Bash: only last iteration of loop index not being stored as new element of array

Dylan,

Your homework give me an idea to do something with my music library too, and I see than you trying do your homework alone, so here is some comments - how to make an musicsort command :)

The main power of shell programming is in his ability pipelining and decomposing the job into small parts, while these small parts can easily play together.

therefore,
It is not a good idea changing the column orders. See for example ls -l. It does not matter by what you want sort the output (e.g. ls -lt = by time, or ls -ltr = by time but reversed) the column orders remain the same. This way, you can easily pipe output from ls to another command without worrying about the column order. And if you really need changing it, here are already tools what can do it effectively.

My 1st suggestion - don't change the output columns order only sort by them.

Second - the header line. Print put only when really want it. (e.g. for normal output), becuse when you later want pipelining output from your brand new musicsort command, headers will cause much problems. So,

my 2nd suggestion - print header only based on command argument.

When we decomposing your problem, we get:

1. need some command-line argument handling
2. need set some defaults, if here are no arguments
3. need find music files in your music directory
4. need sort them by criteria
5. need print them - in sorted order

Skipping 1,2 for now.

finding files in your musicdir is easy.

find "$musicdir" -type f -print

will print out all files recusively. ofc, here can be some cover images and txt lyrics so, need filter them, for example with

find "$musicdir" -type f -print | egrep -i '\.(mp3|aif*|m4p|wav|flac)$'

we have all your music files. Nicely delimited with the '/' character and in order

/path/to/musicdir/genre/artist/album/track.suffix

For the output we need remove the /path/to/musicdir/. It is easy. Here is more way, for example sed.

sed "s:^$musicdir/::;s:\.[^/][^/]*$::"

The above command do two things: 1.) removing the $musicdir path from your filelist, and remove any .suffix too. (like .mp3 .flac etc.). The result is:

genre/artist/album/track

Nice string, clearly separated - so sortable. For the sorting we have the sort command. the sort can sort by any field, and is possible tell him what is the field separator.
E.g.

sort -df -t/ -k2,2

will sort the input separated with '/' by second field (artist). For the -df see man sort.

and finally, we need read the already sorted list of files into variables and print out. (here is ofc another way too). For this bash has the read command. And we must tell bash what is its temporary field separator (IFS), so:

IFS=/; read genre artist album track

and because we have more lines on input need do this in cycle, while we have lines on input.

The final script is here:

musicdir="."
FORMAT="%-20s%-35s%-35s%-35s\n"
sortby=2 #for this example - artist

find "$musicdir" -type f -print |\
egrep -i '\.(aif*|mp3|flac|m4a|wav)$' |\
sed "s:^$musicdir/::;s:\.[^/][^/]*$::" |\
sort -t/ -k$sortby,$sortby | (
IFS=/; while read genre artist album track
do
        printf "$FORMAT" $genre $artist $album $track
done)

As you see, the whole searching, sorting, printing is done in few lines. (parts 3, 4, 5).

For the final, need make some argument handling. I wrote one, it is not 100% ok, but working.

The final script what can handle some arguments, set up defaults, and do the main functionality can look like the next: (ofc, here is possible do zilion optimizations, for example combine the egrep and the sed into only one sed and etc...)

#!/bin/bash
#argument handling - not 100% correct, but working...
while getopts "hHgaltd:" arg
do
case "$arg" in
    g) sortby=1;;
    a) sortby=2;;
    l) sortby=3;;
    t) sortby=4;;
    d) musicdir=$OPTARG;;
    H) header=y;;
    h|?)    echo "Usage: $0 [-d music_dir] [-g|-a|-l|-t] [-H]";
        echo ' -d music_dir = path to your music directory (default ".")'
        echo ' -g|-a|-l|-t = for sorting by Genre/Artist/aLbum/Track (default "-a")'
        echo ' -H print header (default no)'
        exit 1;;
esac
done

#defaults
sortby=${sortby:=2};
musicdir=${musicdir:=.}
FORMAT="%-20s%-35s%-35s%-35s\n"

#header only if want one
if [[ $header == "y" ]]
then
    printf "$FORMAT" genre artist album track
    printf -v line '%*s' 125; echo ${line// /-}
fi

#the main part - search, sort, read into variables, print
find "$musicdir" -type f -print |\
egrep -i '\.(aif*|mp3|flac|m4a|wav)$' |\
sed "s:^$musicdir/::;s:\.[^/][^/]*$::" |\
sort -t/ -k$sortby,$sortby | (
IFS=/; while read genre artist album track
do
    printf "$FORMAT" $genre $artist $album $track
done)

so for example the

$ musicsort -t -H -d .

will produce the output sorted by tracks, print header and music is in current directory

genre               artist                             album                              track                              
-----------------------------------------------------------------------------------------------------------------------------
Electronic          Squarepusher                       Hard Normal Daddy                  Cooper's World                     
Electronic          Squarepusher                       Go Plastic                         Go! Spastic                        
Electronic          Squarepusher                       Go Plastic                         Greenways Trajectory               
Electronic          Squarepusher                       Feed Me Weird Things               Kodack                             
Electronic          Squarepusher                       Go Plastic                         My Red Hot Car                     
Electronic          Squarepusher                       Feed Me Weird Things               North Circular                     
Electronic          Squarepusher                       Hard Normal Daddy                  Papalon                            
Hip-Hop             OutKast                            Stankonia                          Toilet Tisha                       
Electronic          Squarepusher                       Feed Me Weird Things               Tundra                             
Electronic          Squarepusher                       Hard Normal Daddy                  Vic Acid        

As you see, the 3/4 lines are argument handling and other things. The main part is done in few lines.

If you really need change the columns order, can be easily done, by adding few formatting lines...