how to strip the last slash of the directory path?

dir=${1%/}

will take the script's first parameter and remove a trailing slash if there is one.

To remove a trailing slash if there is one, you can use the suffix removal parameter expansion construct present in all POSIX-style shells:

x=${x%/}

There are a few complications. This only removes a single slash, so if you started with a/b/c// then you'll still end up with a slash. Furthermore, if the original path was /, you need to keep the slash. Here's a more complex solution that takes care of these cases:

case $x in
  *[!/]*/) x=${x%"${x##*[!/]}"};;
  *[/]) x="/";;
esac

Alternatively, in ksh, or in bash after shopt -s extglob:

[[ x = *[!/] ]] || x=${x%%*(/)}

Note that in many cases, it doesn't matter that there is a trailing slash. It does matter if the argument is a symbolic link to a directory: with a trailing slash, the argument designates the directory, whereas with no trailing slash, the argument designates the symbolic link itself. It also matters with a few other programs, for example the source argument of rsync is treated differently depending on the presence of a trailing slash.

realpath resolves given path. Among other things it also removes trailing slashes. Use -s to prevent following simlinks

DIR=/tmp/a///
echo $(realpath -s $DIR)
# output: /tmp/a