shell/caucase.sh: Make shellcheck happy.

Do not rely on test's -a & -o.
Escape backslashes which are intended as literals.
Avoid one useless "cat".
Avoid testing $?.
Simplify "is integer ?" test.
Quote a few variable expansions.
Arithmetic expression does not need explicit expansion.
Split declaration and assignment to unmask status.
Disable shellcheck warning about "local" being undefined in POSIX.