$ useradd --help
useradd: invalid option -- -
usage: useradd  [-u uid [-o]] [-g group] [-G group,...] 
                [-d home] [-s shell] [-c comment] [-m [-k template]]
                [-f inactive] [-e expire ] [-p passwd] [-M] [-n] [-r] name
       useradd  -D [-g group] [-b base] [-s shell]
                [-f inactive] [-e expire ]