A Bash script providing a command for computing relative path between two filesystem paths

relpath

#!/bin/bash
# relpath -- Compute relative path from a given DIR to given PATHs
# Usage: relpath DIR PATH...
# 
# Example: relpath /a/b/c /a/d/e/f
#  prints: ../../d/e/f
# 
# Example: relpath /a/b/c /
#  prints: ../../../
# 
# Example: relpath /a/b/c /a/b/c/g/h
#  prints: g/h

relpath() {
    local from=$1; shift
    from=${from//\/\//\/}
    from=${from%/}
    IFS=/ dirs=(${from#/})
    local to=
    for to; do
        to=${to//\/\//\/}
        to=${to%/}
        local commonPrefix=/ d=
        for d in "${dirs[@]}"; do
            case "$to/" in "$commonPrefix$d/"*) ;;
                *)
                    break
                    ;;
            esac
            commonPrefix+="$d/"
        done
        local ancestor="${from#${commonPrefix%/}}"
        ancestor=${ancestor//[^\/]/}
        ancestor=${ancestor//\//..\/}
        echo "$ancestor${to#$commonPrefix}"
    done
}

relpath "$@"