Dual booting with shared DB drive possible?

TripleS, Sorry, my last publication was one of my drafts. For sure it will not work. It was debug version (some syntax error was still there, missing punctuation). After install I didn't use it for long time (multiple versions was on same place). The next is the last actual version:

#!/bin/bash
##############################################################################
#                    Workaround for Dropbox daemon
#                    -----------------------------
# Verified on Dropbox v73.4.118
# Just make it executable (if need, using
#   $ chmod a+x fix_dropbox
# ) and run it.
# Author: Здравко
#   www.dropboxforum.com/t5/user/viewprofilepage/user-id/422790
##############################################################################

# Figure out the target places.
USR_HOME=`realpath ~`
USR_LOCAL="$USR_HOME/.local"
USR_LOCAL_BIN="$USR_LOCAL/bin"
USR_LOCAL_LIB="$USR_LOCAL/lib"
DRB_REPLACEMENT="$USR_LOCAL_BIN/dropbox"
DRB_FIX_LIB="$USR_LOCAL_LIB/libdropbox_ext4.so"
USR_PROFILE="$USR_HOME/.profile"

##############################################################################
# Precheck and fix up some prerequisites.

# Ensure target directories are on their place.
if [[ ! -d $USR_LOCAL_BIN ]]
then
  echo "The Directory \"$USR_LOCAL_BIN\" doesn't exist, going to create it."
  mkdir -p $USR_LOCAL_BIN
  if [[ $? -ne 0 || ! -d $USR_LOCAL_BIN ]]
  then
    echo "Directory \"$USR_LOCAL_BIN\" creation fail! Exit."
    exit 1
  fi
fi
if [[ ! -d $USR_LOCAL_LIB ]]
then
  echo "The Directory \"$USR_LOCAL_LIB\" doesn't exist, going to create it."
  mkdir -p $USR_LOCAL_LIB
  if [[ $? -ne 0 || ! -d $USR_LOCAL_LIB ]]
  then
    echo "Directory \"$USR_LOCAL_LIB\" creation fail! Exit."
    exit 1
  fi
fi

# Ensure backup for any existing files.
if [[ -e $DRB_REPLACEMENT ]]
then
  if [[ -f $DRB_REPLACEMENT ]]
  then
    echo "\"$DRB_REPLACEMENT\" already exist, going to back it up."
    mv --backup=t -T $DRB_REPLACEMENT "${DRB_REPLACEMENT}.backup"
    if [[ $? -ne 0 || -e $DRB_REPLACEMENT ]]
    then
      echo "Error backing up \"$DRB_REPLACEMENT\" file! Exit."
      exit 1
    fi
  else
    echo "$DRB_REPLACEMENT is not regular file! Exit."
    exit 1
  fi
fi
if [[ -e $DRB_FIX_LIB ]]
then
  if [[ -f $DRB_FIX_LIB ]]
  then
    echo "\"$DRB_FIX_LIB\" already exist, going to back it up."
    mv --backup=t -T $DRB_FIX_LIB "${DRB_FIX_LIB}.backup"
    if [[ $? -ne 0 || -e $DRB_FIX_LIB ]]
    then
      echo "Error backing up \"$DRB_FIX_LIB\" file! Exit."
      exit 1
    fi
  else
    echo "$DRB_FIX_LIB is not regular file! Exit."
    exit 1
  fi
fi

# Ensure writing permissions.
if [[ ! -w $USR_LOCAL_BIN || ! -x $USR_LOCAL_BIN ]]
then
  chmod u+rwx $USR_LOCAL_BIN
  if [[ $? -ne 0 ]]
  then
    echo "The Directory \"$USR_LOCAL_BIN\" is inaccessible! Exit."
    exit 1
  fi
fi
if [[ ! -w $USR_LOCAL_LIB || ! -x $USR_LOCAL_LIB ]]
then
  chmod u+rwx $USR_LOCAL_LIB
  if [[ $? -ne 0 ]]
  then
    echo "The Directory \"$USR_LOCAL_LIB\" is inaccessible! Exit."
    exit 1
  fi
fi

##############################################################################
# Generate actual workaround components.

# Fixer - source and building.
gcc -shared -fPIC -ldl -xc -o $DRB_FIX_LIB - << EndOfSrc
// Functions replacements
#define _GNU_SOURCE

