Ensuring a single colon at start of string

Question

I am using getopts to parse options. I want to ensure that the optstring has only a single colon at the start, and if this does not exist I want to introduce it.

Here is an example of the initialisation of optstring

  local optstring="sn:z:" 

Answer 1

optstring=:${optstring#:}

First we unconditionally add a colon, then we use a parameter expansion to expand our string with any preexisting colon removed.