mjhjmtu

I want to know how to count the characters in every line in a document, then subtracting this number from a threshold represents the maximums number of characters allowed per line, then filling the gaps between the maximum number of characters and the count of characters with dashes. For example:

Unix was 
originally meant 
to be a 
co
nvenient p
latform for progra
mmers developing software to
 be run on it and on other 
systems, rather than for non-
programmers.
[7][8] The system grew larger
 as the operating system star
ted spreading in a
cademic circ
les, as users add
ed their own tools to th
e system and shared them wi
th colleagues.

the maximum number of characters in the lines is 28 in line#10, I want the count of characters in every line to be 28 by filling it with dashes

Unix was , .....................
originally meant , .............
to be a , ......................
co, ............................
nvenient p, ....................
latform for progra, ............
mmers developing software to, ..
be run on it and on other , ....
systems, rather than for non-,..
programmers., ..................
[7][8] The system grew larger,..
as the operating system star, ..
ted spreading in a, ............
cademic circ, ..................
les, as users add, .............
ed their own tools to th, ......
e system and shared them wi, ...
th colleagues., ................

How can I do that in bash?
thanks!