#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

# Create symlink after backing up the target.

usage="Usage: ${0##*/} source_file target_file"
src=${1:?$usage}
tgt=${2:?$usage}
bak=~/.backup

[[ ! -d $bak ]] && mkdir $bak

# Check if the desired link already exists.
[[ -L "$tgt" && "$(readlink $tgt)" = "$src" ]] && exit

# Move an existing file to the back up folder.
[[ -e "$tgt" ]] && mv -v "$tgt" "$bak/${tgt##*/}-$(date '+%Y%m%d-%H%M%S')" || true

ln -sfv "$src" "$tgt"