#include <stdlib.h>
#include <sys/vfs.h>
#include <linux/magic.h>
#include <dlfcn.h>
#include <sys/statvfs.h>
#include <stdarg.h>

static int (*orig_statfs)(const char *path, struct statfs *buf) = NULL;
static int (*orig_statfs64)(const char *path, struct statfs64 *buf) = NULL;
static int (*orig_open64)(const char *pathname, int flags, ...) = NULL;

int statfs(const char *path, struct statfs *buf) {
  if (orig_statfs == NULL) {
    orig_statfs = dlsym(RTLD_NEXT, "statfs");
  }
  register int retval = orig_statfs(path, buf);
  if (retval == 0) {
    buf->f_type = EXT4_SUPER_MAGIC;
  }
  return retval;
}
 
int statfs64(const char *path, struct statfs64 *buf) {
  if (orig_statfs64 == NULL) {
    orig_statfs64 = dlsym(RTLD_NEXT, "statfs64");
  }
  register int retval = orig_statfs64(path, buf);
  if (retval == 0) {
    buf->f_type = EXT4_SUPER_MAGIC;
  }
  return retval;
}

int open64(const char *pathname, int flags, ...) {
  if (orig_open64 == NULL) {
    orig_open64 = dlsym(RTLD_NEXT, "open64");
  }
  register const char *p0 = "/proc/filesystems", *p1 = pathname;
  while(*p0 && *p1 && *p0 == *p1) ++p0, ++p1;
  if (*p0 == '\0' && *p1 == '\0') {
    return -1;
  }
  va_list arg;
  va_start(arg, flags);
  mode_t mode = va_arg (arg, int);
  va_end(arg);
  return orig_open64(pathname, flags, mode);
}
EndOfSrc
if [[ $? -ne 0 ]]
then
  echo "Error building \"$DRB_FIX_LIB\"! Exit."
  exit 2
fi
echo "\"$DRB_FIX_LIB\" - build and ready."

# Wrapper generating.
cat > $DRB_REPLACEMENT << WrapperHead
#!/bin/bash
if [ \$1 == "start" ]
then
  sleep 1
  patchpath=\`which \$0\`
  if echo \$(for i in \`ps -C dropbox -o pid\`
            do ls -l /proc/\$i/exe 2>/dev/null | grep -v \$patchpath
            done) | grep dropbox > /dev/null
  then
    timeout=300
    # Make sure there isn't any concurrent start operation in progress.
    /usr/bin/dropbox running
    while [[ \$? -eq 0 && \$timeout -ne 0 ]]
    do
      sleep 1
      ((--timeout))
      if echo \$(for i in \`ps -C dropbox -o pid\`
                do ls -l /proc/\$i/exe 2>/dev/null | grep -v \$patchpath
                done) | grep dropbox > /dev/null
      then
        /usr/bin/dropbox running
      fi
    done
    # If there is running application instance, force its stop.
    /usr/bin/dropbox running
    while [[ \$? -ne 0 && \$timeout -ne 0 ]]
    do
      /usr/bin/dropbox stop
      sleep 1
      ((--timeout))
      /usr/bin/dropbox running
    done
    # If something goes wrong...
    if [[ \$timeout -eq 0 ]]
    then
      # ...notify somehow about.
      title="Problem launching Dropbox"
      mainmsg="Possible existing application instance dead"
      submsg="Check that, clear and try start again"
      if which notify-send > /dev/null
      then notify-send --icon=dropbox "\$title" "<b>\$mainmsg\!</b>\n\$submsg."
      elif which zenity > /dev/null
      then zenity --error --title="\$title" --timeout=10 --width=380 \\
        --text="<span size=\"xx-large\"><b>\$mainmsg\!</b>\n\$submsg.</span>" \\
        2> /dev/null
      else echo "  \$title!"
        echo "\$mainmsg."
        echo "\$submsg."
      fi
      exit 1
    fi
  fi
fi
WrapperHead
declare -p | grep "declare -x LD_PRELOAD" > /dev/null
if [[ $? -ne 0 ]]
then
  echo "export LD_PRELOAD=$DRB_FIX_LIB" >> $DRB_REPLACEMENT
else
  echo "LD_PRELOAD=$DRB_FIX_LIB:\$LD_PRELOAD" >> $DRB_REPLACEMENT
fi
echo "/usr/bin/dropbox \$@" >> $DRB_REPLACEMENT
chmod a+x $DRB_REPLACEMENT
echo "\"$DRB_REPLACEMENT\" - generated."

# Environment check and fixup (if need).
echo ""
echo ""
(
IFS=:
for a in $PATH
do
  if [[ "$a" == "$USR_LOCAL_BIN" ]]
  then
    exit 0
  elif [[ "$a" == "/usr/bin" ]]
  then
    break
  fi
done
exit 1
)
if [[ $? -ne 0 ]]
then
  echo "" >> $USR_PROFILE
  echo "" >> $USR_PROFILE
  echo "# Add $USR_LOCAL_BIN to \$PATH environment variable." >> $USR_PROFILE
  echo "# This addition is dedicated for Dropbox Workaround." >> $USR_PROFILE
  echo "# Remove theses lines when they are no more needed." >> $USR_PROFILE
  echo "PATH=\"$USR_LOCAL_BIN:\$PATH\"" >> $USR_PROFILE
  echo "" >> $USR_PROFILE
  echo "Your Dropbox is almost patched. You have to logout and login"
  echo " (or on Your opinion - restart), so all changes to take effect."
  echo "After Your next account login:"
fi
echo "Your Dropbox is already patched. Just start it from desktop GUI or type:"
echo "\$ dropbox start"
echo "in console."