Say I have the string \"Memory Used: 19.54M\" How would I extract the 19.54 from it? The 19.54 will change frequently so i need to store it in a variable and compare it with
You probably want to extract it rather than remove it. You can use the Parameter Expansion to extract the value:
var="Memory Used: 19.54M"
var=${var#*: } # Remove everything up to a colon and space
var=${var%M} # Remove the M at the end
Note that bash can only compare integers, it has no floating point arithmetics support.