Correct syntax for a variable inside a quotes: MP4Box command

[snippsat]

Use string formatting and now should use f-string.
Example using your working command and put in variables.

>>> first = '/var/www/html/LifeSaverHTML/eg,h264'
>>> last = '/var/www/html/LifeSaverHTML/eg.mp4'
>>> command = f"MP4Box -add '{first}' '{last}'"
>>> print(command)
MP4Box -add '/var/www/html/LifeSaverHTML/eg,h264' '/var/www/html/LifeSaverHTML/eg.mp4'
>>> 
>>> #Assign to a variable the command is like this
>>> print(repr(command))
"MP4Box -add '/var/www/html/LifeSaverHTML/eg,h264' '/var/www/html/LifeSaverHTML/eg.mp4'"