fade_length" | bc`
crossfade_split_length=`echo "scale=2; $fade_length / 2.0" | bc`

fadeout1=$(mktemp).wav
fadeout2=$(mktemp).wav
fadein1=$(mktemp).wav
fadein2=$(mktemp).wav
crossfade=$(mktemp).wav
crossfade1=$(mktemp).wav
crossfade2=$(mktemp).wav
song1=$(mktemp).wav
song2=$(mktemp).wav

sox -D -G "$first_file" -e signed-integer -b 16 "$fadeout1" trim $trim_length $fade_first_opts
if [ "$fade_first" == "auto" ]; then
    RMS=`sox "$fadeout1" 2>&1 -n stat | grep RMS | grep amplitude | cut -d : -f 2 | cut -f 1`
    should_fade=`echo "$RMS > 0.1" | bc`
    if [ $should_fade == 0 ]; then
        fade_first_opts=""
    fi
fi

sox "$fadeout1" "$fadeout2" $fade_first_opts

sox -D -G "$second_file" -e signed-integer -b 16 "$fadein1" trim 0 $fade_length
if [ "$fade_second" == "auto" ]; then
    RMS=`sox "$fadein1" 2>&1 -n stat | grep RMS | grep amplitude | cut -d : -f 2 | cut -f 1`
    should_fade=`echo "$RMS > 0.1" | bc`
    if [ $should_fade == 0 ]; then
        fade_second_opts=""
    fi
fi

sox "$fadein1" "$fadein2" $fade_second_opts
sox -m -D -G -v 1.0 "$fadeout2" -v 1.0 "$fadein2" "$crossfade"

sox "$crossfade" "$crossfade1" trim 0 $crossfade_split_length
sox "$crossfade" "$crossfade2" trim $crossfade_split_length

sox -D -G "$first_file" -e signed-integer -b 16 "$song1" trim 0 $trim_length
sox -D -G "$second_file" -e signed-integer -b 16 "$song2" trim $fade_length

sox "$song1" "$crossfade1" "$first_file"
sox "$crossfade2" "$song2" "$second_file"