[Tkinter] Can't get progressbar to update

[frednet]

I'm trying to create a program that uses a progress bar to show the CPU temperature and it works, but only once!

# importing tkinter module 
from tkinter import *
from tkinter.ttk import *
import subprocess 

# creating tkinter window 
root = Tk() 
  
# Progress bar widget 
root.progress = Progressbar(root, orient = HORIZONTAL, 
              length = 100, mode = 'determinate') 
  
# Function responsible for the updating of the progress bar value 
def bar(): 
    out = subprocess.Popen(['vcgencmd', 'measure_temp'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    stdout,stderr = out.communicate()
    s=(stdout)
    value= (s[5:9])
    f=float(value)
    print('Float Value =', f)
    root.progress['value'] = f
  
root.progress.pack(pady = 10) 
  
# This doesn't work
#while True:
#    bar()

bar()
root.mainloop()

When I delete line 29

bar()

and un-comment line 26 & 27

while True:
    bar()

the window never even appears but the temperature does repeatedly update in the shell. What am I doing wrong?