Write [addpath PATH "/some/path"] instead of [export PATH="/some/path:$PATH"]

add_path_by_variable.sh

#/bin/bash

# License-free

# Example: addpath PATH "/usr/bin"
addpath() {
        varname=$1
        export $1="$2:${!varname}"
}

export X="/fake/path0"

echo Before: $X
addpath X "/fake/path1"
echo After : $X