One liner:
sVar=$(whoami) sVar=${sVar^} sVar=${sVar::1}
It may not be what you were looking for, but it is certainly a single command and uses parameter substitutions. A single command can consist of multiple variable assignments, in which case they are performed left to right.
The subject of a parameter substitution is a variable name, an indirection (a ! followed by a variable name), or a subscript expression (a variable name -- not an indirection -- followed by a subscript within [ and ]. That's a bit limited, to be sure, but that's bash. (Posix shell is even more restrictive; it has no indirection nor arrays, and fewer types of expansion.)
Posix wording from XCU, emphasis added:
When a given simple command is required to be executed… the following expansions, assignments, and redirections shall all be performed from the beginning of the command text to the end:
…
- Each variable assignment shall be expanded for tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal prior to assigning the value.
