This snippet detects various information about the shell script in which
it is being executed. Copy and paste it into the top of your script.

Variables set are as follows:

 * `$__SHELL`: `bash` or `zsh`
 * `$__SOURCED`: `true` if the file was invoked using `source filename` or `. filename`,
                 otherwise it will be blank.
 * `$__SCRIPT_PATH`: the full absolute path to the script.
 * `$__SCRIPT_DIR`: the full absolute path to the script's directory.
 * `$__SCRIPT_PATH_DEREFERENCED`: the full absolute path to the script,
    dereferencing all symbolic links.
 * `$__SCRIPT_DIR_DEREFERENCED`: the full absolute path to the script's directory,
    dereferencing all symbolic links.