Calculate relative path in BASH

_relpath.inc

: relpath A B
# Calculate relative path from A to B, returns true on success
# Example: ln -s "$(relpath "$A" "$B")" "$B"
relpath()
{
local X Y A
# We can create dangling softlinks
X="$(readlink -m -- "$1")" || return
Y="$(readlink -m -- "$2")" || return
X="${X%/}/"
A=""
# See http://stackoverflow.com/questions/2564634/bash-convert-absolute-path-into-relative-path-given-a-current-directory
while   Y="${Y%/*}"
        [ ".${X#"$Y"/}" = ".$X" ]
do
        A="../$A"
done
X="$A${X#"$Y"/}"
X="${X%/}"
echo "${X:-.}"
}

linkit.sh

# example use of relpath.inc, instead of "ln -vs A B" do "lns A B"
# idempotent, so no writes if everything is already OK

. "$(dirname "$0")/_relpath.inc"

OOPS()
{
echo "OOPS: $*" >&2
exit 1
}

NOTE()
{
echo "NOTE: $*" >&2
}

lns()
{
local R
if      [ -L "$2" ]
then
        # Check if both links are the same file
        [ ".$(readlink -m -- "$1")" = ".$(readlink -m -- "$2")" ] && return
        NOTE "replacing $2"
        rm -vf "$2"
else
        [ ! -e "$2" ] || OOPS "$2 exists and is no soflink"
        NOTE "creating $2"
fi
# Note: $2 must not exist, else relpath fails!
R="$(relpath "$1" "$2")" || OOPS "relpath '$1' '$2'"
ln -vs "$R" "$2"
}

lns "$@"

Renamed relpath.inc to _relpath.inc to have GitHub list the correct filename in GIST.

Note that lns is the shell function name, the shell script name is ./linkit.sh A B