fish-shell/share/functions/fish_add_path.fish

function fish_add_path --description "Add paths to the PATH"
    # This is meant to be the easy one-stop shop to adding stuff to $PATH.
    # By default it'll prepend the given paths to a universal $fish_user_paths, excluding the already-included ones.
    #
    # That means it can be executed once in an interactive session, or stuffed in config.fish,
    # and it will do The Right Thing.
    #
    # The options:
    # --prepend or --append to select whether to put the new paths first or last
    # --global or --universal to decide whether to use a universal or global fish_user_paths
    # --path to set $PATH instead
    # --move to move existing entries instead of ignoring them
    # --verbose to print the set-command used
    # --dry-run to print the set-command without running it
    # We do not allow setting $PATH universally.
    #
    # It defaults to keeping $fish_user_paths or creating a universal, prepending and ignoring existing entries.
    argparse -x g,U -x P,U -x a,p g/global U/universal P/path p/prepend a/append h/help m/move v/verbose n/dry-run -- $argv
    or return

    if set -q _flag_help
        __fish_print_help fish_add_path
        return 0
    end

    set -l scope $_flag_global $_flag_universal
    if not set -q scope[1]; and not set -q fish_user_paths
        set scope -U
    end

    set -l var fish_user_paths
    set -q _flag_path
    and set var PATH
    # $PATH should be global
    and set scope -g
    set -l mode $_flag_prepend $_flag_append
    set -q mode[1]; or set mode -p

    # To keep the order of our arguments, go through and save the ones we want to keep.
    set -l newpaths
    set -l indexes
    for path in $argv
        # Realpath allows us to canonicalize the path, which is needed for deduplication.
        # We could add a non-canonical version of the given path if no duplicate exists, but tbh that's a recipe for disaster.

        # realpath complains if a parent directory does not exist, so we silence stderr.
        set -l p (builtin realpath -s -- $path 2>/dev/null)

        # Ignore non-existing paths
        test -d "$p"; or continue

        if set -l ind (contains -i -- $p $$var)
            # In move-mode, we remove it from its current position and add it back.
            if set -q _flag_move
                set -a indexes $ind
                set -a newpaths $p
            end
        else
            # Without move, we only add it if it's not in.
            set -a newpaths $p
        end
    end

    # Ensure the variable is only set once, by constructing a new variable before.
    # This is to stop any handlers or anything from firing more than once.
    set -l newvar $$var
    if set -q _flag_move; and set -q indexes[1]
        # We remove in one step, so the indexes don't move.
        set -e newvar["$indexes"]
    end
    set $mode newvar $newpaths

    # Finally, only set if there is anything *to* set.
    # This saves us from setting, especially in the common case of someone putting this in config.fish
    # to ensure a path is in $PATH.
    if set -q newpaths[1]; or set -q indexes[1]
        if set -q _flag_verbose; or set -q _flag_n
            # The escape helps make it unambiguous - so you see whether an argument includes a space or something.
            echo (string escape -- set $scope $var $newvar)
        end

        not set -q _flag_n
        and set $scope $var $newvar
        return 0
    else
        return 1
    end
end