pathtools.bash

#!/bin/bash




# Functions for manipulating paths

# Counts the number of path elements
function count_path_elems() {
    P=$1
    IFS='/'
    set -f "$P"
    echo $#
}

# Choose the shortest path of the 2 arguments.
# This is expected to be used with equivalent relative and absolute paths.
# Where they are the same length, the first is preferred.
function shortest_path() {
    COUNT1=$(count_path_elems "$1")
    COUNT2=$(count_path_elems "$2")
    if [ "${COUNT1}" -le "${COUNT2}" ]; then
        echo "${1}"
    else
        echo "${2}"
    fi
}

# Portable version of readlink. There are no requirements on path components existing.
if which realpath >&/dev/null &&
   [[ -n "$(realpath --version 2>&1 | grep 'GNU coreutils')" ]]; then
    function bob_realpath {
        realpath -m "$1"
    }
else
    function bob_realpath() {
        python -c "import os, sys; print(os.path.realpath(sys.argv[1]))" "$1"
    }
fi

function bob_abspath() {
    python -c "import os, sys; print(os.path.abspath(sys.argv[1]))" "$1"
}

# Get the target of a symlink. If the target is a relative path, it is intended
# to be relative to the directory containing the link, not the current working
# directory, so it is prefixed with the link's dirname.
function bob_eval_link() {
    local link_target
    link_target="$(readlink "${1}")" link_dir=
    if [[ "${link_target:0:1}" != "/" ]]; then
        link_dir="$(dirname "${1}")"
        if [[ "${link_dir: -1}" != "/" ]]; then
            link_dir="${link_dir}/"
        fi
        link_target="${link_dir}${link_target}"
    fi
    echo "${link_target}"
}

function path_is_parent() {
    local parent="$1" subpath="$2"
    if [[ ${parent} == / ]]; then
        return 0
    elif [[ ${subpath} == ${parent}/* ]]; then
        return 0
    fi
    return 1
}

# Return a path that references $2 from $1
# $1 and $2 must exist
#
# The minimum possible number of symlinks will be followed, in order to
# preserve the filesystem structure as the user sees it. Symlinks inside the
# source path *are* expanded when it is required to access their parent
# directory, because `..` on a soft link returns the parent directory of the
# link's target, rather than the link.
function relative_path() {
    [[ -e $1 ]] || { echo "relative_path: Source path '$1' does not exist" >&2; return 1; }
    [[ -e $2 ]] || { echo "relative_path: Target path '$2' does not exist" >&2; return 1; }
    local SRC_ABS
    local TGT_ABS
    SRC_ABS=$(bob_abspath "${1}")
    TGT_ABS=$(bob_abspath "${2}")
    local BACK='' RESULT='' CMN_PFX='' RELPATH_FROM_LINK=''



    if [[ "${TGT_ABS}" == "${SRC_ABS}" ]]; then
        RESULT=.

    elif path_is_parent "${SRC_ABS}" "${TGT_ABS}"; then
        # SRC_ABS is a parent of TGT_ABS

        # Remove the trailing slash from the prefix if it has one
        SRC_ABS=${SRC_ABS%/}

        RESULT=${TGT_ABS#"${SRC_ABS}"/}

    elif path_is_parent "${TGT_ABS}" "${SRC_ABS}"; then
        # TGT_ABS is a parent of SRC_ABS

        while [[ "${TGT_ABS}" != "${SRC_ABS}" ]]; do
            if [[ -L ${SRC_ABS} ]]; then
                SRC_ABS="$(bob_eval_link "${SRC_ABS}")" || return $?
                RELPATH_FROM_LINK="$(relative_path "${SRC_ABS}" "${TGT_ABS}")" || return $?
                echo "${BACK}${RELPATH_FROM_LINK}"
                return
            fi
            SRC_ABS=$(dirname "${SRC_ABS}")
            BACK="../${BACK}"
        done

        RESULT=${BACK%/}

    else
        CMN_PFX=${SRC_ABS}

        while ! path_is_parent "${CMN_PFX}" "${TGT_ABS}"; do
            if [[ -L ${CMN_PFX} ]]; then
                CMN_PFX="$(bob_eval_link "${CMN_PFX}")" || return $?
                RELPATH_FROM_LINK="$(relative_path "${CMN_PFX}" "${TGT_ABS}")" || return $?
                echo "${BACK}${RELPATH_FROM_LINK}"
                return
            fi
            CMN_PFX=$(dirname "${CMN_PFX}")
            BACK="../${BACK}"
        done

        # Remove the trailing slash from the prefix if it has one
        CMN_PFX=${CMN_PFX%/}

        RESULT=${BACK}${TGT_ABS#"${CMN_PFX}"/}
    fi

    echo "${RESULT}"
}