Run

local UserInputService = game:GetService("UserInputService")

local character = script.Parent
local humanoid = character.Humanoid

local WALKSPEED = 15
local RUNSPEED = 20

local walkAnim = humanoid:LoadAnimation(WALKANIMATION)
local runAnim = humanoid:LoadAnimation(RUNANIMATION)
local idleAnim =  humanoid:LoadAnimation(IDLEANIMATION)

local runKeyCode = Enum.KeyCode.LeftShift

local currentlyMoving = false -- set to true whenever move direction not 0
local currentlyIdle = false -- idle state, true whenever running (idle anim playing)
local currentlyRunning = false -- run state, true whenever running (running anim playing)
local currentlyWalking = false -- walk state, true whenever running (walk anim playing)

local holdingShift = true

local function run()
    if currentlyIdle then
        currentlyIdle = false
        idleAnim:Stop()
    end
    if currentlyWalking then
        currentlyWalking = false
        walkAnim:Stop()
    end
    currentlyRunning = true
    humanoid.WalkSpeed = RUNSPEED
    runAnim:Play()
end

local function walk()
    if currentlyIdle then
        currentlyIdle = false
        idleAnim:Stop()
    end
    if currentlyRunning then
        currentlyRunning = false
        runAnim:Stop()
    end
    currentlyWalking = true
    humanoid.WalkSpeed = WALKSPEED
    walkAnim:Play()
end

local function idle()
    if currentlyWalking then
        currentlyWalking = false
        walkAnim:Stop()
    end
    if currentlyRunning then
        currentlyRunning = false
        runAnim:Stop()
    end
    currentlyIdle = true
    idleAnim:Play()
end


UserInputService.InputBegan:Connect(function(input, gameprocess)
    if gameprocess or not input.KeyCode == runKeyCode then return end
    holdingShift = true
    if currentlyMoving then
        run()
    end
end)

UserInputService.InputEnded:Connect(function(input, gameprocess)
    if gameprocess or not input.KeyCode == runKeyCode then return end
    holdingShift = false
    if currentlyMoving and currentlyRunning then
        walk()
    end
end)

humanoid:GetPropertyChangedSignal("MoveDirection"):Connect(function()
    if humanoid.MoveDirection.Magnitude > 0 then
        if currentlyMoving then return end
        currentlyMoving = true

        if holdingShift then
            run()
        else
            walk()
        end
    else
        currentlyMoving = false
        idle()
    end
end)


idle() -- default state