#!/bin/bash
# Moves and links a file back at its initial location
# It can not cross filesystems!
#
# Usage: mvln PATH1 PATH2
# Depends on: realpath, basename, dirname

if [[ $# -ne 2 ]]
then
  echo "Usage: mvln <PATH1> <PATH2>"
  exit 0
fi

# Step 1: building full path out of both arguments #############################
[[ $1 =~ ^/ ]] && F1=$1 || F1=$(realpath -s $1)
[[ $2 =~ ^/ ]] && F2=$2 || F2=$(realpath -s $2)

# Step 2: enumerating all interesting cases ####################################
# PATH1 can be:
if    [[ -f $F1 ]]
then STATE1="0"         # a regular file
elif  [[ -d $PWD/$F1 ]]
then STATE1="1"         # a directory
elif  [[ -e $F1 ]]
then STATE1="8"         # an other kind of file
else STATE1="9"         # a non-existent file
fi

# PATH2 can be:
if    [[ -d $F2 ]]
then STATE2="0"         # a directory
elif  [[ -d $(dirname $F2) ]]
then STATE2="1"         # a non-existent file in an existent directory
elif  [[ -e $F2 ]]
then STATE2="8"         # any other kind of file
else STATE2="9"         # a non-existent file in a non-existent directory
fi

# Step 3: eat this my little state automaton ###################################
STATE="$STATE1$STATE2"
case $STATE in
  [89]* )
    >&2 echo "$F1 is not a proper file (or a non-existent one)"
    >&2 echo "Note that only regular files and directories are allowed"
    exit $STATE
    ;;
  *[89] )
    >&2 echo "$F2 can not be created (containing directory does not exist)"
    >&2 echo "or exists already (refusing to overwrite)"
    exit $STATE
    ;;
  [01]0 ) # file or dir to an existing dir
    DEGUB="File/Dir to same name in existing dir"
    PATH1=$F1
    PATH2=$F2/$(basename $F1)
    ;;
  [01]1 ) # file to new file name in an existing dir
    DEGUB="File/Dir to new name in existing dir"
    PATH1=$F1
    PATH2=$F2
    ;;
  *     )
    echo "Uncaught error (this is f*****g bad)"
    exit $STATE
    ;;
esac

#echo $DEGUB

# Step 4: eventually act upon gathered knowledge ###############################
mv $PATH1 $PATH2      # Works across filesystems (fast enough!)
ln -s $PATH2 $PATH1   # Soft link back to old location