Getting relative paths in BASH

Question

I already searched for this, but I guess there was no great demand on working with paths. So I'm trying two write a bash script to convert my music collection using tta and cue files. My directory structure is as following: /Volumes/External/Music/Just/Some/Dirs/Album.tta for the tta files and /Volumes/External/Cuesheets/Just/Some/Dirs/Album.cue for cue sheets.

My current approach is setting /Volumes/External as "root_dir" and get the relative path of the album.tta file to $ROOT_DIR/Music (in this case this would be Just/Some/Dirs/Album.tta), then add this result to $ROOT_DIR/Cuesheets and change the suffix from .tta to .cue.

My current problem is, that dirname returns paths as they are, which means /Volumes/External/Music/Just/Some/Dirs does not get converted to ./Just/Some/Dirs/ when my current folder is $ROOT_DIR/Music and the absolute path was given.

Add://Here is the script if anybody has similar problems:

#!/bin/bash
ROOT_DIR=/Volumes/External
BASE="$1"

if [ ! -f "$BASE" ]
then
    echo "Not a file"
    exit 1
fi

if [ -n "$2" ]
then
    OUTPUT_DIR="$HOME/tmp"
else
    OUTPUT_DIR="$2"
fi
mkfdir -p "$OUTPUT_DIR" || exit 1

BASE=${BASE#"$ROOT_DIR/Music/"}
BASE=${BASE%.*}

TTA_FILE="$ROOT_DIR/Music/$BASE.tta"
CUE_FILE="$ROOT_DIR/Cuesheets/$BASE.cue"
shntool split -f "${CUE_FILE}" -o aiff -t "%n %t" -d "${OUTPUT_DIR}" "${TTA_FILE}"
exit 0

Answer 1

If your Cuesheets dir is always in the same directory as your Music, you can just remove root_dir from the path, and what is left is the relative path. If you have the path to your album.tta in album_path (album_path=/Volumes/External/Music/Just/Some/Dirs/Album.tta) and your root_dir set(root_dir=/Volumes/External), just do ${album_path#$root_dir}. This trims root_dir from the front of album_path, so you are left with album_path=Just/Some/Dirs/Album.tta.

See bash docs for more information on bash string manipulation

EDIT:// Changed ${$album_path#$root_dir} to ${album_path#$root_dir}

Answer 2

Okay so I've tackled this a couple of ways in the past. I don't recommend screwing with paths and pwd environment variables, I've seen some catastrophic events because of it.

Here's what I would do

CURRENTDIR=/Volumes/External/Music # make sure you check the existence in your script
...
SEDVAL=$(echo $CURRENTDIR | sed s/'\/'/'\\\/'/g)
#run your loops for iterating through files
for a in $(find ./ -name \*ogg); do
  FILE=`echo $a | sed s/$SEDVAL/./g`  # strip the initial directory and replace it with .
  convert_file $FILE # whatever action to be performed
done

If this is something you might do frequently I would actually just write a separate script just for this.