#!/bin/bash
# Copyright (C) 2014-2016 Ian Kelling
# This program is under GPL v. 3 or later, see <http://www.gnu.org/licenses/>

_lnf_existing_link() {
    local target dest_file dest_dir
    target="$1"
    dest_file="$2"
    dest_dir="$3"
    if [[ -L $dest_file ]]; then
        if [[ $(readlink $dest_file) == "$target" ]]; then
            # Leave the link in place, but make sure it's
            # owner & group is as if we created it.
            # links all get 777 perms, so
            # we already know that is right.

            # test for setgid.
            if [[ $(stat -L -c%a "$dest_dir") == 2??? ]]; then
                grp=$(stat -L -c%g "$dest_dir") || return $?
            else
                grp=$(id -g) || return $?
            fi
            if [[ $EUID == 0 && $(stat -c%u "$dest_file") != 0 ]]; then
                chown -h 0:$grp "$dest_file" || return $?
            elif [[ $(stat -c%g "$dest_file") != "$grp" ]]; then
                chgrp -h $grp "$dest_file" || return $?
            fi
            do_exit=true
            return 0
        fi
        to_remove+=("$dest_file")
    elif [[ -e $dest_file ]]; then
        to_remove+=("$dest_file")
    fi
    to_link+=("$target")
}
lnf() {
    local help="Usage:
       lnf [OPTIONS] -T TARGET LINK_NAME     (1st form)
       lnf [OPTIONS] TARGET                  (2nd form)
       lnf [OPTIONS] TARGET... DIRECTORY     (3rd form)
Create symlinks forcefully

If the link already exists, make it's ownership be the same as if it was
newly created (only chown if we are root).  Removes existing files using
trash-put or rm -rf if it is not available, or if trash-put fails due to a
limitation such as a cross-filesystem link. Create directory of link if
needed. Slightly more restrictive arguments than ln.

In the 1st form, create a link to TARGET with the name LINK_NAME.  In the 2nd
form, create a link to TARGET in the current directory.  In the 3rd form, create
links to each TARGET in DIRECTORY.

-n|--dry-run     Do verbose dry run.
-v|--verbose     Print commands which modify the filesystem.
-h|--help        Print help and exit.
"


    local temp nodir
    local verbose=false
    local dry_run=false
    local do_exit=false
    temp=$(getopt -l help,dry-run,verbose hnTv "$@") || usage 1
    eval set -- "$temp"
    while true; do
        case $1 in
            -n|--dry-run) dry_run=true; verbose=true; shift ;;
            -T) nodir=-T; shift ;;
            -v|--verbose) verbose=true; shift ;;
            -h|--help) echo "$help"; return 0 ;;
            --) shift; break ;;
            *) echo "$0: Internal error! unexpected args: $*" ; exit 1 ;;
        esac
    done

    if (( $# == 0 )); then
        echo "$help"
        return 1
    fi

    if [[ $nodir ]]; then
        if (( $# != 2 )); then
            echo "lnf: error: expected 2 arguments with -T flag. Got $#"
            return 1
        fi
    fi

    local reset_extglob=false
    ! shopt extglob >/dev/null && reset_extglob=true
    shopt -s extglob


    local -a to_remove to_link
    local x ret prefix dest_dir grp target
    local mkdir=false

    if [[ $nodir ]]; then
        dest_file="$2"
        dest_dir="$(dirname "$dest_file")"
        _lnf_existing_link "$1" "$dest_file" "$dest_dir" || return $?
        if $do_exit; then return 0; fi
        if [[ ! -d $dest_dir ]]; then
            mkdir=true
            if [[ -e $dest_dir || -L $dest_dir ]]; then
                to_remove+=("$dest_dir")
            fi
        fi
        to_link+=("$dest_file")
    elif (( $# >= 2 )); then
        dest_dir="${!#}"
        if [[ -d $dest_dir ]]; then
            prefix="$dest_dir" # last arg
            for target in "${@:1:$(( $# - 1 ))}"; do # all but last arg
                # Remove 1 or more trailing slashes, using.
                dest_file="${target%%+(/)}"
                # remove any leading directory components, add prefix
                dest_file="$prefix/${target##*/}"
                _lnf_existing_link "$target" "$dest_file" "$dest_dir"
            done
        else
            to_link+=("${@:1:$(( $# - 1 ))}")
            mkdir=true
        fi
        if (( ${#to_link[@]} == 0 )); then
            return 0
        fi
        to_link+=("$dest_dir")
    elif  [[ $# -eq 1 ]]; then
        dest_file="${1##*/}"
        _lnf_existing_link "$1" "$dest_file" . || return $?
        if $do_exit; then return 0; fi
    fi
    if (( ${#to_remove[@]} >= 1 )); then
        if type -P trash-put >/dev/null; then
            if $verbose; then
                echo "lnf: trash-put -- ${to_remove[*]}"
            fi
            if ! $dry_run; then
                trash-put -- "${to_remove[@]}" || ret=$?
            fi
            # trash-put will fail to trash a link that goes across filesystems (72),
            # and for empty files (74)
            # so revert to rm -rf in that case
            if [[ $ret == 72 ]]; then
                echo "lnf: using rm -rf to overcome cross filesystem trash-put limitation"
                rm -rf -- "${to_remove[@]}" || return $?
            elif [[ $ret == 74 ]]; then
                echo "lnf: using rm -rf to overcome empty file & hardlink trash-put limitation"
                rm -rf -- "${to_remove[@]}"
            elif [[ $ret && $ret != 0 ]]; then
                return $x
            fi
        else
            if $verbose; then
                echo "lnf: rm -rf -- ${to_remove[*]}"
            fi
            if ! $dry_run; then
                rm -rf -- "${to_remove[@]}"
            fi
        fi
    fi

    $reset_extglob && shopt -u extglob

    if $mkdir; then
        if $verbose; then
            echo "lnf: mkdir -p $dest_dir"
        fi

        if ! $dry_run && ! mkdir -p "$dest_dir"; then
            echo "lnf error: failed to make directory $dest_dir"
            return 1
        fi
    fi

    if $verbose; then
        echo "lnf: ln -s $nodir -- ${to_link[*]}"
    fi
    if ! $dry_run; then
        ln -s $nodir -- "${to_link[@]}"
    fi
}
lnf "$@"