#!/bin/bash
PROGNAME=${0##*/}

DSTGPX=$(mktemp /tmp/$PROGNAME.XXXXXXXX.gpx)
SRCGPXS=( ~/doc/def/travel/holidays/*/tracks/alexis/*simpl*.gpx )

#  Combine all tracks into one file. 
gpsbabel -i gpx $(for SRCGPX in "${SRCGPXS[@]}"; do echo -n "-f $SRCGPX "; done) -x simplify,count=800 -o gpx -F $DSTGPX
#  Strip out an GPX extensions (which, in my experience, only define
#  track colours).
perl -0777 -pi -e 's/<extensions>.*?<\/extensions>\n//sg' $DSTGPX
#  Insert new colours. I've spread the RHS of the s/.../.../ over 
#  several lines and added '/x' modified but this doesn't entirely
#  behave as I wanted (extra newlines are in the output). But
#  it still is valid GPX and it is more readable.
perl -0777 -pi -e '
    BEGIN { 
        #  Colours list taken from GORP.
        @colours=qw/DarkRed DarkGreen DarkBlue DarkMagenta DarkCyan Red Green Yellow Blue Magenta Cyan/; 
        #  We will start with colour #0.
        $i=0; 
    } 
    #  Insert colour-defining GPX extension block immediately after
    #  track name.
    s/
        (<trk>.*?<name>.*?<\/name>\n)
    /
        $1
        <extensions>
          <gpxx:TrackExtension>
            <gpxx:DisplayColor>$colours[($i++)%($#colours+1)]<\/gpxx:DisplayColor>
          <\/gpxx:TrackExtension>
        <\/extensions>
    /sgx
' $DSTGPX
echo $DSTGPX